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<!--{
"title": "About the go command"
}-->
<p>The Go distribution includes a command, named
"<code><a href="/cmd/go/">go</a></code>", that
automates the downloading, building, installation, and testing of Go packages
and commands. This document talks about why we wrote a new command, what it
is, what it's not, and how to use it.</p>
<h2>Motivation</h2>
<p>You might have seen early Go talks in which Rob Pike jokes that the idea
for Go arose while waiting for a large Google server to compile. That
really was the motivation for Go: to build a language that worked well
for building the large software that Google writes and runs. It was
clear from the start that such a language must provide a way to
express dependencies between code libraries clearly, hence the package
grouping and the explicit import blocks. It was also clear from the
start that you might want arbitrary syntax for describing the code
being imported; this is why import paths are string literals.</p>
<p>An explicit goal for Go from the beginning was to be able to build Go
code using only the information found in the source itself, not
needing to write a makefile or one of the many modern replacements for
makefiles. If Go needed a configuration file to explain how to build
your program, then Go would have failed.</p>
<p>At first, there was no Go compiler, and the initial development
focused on building one and then building libraries for it. For
expedience, we postponed the automation of building Go code by using
make and writing makefiles. When compiling a single package involved
multiple invocations of the Go compiler, we even used a program to
write the makefiles for us. You can find it if you dig through the
repository history.</p>
<p>The purpose of the new go command is our return to this ideal, that Go
programs should compile without configuration or additional effort on
the part of the developer beyond writing the necessary import
statements.</p>
<h2>Configuration versus convention</h2>
<p>The way to achieve the simplicity of a configuration-free system is to
establish conventions. The system works only to the extent that those conventions
are followed. When we first launched Go, many people published packages that
had to be installed in certain places, under certain names, using certain build
tools, in order to be used. That's understandable: that's the way it works in
most other languages. Over the last few years we consistently reminded people
about the <code>goinstall</code> command
(now replaced by <a href="/cmd/go/#hdr-Download_and_install_packages_and_dependencies"><code>go get</code></a>)
and its conventions: first, that the import path is derived in a known way from
the URL of the source code; second, that the place to store the sources in
the local file system is derived in a known way from the import path; third,
that each directory in a source tree corresponds to a single package; and
fourth, that the package is built using only information in the source code.
Today, the vast majority of packages follow these conventions.
The Go ecosystem is simpler and more powerful as a result.</p>
<p>We received many requests to allow a makefile in a package directory to
provide just a little extra configuration beyond what's in the source code.
But that would have introduced new rules. Because we did not accede to such
requests, we were able to write the go command and eliminate our use of make
or any other build system.</p>
<p>It is important to understand that the go command is not a general
build tool. It cannot be configured and it does not attempt to build
anything but Go packages. These are important simplifying
assumptions: they simplify not only the implementation but also, more
important, the use of the tool itself.</p>
<h2>Go's conventions</h2>
<p>The <code>go</code> command requires that code adheres to a few key,
well-established conventions.</p>
<p>First, the import path is derived in a known way from the URL of the
source code. For Bitbucket, GitHub, Google Code, and Launchpad, the
root directory of the repository is identified by the repository's
main URL, without the <code>http://</code> prefix. Subdirectories are named by
adding to that path.
For example, the Go example programs are obtained by running</p>
<pre>
git clone https://github.com/golang/example
</pre>
<p>and thus the import path for the root directory of that repository is
"<code>github.com/golang/example</code>".
The <a href="https://godoc.org/github.com/golang/example/stringutil">stringutil</a>
package is stored in a subdirectory, so its import path is
"<code>github.com/golang/example/stringutil</code>".</p>
<p>These paths are on the long side, but in exchange we get an
automatically managed name space for import paths and the ability for
a tool like the go command to look at an unfamiliar import path and
deduce where to obtain the source code.</p>
<p>Second, the place to store sources in the local file system is derived
in a known way from the import path, specifically
<code>$GOPATH/src/&lt;import-path&gt;</code>.
If unset, <code>$GOPATH</code> defaults to a subdirectory
named <code>go</code> in the user's home directory.
If <code>$GOPATH</code> is set to a list of paths, the go command tries
<code>&lt;dir&gt;/src/&lt;import-path&gt;</code> for each of the directories in
that list.
</p>
<p>Each of those trees contains, by convention, a top-level directory named
"<code>bin</code>", for holding compiled executables, and a top-level directory
named "<code>pkg</code>", for holding compiled packages that can be imported,
and the "<code>src</code>" directory, for holding package source files.
Imposing this structure lets us keep each of these directory trees
self-contained: the compiled form and the sources are always near each
other.</p>
<p>These naming conventions also let us work in the reverse direction,
from a directory name to its import path. This mapping is important
for many of the go command's subcommands, as we'll see below.</p>
<p>Third, each directory in a source tree corresponds to a single
package. By restricting a directory to a single package, we don't have
to create hybrid import paths that specify first the directory and
then the package within that directory. Also, most file management
tools and UIs work on directories as fundamental units. Tying the
fundamental Go unit&mdash;the package&mdash;to file system structure means
that file system tools become Go package tools. Copying, moving, or
deleting a package corresponds to copying, moving, or deleting a
directory.</p>
<p>Fourth, each package is built using only the information present in
the source files. This makes it much more likely that the tool will
be able to adapt to changing build environments and conditions. For
example, if we allowed extra configuration such as compiler flags or
command line recipes, then that configuration would need to be updated
each time the build tools changed; it would also be inherently tied
to the use of a specific toolchain.</p>
<h2>Getting started with the go command</h2>
<p>Finally, a quick tour of how to use the go command.
As mentioned above, the default <code>$GOPATH</code> on Unix is <code>$HOME/go</code>.
We'll store our programs there.
To use a different location, you can set <code>$GOPATH</code>;
see <a href="/doc/code.html">How to Write Go Code</a> for details.
<p>We first add some source code. Suppose we want to use
the indexing library from the codesearch project along with a left-leaning
red-black tree. We can install both with the "<code>go get</code>"
subcommand:</p>
<pre>
$ go get github.com/google/codesearch/index
$ go get github.com/petar/GoLLRB/llrb
$
</pre>
<p>Both of these projects are now downloaded and installed into <code>$HOME/go</code>,
which contains the two directories
<code>src/github.com/google/codesearch/index/</code> and
<code>src/github.com/petar/GoLLRB/llrb/</code>, along with the compiled
packages (in <code>pkg/</code>) for those libraries and their dependencies.</p>
<p>Because we used version control systems (Mercurial and Git) to check
out the sources, the source tree also contains the other files in the
corresponding repositories, such as related packages. The "<code>go list</code>"
subcommand lists the import paths corresponding to its arguments, and
the pattern "<code>./...</code>" means start in the current directory
("<code>./</code>") and find all packages below that directory
("<code>...</code>"):</p>
<pre>
$ cd $HOME/go/src
$ go list ./...
github.com/google/codesearch/cmd/cgrep
github.com/google/codesearch/cmd/cindex
github.com/google/codesearch/cmd/csearch
github.com/google/codesearch/index
github.com/google/codesearch/regexp
github.com/google/codesearch/sparse
github.com/petar/GoLLRB/example
github.com/petar/GoLLRB/llrb
$
</pre>
<p>We can also test those packages:</p>
<pre>
$ go test ./...
? github.com/google/codesearch/cmd/cgrep [no test files]
? github.com/google/codesearch/cmd/cindex [no test files]
? github.com/google/codesearch/cmd/csearch [no test files]
ok github.com/google/codesearch/index 0.203s
ok github.com/google/codesearch/regexp 0.017s
? github.com/google/codesearch/sparse [no test files]
? github.com/petar/GoLLRB/example [no test files]
ok github.com/petar/GoLLRB/llrb 0.231s
$
</pre>
<p>If a go subcommand is invoked with no paths listed, it operates on the
current directory:</p>
<pre>
$ cd github.com/google/codesearch/regexp
$ go list
github.com/google/codesearch/regexp
$ go test -v
=== RUN TestNstateEnc
--- PASS: TestNstateEnc (0.00s)
=== RUN TestMatch
--- PASS: TestMatch (0.00s)
=== RUN TestGrep
--- PASS: TestGrep (0.00s)
PASS
ok github.com/google/codesearch/regexp 0.018s
$ go install
$
</pre>
<p>That "<code>go install</code>" subcommand installs the latest copy of the
package into the pkg directory. Because the go command can analyze the
dependency graph, "<code>go install</code>" also installs any packages that
this package imports but that are out of date, recursively.</p>
<p>Notice that "<code>go install</code>" was able to determine the name of the
import path for the package in the current directory, because of the convention
for directory naming. It would be a little more convenient if we could pick
the name of the directory where we kept source code, and we probably wouldn't
pick such a long name, but that ability would require additional configuration
and complexity in the tool. Typing an extra directory name or two is a small
price to pay for the increased simplicity and power.</p>
<h2>Limitations</h2>
<p>As mentioned above, the go command is not a general-purpose build
tool.
In particular, it does not have any facility for generating Go
source files <em>during</em> a build, although it does provide
<a href="/cmd/go/#hdr-Generate_Go_files_by_processing_source"><code>go</code>
<code>generate</code></a>,
which can automate the creation of Go files <em>before</em> the build.
For more advanced build setups, you may need to write a
makefile (or a configuration file for the build tool of your choice)
to run whatever tool creates the Go files and then check those generated source files
into your repository. This is more work for you, the package author,
but it is significantly less work for your users, who can use
"<code>go get</code>" without needing to obtain and build
any additional tools.</p>
<h2>More information</h2>
<p>For more information, read <a href="/doc/code.html">How to Write Go Code</a>
and see the <a href="/cmd/go/">go command documentation</a>.</p>

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<!--{
"Title": "/doc/articles/"
}-->
<p>
See the <a href="/doc/#articles">Documents page</a> and the
<a href="/blog/index">Blog index</a> for a complete list of Go articles.
</p>

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<!--{
"Title": "Data Race Detector",
"Template": true
}-->
<h2 id="Introduction">Introduction</h2>
<p>
Data races are among the most common and hardest to debug types of bugs in concurrent systems.
A data race occurs when two goroutines access the same variable concurrently and at least one of the accesses is a write.
See the <a href="/ref/mem/">The Go Memory Model</a> for details.
</p>
<p>
Here is an example of a data race that can lead to crashes and memory corruption:
</p>
<pre>
func main() {
c := make(chan bool)
m := make(map[string]string)
go func() {
m["1"] = "a" // First conflicting access.
c &lt;- true
}()
m["2"] = "b" // Second conflicting access.
&lt;-c
for k, v := range m {
fmt.Println(k, v)
}
}
</pre>
<h2 id="Usage">Usage</h2>
<p>
To help diagnose such bugs, Go includes a built-in data race detector.
To use it, add the <code>-race</code> flag to the go command:
</p>
<pre>
$ go test -race mypkg // to test the package
$ go run -race mysrc.go // to run the source file
$ go build -race mycmd // to build the command
$ go install -race mypkg // to install the package
</pre>
<h2 id="Report_Format">Report Format</h2>
<p>
When the race detector finds a data race in the program, it prints a report.
The report contains stack traces for conflicting accesses, as well as stacks where the involved goroutines were created.
Here is an example:
</p>
<pre>
WARNING: DATA RACE
Read by goroutine 185:
net.(*pollServer).AddFD()
src/net/fd_unix.go:89 +0x398
net.(*pollServer).WaitWrite()
src/net/fd_unix.go:247 +0x45
net.(*netFD).Write()
src/net/fd_unix.go:540 +0x4d4
net.(*conn).Write()
src/net/net.go:129 +0x101
net.func·060()
src/net/timeout_test.go:603 +0xaf
Previous write by goroutine 184:
net.setWriteDeadline()
src/net/sockopt_posix.go:135 +0xdf
net.setDeadline()
src/net/sockopt_posix.go:144 +0x9c
net.(*conn).SetDeadline()
src/net/net.go:161 +0xe3
net.func·061()
src/net/timeout_test.go:616 +0x3ed
Goroutine 185 (running) created at:
net.func·061()
src/net/timeout_test.go:609 +0x288
Goroutine 184 (running) created at:
net.TestProlongTimeout()
src/net/timeout_test.go:618 +0x298
testing.tRunner()
src/testing/testing.go:301 +0xe8
</pre>
<h2 id="Options">Options</h2>
<p>
The <code>GORACE</code> environment variable sets race detector options.
The format is:
</p>
<pre>
GORACE="option1=val1 option2=val2"
</pre>
<p>
The options are:
</p>
<ul>
<li>
<code>log_path</code> (default <code>stderr</code>): The race detector writes
its report to a file named <code>log_path.<em>pid</em></code>.
The special names <code>stdout</code>
and <code>stderr</code> cause reports to be written to standard output and
standard error, respectively.
</li>
<li>
<code>exitcode</code> (default <code>66</code>): The exit status to use when
exiting after a detected race.
</li>
<li>
<code>strip_path_prefix</code> (default <code>""</code>): Strip this prefix
from all reported file paths, to make reports more concise.
</li>
<li>
<code>history_size</code> (default <code>1</code>): The per-goroutine memory
access history is <code>32K * 2**history_size elements</code>.
Increasing this value can avoid a "failed to restore the stack" error in reports, at the
cost of increased memory usage.
</li>
<li>
<code>halt_on_error</code> (default <code>0</code>): Controls whether the program
exits after reporting first data race.
</li>
<li>
<code>atexit_sleep_ms</code> (default <code>1000</code>): Amount of milliseconds
to sleep in the main goroutine before exiting.
</li>
</ul>
<p>
Example:
</p>
<pre>
$ GORACE="log_path=/tmp/race/report strip_path_prefix=/my/go/sources/" go test -race
</pre>
<h2 id="Excluding_Tests">Excluding Tests</h2>
<p>
When you build with <code>-race</code> flag, the <code>go</code> command defines additional
<a href="/pkg/go/build/#hdr-Build_Constraints">build tag</a> <code>race</code>.
You can use the tag to exclude some code and tests when running the race detector.
Some examples:
</p>
<pre>
// +build !race
package foo
// The test contains a data race. See issue 123.
func TestFoo(t *testing.T) {
// ...
}
// The test fails under the race detector due to timeouts.
func TestBar(t *testing.T) {
// ...
}
// The test takes too long under the race detector.
func TestBaz(t *testing.T) {
// ...
}
</pre>
<h2 id="How_To_Use">How To Use</h2>
<p>
To start, run your tests using the race detector (<code>go test -race</code>).
The race detector only finds races that happen at runtime, so it can't find
races in code paths that are not executed.
If your tests have incomplete coverage,
you may find more races by running a binary built with <code>-race</code> under a realistic
workload.
</p>
<h2 id="Typical_Data_Races">Typical Data Races</h2>
<p>
Here are some typical data races. All of them can be detected with the race detector.
</p>
<h3 id="Race_on_loop_counter">Race on loop counter</h3>
<pre>
func main() {
var wg sync.WaitGroup
wg.Add(5)
for i := 0; i < 5; i++ {
go func() {
fmt.Println(i) // Not the 'i' you are looking for.
wg.Done()
}()
}
wg.Wait()
}
</pre>
<p>
The variable <code>i</code> in the function literal is the same variable used by the loop, so
the read in the goroutine races with the loop increment.
(This program typically prints 55555, not 01234.)
The program can be fixed by making a copy of the variable:
</p>
<pre>
func main() {
var wg sync.WaitGroup
wg.Add(5)
for i := 0; i < 5; i++ {
go func(j int) {
fmt.Println(j) // Good. Read local copy of the loop counter.
wg.Done()
}(i)
}
wg.Wait()
}
</pre>
<h3 id="Accidentally_shared_variable">Accidentally shared variable</h3>
<pre>
// ParallelWrite writes data to file1 and file2, returns the errors.
func ParallelWrite(data []byte) chan error {
res := make(chan error, 2)
f1, err := os.Create("file1")
if err != nil {
res &lt;- err
} else {
go func() {
// This err is shared with the main goroutine,
// so the write races with the write below.
_, err = f1.Write(data)
res &lt;- err
f1.Close()
}()
}
f2, err := os.Create("file2") // The second conflicting write to err.
if err != nil {
res &lt;- err
} else {
go func() {
_, err = f2.Write(data)
res &lt;- err
f2.Close()
}()
}
return res
}
</pre>
<p>
The fix is to introduce new variables in the goroutines (note the use of <code>:=</code>):
</p>
<pre>
...
_, err := f1.Write(data)
...
_, err := f2.Write(data)
...
</pre>
<h3 id="Unprotected_global_variable">Unprotected global variable</h3>
<p>
If the following code is called from several goroutines, it leads to races on the <code>service</code> map.
Concurrent reads and writes of the same map are not safe:
</p>
<pre>
var service map[string]net.Addr
func RegisterService(name string, addr net.Addr) {
service[name] = addr
}
func LookupService(name string) net.Addr {
return service[name]
}
</pre>
<p>
To make the code safe, protect the accesses with a mutex:
</p>
<pre>
var (
service map[string]net.Addr
serviceMu sync.Mutex
)
func RegisterService(name string, addr net.Addr) {
serviceMu.Lock()
defer serviceMu.Unlock()
service[name] = addr
}
func LookupService(name string) net.Addr {
serviceMu.Lock()
defer serviceMu.Unlock()
return service[name]
}
</pre>
<h3 id="Primitive_unprotected_variable">Primitive unprotected variable</h3>
<p>
Data races can happen on variables of primitive types as well (<code>bool</code>, <code>int</code>, <code>int64</code>, etc.),
as in this example:
</p>
<pre>
type Watchdog struct{ last int64 }
func (w *Watchdog) KeepAlive() {
w.last = time.Now().UnixNano() // First conflicting access.
}
func (w *Watchdog) Start() {
go func() {
for {
time.Sleep(time.Second)
// Second conflicting access.
if w.last < time.Now().Add(-10*time.Second).UnixNano() {
fmt.Println("No keepalives for 10 seconds. Dying.")
os.Exit(1)
}
}
}()
}
</pre>
<p>
Even such "innocent" data races can lead to hard-to-debug problems caused by
non-atomicity of the memory accesses,
interference with compiler optimizations,
or reordering issues accessing processor memory .
</p>
<p>
A typical fix for this race is to use a channel or a mutex.
To preserve the lock-free behavior, one can also use the
<a href="/pkg/sync/atomic/"><code>sync/atomic</code></a> package.
</p>
<pre>
type Watchdog struct{ last int64 }
func (w *Watchdog) KeepAlive() {
atomic.StoreInt64(&amp;w.last, time.Now().UnixNano())
}
func (w *Watchdog) Start() {
go func() {
for {
time.Sleep(time.Second)
if atomic.LoadInt64(&amp;w.last) < time.Now().Add(-10*time.Second).UnixNano() {
fmt.Println("No keepalives for 10 seconds. Dying.")
os.Exit(1)
}
}
}()
}
</pre>
<h3 id="Unsynchronized_send_and_close_operations">Unsynchronized send and close operations</h3>
<p>
As this example demonstrates, unsynchronized send and close operations
on the same channel can also be a race condition:
</p>
<pre>
c := make(chan struct{}) // or buffered channel
// The race detector cannot derive the happens before relation
// for the following send and close operations. These two operations
// are unsynchronized and happen concurrently.
go func() { c <- struct{}{} }()
close(c)
</pre>
<p>
According to the Go memory model, a send on a channel happens before
the corresponding receive from that channel completes. To synchronize
send and close operations, use a receive operation that guarantees
the send is done before the close:
</p>
<pre>
c := make(chan struct{}) // or buffered channel
go func() { c <- struct{}{} }()
<-c
close(c)
</pre>
<h2 id="Supported_Systems">Supported Systems</h2>
<p>
The race detector runs on
<code>linux/amd64</code>, <code>linux/ppc64le</code>,
<code>linux/arm64</code>, <code>freebsd/amd64</code>,
<code>netbsd/amd64</code>, <code>darwin/amd64</code>,
<code>darwin/arm64</code>, and <code>windows/amd64</code>.
</p>
<h2 id="Runtime_Overheads">Runtime Overhead</h2>
<p>
The cost of race detection varies by program, but for a typical program, memory
usage may increase by 5-10x and execution time by 2-20x.
</p>
<p>
The race detector currently allocates an extra 8 bytes per <code>defer</code>
and <code>recover</code> statement. Those extra allocations <a
href="https://golang.org/issue/26813">are not recovered until the goroutine
exits</a>. This means that if you have a long-running goroutine that is
periodically issuing <code>defer</code> and <code>recover</code> calls,
the program memory usage may grow without bound. These memory allocations
will not show up in the output of <code>runtime.ReadMemStats</code> or
<code>runtime/pprof</code>.
</p>

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<h1>Editing {{.Title}}</h1>
<form action="/save/{{.Title}}" method="POST">
<div><textarea name="body" rows="20" cols="80">{{printf "%s" .Body}}</textarea></div>
<div><input type="submit" value="Save"></div>
</form>

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"errors"
"html/template"
"io/ioutil"
"log"
"net/http"
"regexp"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, "/edit/"+title, http.StatusFound)
return
}
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
body := r.FormValue("body")
p := &Page{Title: title, Body: []byte(body)}
err = p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, err := template.ParseFiles(tmpl + ".html")
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
err = t.Execute(w, p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
var validPath = regexp.MustCompile("^/(edit|save|view)/([a-zA-Z0-9]+)$")
func getTitle(w http.ResponseWriter, r *http.Request) (string, error) {
m := validPath.FindStringSubmatch(r.URL.Path)
if m == nil {
http.NotFound(w, r)
return "", errors.New("invalid Page Title")
}
return m[2], nil // The title is the second subexpression.
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
http.HandleFunc("/save/", saveHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/edit/"):]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
t, _ := template.ParseFiles("edit.html")
t.Execute(w, p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, _ := loadPage(title)
t, _ := template.ParseFiles("view.html")
t.Execute(w, p)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
"regexp"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func viewHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, "/edit/"+title, http.StatusFound)
return
}
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
body := r.FormValue("body")
p := &Page{Title: title, Body: []byte(body)}
err := p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, err := template.ParseFiles(tmpl + ".html")
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
err = t.Execute(w, p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
var validPath = regexp.MustCompile("^/(edit|save|view)/([a-zA-Z0-9]+)$")
func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
m := validPath.FindStringSubmatch(r.URL.Path)
if m == nil {
http.NotFound(w, r)
return
}
fn(w, r, m[2])
}
}
func main() {
http.HandleFunc("/view/", makeHandler(viewHandler))
http.HandleFunc("/edit/", makeHandler(editHandler))
http.HandleFunc("/save/", makeHandler(saveHandler))
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/edit/"):]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, _ := loadPage(title)
renderTemplate(w, "view", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/save/"):]
body := r.FormValue("body")
p := &Page{Title: title, Body: []byte(body)}
p.save()
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + ".html")
t.Execute(w, p)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
http.HandleFunc("/save/", saveHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
"regexp"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func viewHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, "/edit/"+title, http.StatusFound)
return
}
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
body := r.FormValue("body")
p := &Page{Title: title, Body: []byte(body)}
err := p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
var templates = template.Must(template.ParseFiles("edit.html", "view.html"))
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
err := templates.ExecuteTemplate(w, tmpl+".html", p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
var validPath = regexp.MustCompile("^/(edit|save|view)/([a-zA-Z0-9]+)$")
func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
m := validPath.FindStringSubmatch(r.URL.Path)
if m == nil {
http.NotFound(w, r)
return
}
fn(w, r, m[2])
}
}
func main() {
http.HandleFunc("/view/", makeHandler(viewHandler))
http.HandleFunc("/edit/", makeHandler(editHandler))
http.HandleFunc("/save/", makeHandler(saveHandler))
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"fmt"
"log"
"net"
"net/http"
)
func serve() error {
l, err := net.Listen("tcp", "127.0.0.1:0")
if err != nil {
log.Fatal(err)
}
fmt.Println(l.Addr().String())
s := &http.Server{}
return s.Serve(l)
}

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module doc/articles/wiki
go 1.14

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// +build ignore
package main
import (
"fmt"
"log"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:])
}
func main() {
http.HandleFunc("/", handler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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<!--{
"Title": "Writing Web Applications",
"Template": true
}-->
<h2>Introduction</h2>
<p>
Covered in this tutorial:
</p>
<ul>
<li>Creating a data structure with load and save methods</li>
<li>Using the <code>net/http</code> package to build web applications
<li>Using the <code>html/template</code> package to process HTML templates</li>
<li>Using the <code>regexp</code> package to validate user input</li>
<li>Using closures</li>
</ul>
<p>
Assumed knowledge:
</p>
<ul>
<li>Programming experience</li>
<li>Understanding of basic web technologies (HTTP, HTML)</li>
<li>Some UNIX/DOS command-line knowledge</li>
</ul>
<h2>Getting Started</h2>
<p>
At present, you need to have a FreeBSD, Linux, OS X, or Windows machine to run Go.
We will use <code>$</code> to represent the command prompt.
</p>
<p>
Install Go (see the <a href="/doc/install">Installation Instructions</a>).
</p>
<p>
Make a new directory for this tutorial inside your <code>GOPATH</code> and cd to it:
</p>
<pre>
$ mkdir gowiki
$ cd gowiki
</pre>
<p>
Create a file named <code>wiki.go</code>, open it in your favorite editor, and
add the following lines:
</p>
<pre>
package main
import (
"fmt"
"io/ioutil"
)
</pre>
<p>
We import the <code>fmt</code> and <code>ioutil</code> packages from the Go
standard library. Later, as we implement additional functionality, we will
add more packages to this <code>import</code> declaration.
</p>
<h2>Data Structures</h2>
<p>
Let's start by defining the data structures. A wiki consists of a series of
interconnected pages, each of which has a title and a body (the page content).
Here, we define <code>Page</code> as a struct with two fields representing
the title and body.
</p>
{{code "doc/articles/wiki/part1.go" `/^type Page/` `/}/`}}
<p>
The type <code>[]byte</code> means "a <code>byte</code> slice".
(See <a href="/doc/articles/slices_usage_and_internals.html">Slices: usage and
internals</a> for more on slices.)
The <code>Body</code> element is a <code>[]byte</code> rather than
<code>string</code> because that is the type expected by the <code>io</code>
libraries we will use, as you'll see below.
</p>
<p>
The <code>Page</code> struct describes how page data will be stored in memory.
But what about persistent storage? We can address that by creating a
<code>save</code> method on <code>Page</code>:
</p>
{{code "doc/articles/wiki/part1.go" `/^func.*Page.*save/` `/}/`}}
<p>
This method's signature reads: "This is a method named <code>save</code> that
takes as its receiver <code>p</code>, a pointer to <code>Page</code> . It takes
no parameters, and returns a value of type <code>error</code>."
</p>
<p>
This method will save the <code>Page</code>'s <code>Body</code> to a text
file. For simplicity, we will use the <code>Title</code> as the file name.
</p>
<p>
The <code>save</code> method returns an <code>error</code> value because
that is the return type of <code>WriteFile</code> (a standard library function
that writes a byte slice to a file). The <code>save</code> method returns the
error value, to let the application handle it should anything go wrong while
writing the file. If all goes well, <code>Page.save()</code> will return
<code>nil</code> (the zero-value for pointers, interfaces, and some other
types).
</p>
<p>
The octal integer literal <code>0600</code>, passed as the third parameter to
<code>WriteFile</code>, indicates that the file should be created with
read-write permissions for the current user only. (See the Unix man page
<code>open(2)</code> for details.)
</p>
<p>
In addition to saving pages, we will want to load pages, too:
</p>
{{code "doc/articles/wiki/part1-noerror.go" `/^func loadPage/` `/^}/`}}
<p>
The function <code>loadPage</code> constructs the file name from the title
parameter, reads the file's contents into a new variable <code>body</code>, and
returns a pointer to a <code>Page</code> literal constructed with the proper
title and body values.
</p>
<p>
Functions can return multiple values. The standard library function
<code>io.ReadFile</code> returns <code>[]byte</code> and <code>error</code>.
In <code>loadPage</code>, error isn't being handled yet; the "blank identifier"
represented by the underscore (<code>_</code>) symbol is used to throw away the
error return value (in essence, assigning the value to nothing).
</p>
<p>
But what happens if <code>ReadFile</code> encounters an error? For example,
the file might not exist. We should not ignore such errors. Let's modify the
function to return <code>*Page</code> and <code>error</code>.
</p>
{{code "doc/articles/wiki/part1.go" `/^func loadPage/` `/^}/`}}
<p>
Callers of this function can now check the second parameter; if it is
<code>nil</code> then it has successfully loaded a Page. If not, it will be an
<code>error</code> that can be handled by the caller (see the
<a href="/ref/spec#Errors">language specification</a> for details).
</p>
<p>
At this point we have a simple data structure and the ability to save to and
load from a file. Let's write a <code>main</code> function to test what we've
written:
</p>
{{code "doc/articles/wiki/part1.go" `/^func main/` `/^}/`}}
<p>
After compiling and executing this code, a file named <code>TestPage.txt</code>
would be created, containing the contents of <code>p1</code>. The file would
then be read into the struct <code>p2</code>, and its <code>Body</code> element
printed to the screen.
</p>
<p>
You can compile and run the program like this:
</p>
<pre>
$ go build wiki.go
$ ./wiki
This is a sample Page.
</pre>
<p>
(If you're using Windows you must type "<code>wiki</code>" without the
"<code>./</code>" to run the program.)
</p>
<p>
<a href="part1.go">Click here to view the code we've written so far.</a>
</p>
<h2>Introducing the <code>net/http</code> package (an interlude)</h2>
<p>
Here's a full working example of a simple web server:
</p>
{{code "doc/articles/wiki/http-sample.go"}}
<p>
The <code>main</code> function begins with a call to
<code>http.HandleFunc</code>, which tells the <code>http</code> package to
handle all requests to the web root (<code>"/"</code>) with
<code>handler</code>.
</p>
<p>
It then calls <code>http.ListenAndServe</code>, specifying that it should
listen on port 8080 on any interface (<code>":8080"</code>). (Don't
worry about its second parameter, <code>nil</code>, for now.)
This function will block until the program is terminated.
</p>
<p>
<code>ListenAndServe</code> always returns an error, since it only returns when an
unexpected error occurs.
In order to log that error we wrap the function call with <code>log.Fatal</code>.
</p>
<p>
The function <code>handler</code> is of the type <code>http.HandlerFunc</code>.
It takes an <code>http.ResponseWriter</code> and an <code>http.Request</code> as
its arguments.
</p>
<p>
An <code>http.ResponseWriter</code> value assembles the HTTP server's response; by writing
to it, we send data to the HTTP client.
</p>
<p>
An <code>http.Request</code> is a data structure that represents the client
HTTP request. <code>r.URL.Path</code> is the path component
of the request URL. The trailing <code>[1:]</code> means
"create a sub-slice of <code>Path</code> from the 1st character to the end."
This drops the leading "/" from the path name.
</p>
<p>
If you run this program and access the URL:
</p>
<pre>http://localhost:8080/monkeys</pre>
<p>
the program would present a page containing:
</p>
<pre>Hi there, I love monkeys!</pre>
<h2>Using <code>net/http</code> to serve wiki pages</h2>
<p>
To use the <code>net/http</code> package, it must be imported:
</p>
<pre>
import (
"fmt"
"io/ioutil"
"log"
<b>"net/http"</b>
)
</pre>
<p>
Let's create a handler, <code>viewHandler</code> that will allow users to
view a wiki page. It will handle URLs prefixed with "/view/".
</p>
{{code "doc/articles/wiki/part2.go" `/^func viewHandler/` `/^}/`}}
<p>
Again, note the use of <code>_</code> to ignore the <code>error</code>
return value from <code>loadPage</code>. This is done here for simplicity
and generally considered bad practice. We will attend to this later.
</p>
<p>
First, this function extracts the page title from <code>r.URL.Path</code>,
the path component of the request URL.
The <code>Path</code> is re-sliced with <code>[len("/view/"):]</code> to drop
the leading <code>"/view/"</code> component of the request path.
This is because the path will invariably begin with <code>"/view/"</code>,
which is not part of the page's title.
</p>
<p>
The function then loads the page data, formats the page with a string of simple
HTML, and writes it to <code>w</code>, the <code>http.ResponseWriter</code>.
</p>
<p>
To use this handler, we rewrite our <code>main</code> function to
initialize <code>http</code> using the <code>viewHandler</code> to handle
any requests under the path <code>/view/</code>.
</p>
{{code "doc/articles/wiki/part2.go" `/^func main/` `/^}/`}}
<p>
<a href="part2.go">Click here to view the code we've written so far.</a>
</p>
<p>
Let's create some page data (as <code>test.txt</code>), compile our code, and
try serving a wiki page.
</p>
<p>
Open <code>test.txt</code> file in your editor, and save the string "Hello world" (without quotes)
in it.
</p>
<pre>
$ go build wiki.go
$ ./wiki
</pre>
<p>
(If you're using Windows you must type "<code>wiki</code>" without the
"<code>./</code>" to run the program.)
</p>
<p>
With this web server running, a visit to <code><a
href="http://localhost:8080/view/test">http://localhost:8080/view/test</a></code>
should show a page titled "test" containing the words "Hello world".
</p>
<h2>Editing Pages</h2>
<p>
A wiki is not a wiki without the ability to edit pages. Let's create two new
handlers: one named <code>editHandler</code> to display an 'edit page' form,
and the other named <code>saveHandler</code> to save the data entered via the
form.
</p>
<p>
First, we add them to <code>main()</code>:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^func main/` `/^}/`}}
<p>
The function <code>editHandler</code> loads the page
(or, if it doesn't exist, create an empty <code>Page</code> struct),
and displays an HTML form.
</p>
{{code "doc/articles/wiki/notemplate.go" `/^func editHandler/` `/^}/`}}
<p>
This function will work fine, but all that hard-coded HTML is ugly.
Of course, there is a better way.
</p>
<h2>The <code>html/template</code> package</h2>
<p>
The <code>html/template</code> package is part of the Go standard library.
We can use <code>html/template</code> to keep the HTML in a separate file,
allowing us to change the layout of our edit page without modifying the
underlying Go code.
</p>
<p>
First, we must add <code>html/template</code> to the list of imports. We
also won't be using <code>fmt</code> anymore, so we have to remove that.
</p>
<pre>
import (
<b>"html/template"</b>
"io/ioutil"
"net/http"
)
</pre>
<p>
Let's create a template file containing the HTML form.
Open a new file named <code>edit.html</code>, and add the following lines:
</p>
{{code "doc/articles/wiki/edit.html"}}
<p>
Modify <code>editHandler</code> to use the template, instead of the hard-coded
HTML:
</p>
{{code "doc/articles/wiki/final-noerror.go" `/^func editHandler/` `/^}/`}}
<p>
The function <code>template.ParseFiles</code> will read the contents of
<code>edit.html</code> and return a <code>*template.Template</code>.
</p>
<p>
The method <code>t.Execute</code> executes the template, writing the
generated HTML to the <code>http.ResponseWriter</code>.
The <code>.Title</code> and <code>.Body</code> dotted identifiers refer to
<code>p.Title</code> and <code>p.Body</code>.
</p>
<p>
Template directives are enclosed in double curly braces.
The <code>printf "%s" .Body</code> instruction is a function call
that outputs <code>.Body</code> as a string instead of a stream of bytes,
the same as a call to <code>fmt.Printf</code>.
The <code>html/template</code> package helps guarantee that only safe and
correct-looking HTML is generated by template actions. For instance, it
automatically escapes any greater than sign (<code>&gt;</code>), replacing it
with <code>&amp;gt;</code>, to make sure user data does not corrupt the form
HTML.
</p>
<p>
Since we're working with templates now, let's create a template for our
<code>viewHandler</code> called <code>view.html</code>:
</p>
{{code "doc/articles/wiki/view.html"}}
<p>
Modify <code>viewHandler</code> accordingly:
</p>
{{code "doc/articles/wiki/final-noerror.go" `/^func viewHandler/` `/^}/`}}
<p>
Notice that we've used almost exactly the same templating code in both
handlers. Let's remove this duplication by moving the templating code
to its own function:
</p>
{{code "doc/articles/wiki/final-template.go" `/^func renderTemplate/` `/^}/`}}
<p>
And modify the handlers to use that function:
</p>
{{code "doc/articles/wiki/final-template.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-template.go" `/^func editHandler/` `/^}/`}}
<p>
If we comment out the registration of our unimplemented save handler in
<code>main</code>, we can once again build and test our program.
<a href="part3.go">Click here to view the code we've written so far.</a>
</p>
<h2>Handling non-existent pages</h2>
<p>
What if you visit <a href="http://localhost:8080/view/APageThatDoesntExist">
<code>/view/APageThatDoesntExist</code></a>? You'll see a page containing
HTML. This is because it ignores the error return value from
<code>loadPage</code> and continues to try and fill out the template
with no data. Instead, if the requested Page doesn't exist, it should
redirect the client to the edit Page so the content may be created:
</p>
{{code "doc/articles/wiki/part3-errorhandling.go" `/^func viewHandler/` `/^}/`}}
<p>
The <code>http.Redirect</code> function adds an HTTP status code of
<code>http.StatusFound</code> (302) and a <code>Location</code>
header to the HTTP response.
</p>
<h2>Saving Pages</h2>
<p>
The function <code>saveHandler</code> will handle the submission of forms
located on the edit pages. After uncommenting the related line in
<code>main</code>, let's implement the handler:
</p>
{{code "doc/articles/wiki/final-template.go" `/^func saveHandler/` `/^}/`}}
<p>
The page title (provided in the URL) and the form's only field,
<code>Body</code>, are stored in a new <code>Page</code>.
The <code>save()</code> method is then called to write the data to a file,
and the client is redirected to the <code>/view/</code> page.
</p>
<p>
The value returned by <code>FormValue</code> is of type <code>string</code>.
We must convert that value to <code>[]byte</code> before it will fit into
the <code>Page</code> struct. We use <code>[]byte(body)</code> to perform
the conversion.
</p>
<h2>Error handling</h2>
<p>
There are several places in our program where errors are being ignored. This
is bad practice, not least because when an error does occur the program will
have unintended behavior. A better solution is to handle the errors and return
an error message to the user. That way if something does go wrong, the server
will function exactly how we want and the user can be notified.
</p>
<p>
First, let's handle the errors in <code>renderTemplate</code>:
</p>
{{code "doc/articles/wiki/final-parsetemplate.go" `/^func renderTemplate/` `/^}/`}}
<p>
The <code>http.Error</code> function sends a specified HTTP response code
(in this case "Internal Server Error") and error message.
Already the decision to put this in a separate function is paying off.
</p>
<p>
Now let's fix up <code>saveHandler</code>:
</p>
{{code "doc/articles/wiki/part3-errorhandling.go" `/^func saveHandler/` `/^}/`}}
<p>
Any errors that occur during <code>p.save()</code> will be reported
to the user.
</p>
<h2>Template caching</h2>
<p>
There is an inefficiency in this code: <code>renderTemplate</code> calls
<code>ParseFiles</code> every time a page is rendered.
A better approach would be to call <code>ParseFiles</code> once at program
initialization, parsing all templates into a single <code>*Template</code>.
Then we can use the
<a href="/pkg/html/template/#Template.ExecuteTemplate"><code>ExecuteTemplate</code></a>
method to render a specific template.
</p>
<p>
First we create a global variable named <code>templates</code>, and initialize
it with <code>ParseFiles</code>.
</p>
{{code "doc/articles/wiki/final.go" `/var templates/`}}
<p>
The function <code>template.Must</code> is a convenience wrapper that panics
when passed a non-nil <code>error</code> value, and otherwise returns the
<code>*Template</code> unaltered. A panic is appropriate here; if the templates
can't be loaded the only sensible thing to do is exit the program.
</p>
<p>
The <code>ParseFiles</code> function takes any number of string arguments that
identify our template files, and parses those files into templates that are
named after the base file name. If we were to add more templates to our
program, we would add their names to the <code>ParseFiles</code> call's
arguments.
</p>
<p>
We then modify the <code>renderTemplate</code> function to call the
<code>templates.ExecuteTemplate</code> method with the name of the appropriate
template:
</p>
{{code "doc/articles/wiki/final.go" `/func renderTemplate/` `/^}/`}}
<p>
Note that the template name is the template file name, so we must
append <code>".html"</code> to the <code>tmpl</code> argument.
</p>
<h2>Validation</h2>
<p>
As you may have observed, this program has a serious security flaw: a user
can supply an arbitrary path to be read/written on the server. To mitigate
this, we can write a function to validate the title with a regular expression.
</p>
<p>
First, add <code>"regexp"</code> to the <code>import</code> list.
Then we can create a global variable to store our validation
expression:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^var validPath/`}}
<p>
The function <code>regexp.MustCompile</code> will parse and compile the
regular expression, and return a <code>regexp.Regexp</code>.
<code>MustCompile</code> is distinct from <code>Compile</code> in that it will
panic if the expression compilation fails, while <code>Compile</code> returns
an <code>error</code> as a second parameter.
</p>
<p>
Now, let's write a function that uses the <code>validPath</code>
expression to validate path and extract the page title:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/func getTitle/` `/^}/`}}
<p>
If the title is valid, it will be returned along with a <code>nil</code>
error value. If the title is invalid, the function will write a
"404 Not Found" error to the HTTP connection, and return an error to the
handler. To create a new error, we have to import the <code>errors</code>
package.
</p>
<p>
Let's put a call to <code>getTitle</code> in each of the handlers:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-noclosure.go" `/^func editHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}}
<h2>Introducing Function Literals and Closures</h2>
<p>
Catching the error condition in each handler introduces a lot of repeated code.
What if we could wrap each of the handlers in a function that does this
validation and error checking? Go's
<a href="/ref/spec#Function_literals">function
literals</a> provide a powerful means of abstracting functionality
that can help us here.
</p>
<p>
First, we re-write the function definition of each of the handlers to accept
a title string:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request, title string)
func editHandler(w http.ResponseWriter, r *http.Request, title string)
func saveHandler(w http.ResponseWriter, r *http.Request, title string)
</pre>
<p>
Now let's define a wrapper function that <i>takes a function of the above
type</i>, and returns a function of type <code>http.HandlerFunc</code>
(suitable to be passed to the function <code>http.HandleFunc</code>):
</p>
<pre>
func makeHandler(fn func (http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
// Here we will extract the page title from the Request,
// and call the provided handler 'fn'
}
}
</pre>
<p>
The returned function is called a closure because it encloses values defined
outside of it. In this case, the variable <code>fn</code> (the single argument
to <code>makeHandler</code>) is enclosed by the closure. The variable
<code>fn</code> will be one of our save, edit, or view handlers.
</p>
<p>
Now we can take the code from <code>getTitle</code> and use it here
(with some minor modifications):
</p>
{{code "doc/articles/wiki/final.go" `/func makeHandler/` `/^}/`}}
<p>
The closure returned by <code>makeHandler</code> is a function that takes
an <code>http.ResponseWriter</code> and <code>http.Request</code> (in other
words, an <code>http.HandlerFunc</code>).
The closure extracts the <code>title</code> from the request path, and
validates it with the <code>validPath</code> regexp. If the
<code>title</code> is invalid, an error will be written to the
<code>ResponseWriter</code> using the <code>http.NotFound</code> function.
If the <code>title</code> is valid, the enclosed handler function
<code>fn</code> will be called with the <code>ResponseWriter</code>,
<code>Request</code>, and <code>title</code> as arguments.
</p>
<p>
Now we can wrap the handler functions with <code>makeHandler</code> in
<code>main</code>, before they are registered with the <code>http</code>
package:
</p>
{{code "doc/articles/wiki/final.go" `/func main/` `/^}/`}}
<p>
Finally we remove the calls to <code>getTitle</code> from the handler functions,
making them much simpler:
</p>
{{code "doc/articles/wiki/final.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final.go" `/^func editHandler/` `/^}/`}}
{{code "doc/articles/wiki/final.go" `/^func saveHandler/` `/^}/`}}
<h2>Try it out!</h2>
<p>
<a href="final.go">Click here to view the final code listing.</a>
</p>
<p>
Recompile the code, and run the app:
</p>
<pre>
$ go build wiki.go
$ ./wiki
</pre>
<p>
Visiting <a href="http://localhost:8080/view/ANewPage">http://localhost:8080/view/ANewPage</a>
should present you with the page edit form. You should then be able to
enter some text, click 'Save', and be redirected to the newly created page.
</p>
<h2>Other tasks</h2>
<p>
Here are some simple tasks you might want to tackle on your own:
</p>
<ul>
<li>Store templates in <code>tmpl/</code> and page data in <code>data/</code>.
<li>Add a handler to make the web root redirect to
<code>/view/FrontPage</code>.</li>
<li>Spruce up the page templates by making them valid HTML and adding some
CSS rules.</li>
<li>Implement inter-page linking by converting instances of
<code>[PageName]</code> to <br>
<code>&lt;a href="/view/PageName"&gt;PageName&lt;/a&gt;</code>.
(hint: you could use <code>regexp.ReplaceAllFunc</code> to do this)
</li>
</ul>

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"fmt"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, _ := loadPage(title)
fmt.Fprintf(w, "<h1>%s</h1><div>%s</div>", p.Title, p.Body)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/edit/"):]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
fmt.Fprintf(w, "<h1>Editing %s</h1>"+
"<form action=\"/save/%s\" method=\"POST\">"+
"<textarea name=\"body\">%s</textarea><br>"+
"<input type=\"submit\" value=\"Save\">"+
"</form>",
p.Title, p.Title, p.Body)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"fmt"
"io/ioutil"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) *Page {
filename := title + ".txt"
body, _ := ioutil.ReadFile(filename)
return &Page{Title: title, Body: body}
}
func main() {
p1 := &Page{Title: "TestPage", Body: []byte("This is a sample page.")}
p1.save()
p2 := loadPage("TestPage")
fmt.Println(string(p2.Body))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"fmt"
"io/ioutil"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func main() {
p1 := &Page{Title: "TestPage", Body: []byte("This is a sample Page.")}
p1.save()
p2, _ := loadPage("TestPage")
fmt.Println(string(p2.Body))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"fmt"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, _ := loadPage(title)
fmt.Fprintf(w, "<h1>%s</h1><div>%s</div>", p.Title, p.Body)
}
func main() {
http.HandleFunc("/view/", viewHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + ".html")
t.Execute(w, p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, "/edit/"+title, http.StatusFound)
return
}
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/edit/"):]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/save/"):]
body := r.FormValue("body")
p := &Page{Title: title, Body: []byte(body)}
err := p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
http.HandleFunc("/save/", saveHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build ignore
package main
import (
"html/template"
"io/ioutil"
"log"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + ".html")
t.Execute(w, p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/view/"):]
p, _ := loadPage(title)
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[len("/edit/"):]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
//http.HandleFunc("/save/", saveHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}

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some content

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_content/doc/articles/wiki/test_edit.good Normal file
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<h1>Editing Test</h1>
<form action="/save/Test" method="POST">
<div><textarea name="body" rows="20" cols="80"></textarea></div>
<div><input type="submit" value="Save"></div>
</form>

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<h1>Test</h1>
<p>[<a href="/edit/Test">edit</a>]</p>
<div>some content</div>

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<h1>{{.Title}}</h1>
<p>[<a href="/edit/{{.Title}}">edit</a>]</p>
<div>{{printf "%s" .Body}}</div>

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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main_test
import (
"bytes"
"fmt"
"io/ioutil"
"net/http"
"os"
"os/exec"
"path/filepath"
"strings"
"testing"
)
func TestSnippetsCompile(t *testing.T) {
if testing.Short() {
t.Skip("skipping slow builds in short mode")
}
goFiles, err := filepath.Glob("*.go")
if err != nil {
t.Fatal(err)
}
for _, f := range goFiles {
if strings.HasSuffix(f, "_test.go") {
continue
}
f := f
t.Run(f, func(t *testing.T) {
t.Parallel()
cmd := exec.Command("go", "build", "-o", os.DevNull, f)
out, err := cmd.CombinedOutput()
if err != nil {
t.Errorf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, out)
}
})
}
}
func TestWikiServer(t *testing.T) {
must := func(err error) {
if err != nil {
t.Helper()
t.Fatal(err)
}
}
dir, err := ioutil.TempDir("", t.Name())
must(err)
defer os.RemoveAll(dir)
// We're testing a walkthrough example of how to write a server.
//
// That server hard-codes a port number to make the walkthrough simpler, but
// we can't assume that the hard-coded port is available on an arbitrary
// builder. So we'll patch out the hard-coded port, and replace it with a
// function that writes the server's address to stdout
// so that we can read it and know where to send the test requests.
finalGo, err := ioutil.ReadFile("final.go")
must(err)
const patchOld = `log.Fatal(http.ListenAndServe(":8080", nil))`
patched := bytes.ReplaceAll(finalGo, []byte(patchOld), []byte(`log.Fatal(serve())`))
if bytes.Equal(patched, finalGo) {
t.Fatalf("Can't patch final.go: %q not found.", patchOld)
}
must(ioutil.WriteFile(filepath.Join(dir, "final_patched.go"), patched, 0644))
// Build the server binary from the patched sources.
// The 'go' command requires that they all be in the same directory.
// final_test.go provides the implemtation for our serve function.
must(copyFile(filepath.Join(dir, "final_srv.go"), "final_test.go"))
cmd := exec.Command("go", "build",
"-o", filepath.Join(dir, "final.exe"),
filepath.Join(dir, "final_patched.go"),
filepath.Join(dir, "final_srv.go"))
out, err := cmd.CombinedOutput()
if err != nil {
t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, out)
}
// Run the server in our temporary directory so that it can
// write its content there. It also needs a couple of template files,
// and looks for them in the same directory.
must(copyFile(filepath.Join(dir, "edit.html"), "edit.html"))
must(copyFile(filepath.Join(dir, "view.html"), "view.html"))
cmd = exec.Command(filepath.Join(dir, "final.exe"))
cmd.Dir = dir
stderr := bytes.NewBuffer(nil)
cmd.Stderr = stderr
stdout, err := cmd.StdoutPipe()
must(err)
must(cmd.Start())
defer func() {
cmd.Process.Kill()
err := cmd.Wait()
if stderr.Len() > 0 {
t.Logf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, stderr)
}
}()
var addr string
if _, err := fmt.Fscanln(stdout, &addr); err != nil || addr == "" {
t.Fatalf("Failed to read server address: %v", err)
}
// The server is up and has told us its address.
// Make sure that its HTTP API works as described in the article.
r, err := http.Get(fmt.Sprintf("http://%s/edit/Test", addr))
must(err)
responseMustMatchFile(t, r, "test_edit.good")
r, err = http.Post(fmt.Sprintf("http://%s/save/Test", addr),
"application/x-www-form-urlencoded",
strings.NewReader("body=some%20content"))
must(err)
responseMustMatchFile(t, r, "test_view.good")
gotTxt, err := ioutil.ReadFile(filepath.Join(dir, "Test.txt"))
must(err)
wantTxt, err := ioutil.ReadFile("test_Test.txt.good")
must(err)
if !bytes.Equal(wantTxt, gotTxt) {
t.Fatalf("Test.txt differs from expected after posting to /save.\ngot:\n%s\nwant:\n%s", gotTxt, wantTxt)
}
r, err = http.Get(fmt.Sprintf("http://%s/view/Test", addr))
must(err)
responseMustMatchFile(t, r, "test_view.good")
}
func responseMustMatchFile(t *testing.T, r *http.Response, filename string) {
t.Helper()
defer r.Body.Close()
body, err := ioutil.ReadAll(r.Body)
if err != nil {
t.Fatal(err)
}
wantBody, err := ioutil.ReadFile(filename)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(body, wantBody) {
t.Fatalf("%v: body does not match %s.\ngot:\n%s\nwant:\n%s", r.Request.URL, filename, body, wantBody)
}
}
func copyFile(dst, src string) error {
buf, err := ioutil.ReadFile(src)
if err != nil {
return err
}
return ioutil.WriteFile(dst, buf, 0644)
}

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<!--{
"Title": "Command Documentation",
"Path": "/doc/cmd"
}-->
<p>
There is a suite of programs to build and process Go source code.
Instead of being run directly, programs in the suite are usually invoked
by the <a href="/cmd/go/">go</a> program.
</p>
<p>
The most common way to run these programs is as a subcommand of the go program,
for instance as <code>go fmt</code>. Run like this, the command operates on
complete packages of Go source code, with the go program invoking the
underlying binary with arguments appropriate to package-level processing.
</p>
<p>
The programs can also be run as stand-alone binaries, with unmodified arguments,
using the go <code>tool</code> subcommand, such as <code>go tool cgo</code>.
For most commands this is mainly useful for debugging.
Some of the commands, such as <code>pprof</code>, are accessible only through
the go <code>tool</code> subcommand.
</p>
<p>
Finally the <code>fmt</code> and <code>godoc</code> commands are installed
as regular binaries called <code>gofmt</code> and <code>godoc</code> because
they are so often referenced.
</p>
<p>
Click on the links for more documentation, invocation methods, and usage details.
</p>
<table class="dir">
<tr>
<th>Name</th>
<th>&nbsp;&nbsp;&nbsp;&nbsp;</th>
<th>Synopsis</th>
</tr>
<tr>
<td><a href="/cmd/go/">go</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>
The <code>go</code> program manages Go source code and runs the other
commands listed here.
See the command docs for usage
details.
</td>
</tr>
<tr>
<td><a href="/cmd/cgo/">cgo</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Cgo enables the creation of Go packages that call C code.</td>
</tr>
<tr>
<td><a href="/cmd/cover/">cover</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Cover is a program for creating and analyzing the coverage profiles
generated by <code>"go test -coverprofile"</code>.</td>
</tr>
<tr>
<td><a href="/cmd/fix/">fix</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Fix finds Go programs that use old features of the language and libraries
and rewrites them to use newer ones.</td>
</tr>
<tr>
<td><a href="/cmd/gofmt/">fmt</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Fmt formats Go packages, it is also available as an independent <a href="/cmd/gofmt/">
gofmt</a> command with more general options.</td>
</tr>
<tr>
<td><a href="//godoc.org/golang.org/x/tools/cmd/godoc/">godoc</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Godoc extracts and generates documentation for Go packages.</td>
</tr>
<tr>
<td><a href="/cmd/vet/">vet</a></td>
<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
<td>Vet examines Go source code and reports suspicious constructs, such as Printf
calls whose arguments do not align with the format string.</td>
</tr>
</table>
<p>
This is an abridged list. See the <a href="/cmd/">full command reference</a>
for documentation of the compilers and more.
</p>

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/*
Copyright 2010 The Go Authors. All rights reserved.
Use of this source code is governed by a BSD-style
license that can be found in the LICENSE file.
*/
#codewalk-main {
text-align: left;
width: 100%;
overflow: auto;
}
#code-display {
border: 0;
width: 100%;
}
.setting {
font-size: 8pt;
color: #888888;
padding: 5px;
}
.hotkey {
text-decoration: underline;
}
/* Style for Comments (the left-hand column) */
#comment-column {
margin: 0pt;
width: 30%;
}
#comment-column.right {
float: right;
}
#comment-column.left {
float: left;
}
#comment-area {
overflow-x: hidden;
overflow-y: auto;
}
.comment {
cursor: pointer;
font-size: 16px;
border: 2px solid #ba9836;
margin-bottom: 10px;
margin-right: 10px; /* yes, for both .left and .right */
}
.comment:last-child {
margin-bottom: 0px;
}
.right .comment {
margin-left: 10px;
}
.right .comment.first {
}
.right .comment.last {
}
.left .comment.first {
}
.left .comment.last {
}
.comment.selected {
border-color: #99b2cb;
}
.right .comment.selected {
border-left-width: 12px;
margin-left: 0px;
}
.left .comment.selected {
border-right-width: 12px;
margin-right: 0px;
}
.comment-link {
display: none;
}
.comment-title {
font-size: small;
font-weight: bold;
background-color: #fffff0;
padding-right: 10px;
padding-left: 10px;
padding-top: 5px;
padding-bottom: 5px;
}
.right .comment-title {
}
.left .comment-title {
}
.comment.selected .comment-title {
background-color: #f8f8ff;
}
.comment-text {
overflow: auto;
padding-left: 10px;
padding-right: 10px;
padding-top: 10px;
padding-bottom: 5px;
font-size: small;
line-height: 1.3em;
}
.comment-text p {
margin-top: 0em;
margin-bottom: 0.5em;
}
.comment-text p:last-child {
margin-bottom: 0em;
}
.file-name {
font-size: x-small;
padding-top: 0px;
padding-bottom: 5px;
}
.hidden-filepaths .file-name {
display: none;
}
.path-dir {
color: #555;
}
.path-file {
color: #555;
}
/* Style for Code (the right-hand column) */
/* Wrapper for the code column to make widths get calculated correctly */
#code-column {
display: block;
position: relative;
margin: 0pt;
width: 70%;
}
#code-column.left {
float: left;
}
#code-column.right {
float: right;
}
#code-area {
background-color: #f8f8ff;
border: 2px solid #99b2cb;
padding: 5px;
}
.left #code-area {
margin-right: -1px;
}
.right #code-area {
margin-left: -1px;
}
#code-header {
margin-bottom: 5px;
}
#code {
background-color: white;
}
code {
font-size: 100%;
}
.codewalkhighlight {
font-weight: bold;
background-color: #f8f8ff;
}
#code-display {
margin-top: 0px;
margin-bottom: 0px;
}
#sizer {
position: absolute;
cursor: col-resize;
left: 0px;
top: 0px;
width: 8px;
}
/* Style for options (bottom strip) */
#code-options {
display: none;
}
#code-options > span {
padding-right: 20px;
}
#code-options .selected {
border-bottom: 1px dotted;
}
#comment-options {
text-align: center;
}
div#content {
padding-bottom: 0em;
}

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/**
* A class to hold information about the Codewalk Viewer.
* @param {jQuery} context The top element in whose context the viewer should
* operate. It will not touch any elements above this one.
* @constructor
*/
var CodewalkViewer = function(context) {
this.context = context;
/**
* The div that contains all of the comments and their controls.
*/
this.commentColumn = this.context.find('#comment-column');
/**
* The div that contains the comments proper.
*/
this.commentArea = this.context.find('#comment-area');
/**
* The div that wraps the iframe with the code, as well as the drop down menu
* listing the different files.
* @type {jQuery}
*/
this.codeColumn = this.context.find('#code-column');
/**
* The div that contains the code but excludes the options strip.
* @type {jQuery}
*/
this.codeArea = this.context.find('#code-area');
/**
* The iframe that holds the code (from Sourcerer).
* @type {jQuery}
*/
this.codeDisplay = this.context.find('#code-display');
/**
* The overlaid div used as a grab handle for sizing the code/comment panes.
* @type {jQuery}
*/
this.sizer = this.context.find('#sizer');
/**
* The full-screen overlay that ensures we don't lose track of the mouse
* while dragging.
* @type {jQuery}
*/
this.overlay = this.context.find('#overlay');
/**
* The hidden input field that we use to hold the focus so that we can detect
* shortcut keypresses.
* @type {jQuery}
*/
this.shortcutInput = this.context.find('#shortcut-input');
/**
* The last comment that was selected.
* @type {jQuery}
*/
this.lastSelected = null;
};
/**
* Minimum width of the comments or code pane, in pixels.
* @type {number}
*/
CodewalkViewer.MIN_PANE_WIDTH = 200;
/**
* Navigate the code iframe to the given url and update the code popout link.
* @param {string} url The target URL.
* @param {Object} opt_window Window dependency injection for testing only.
*/
CodewalkViewer.prototype.navigateToCode = function(url, opt_window) {
if (!opt_window) opt_window = window;
// Each iframe is represented by two distinct objects in the DOM: an iframe
// object and a window object. These do not expose the same capabilities.
// Here we need to get the window representation to get the location member,
// so we access it directly through window[] since jQuery returns the iframe
// representation.
// We replace location rather than set so as not to create a history for code
// navigation.
opt_window['code-display'].location.replace(url);
var k = url.indexOf('&');
if (k != -1) url = url.slice(0, k);
k = url.indexOf('fileprint=');
if (k != -1) url = url.slice(k+10, url.length);
this.context.find('#code-popout-link').attr('href', url);
};
/**
* Selects the first comment from the list and forces a refresh of the code
* view.
*/
CodewalkViewer.prototype.selectFirstComment = function() {
// TODO(rsc): handle case where there are no comments
var firstSourcererLink = this.context.find('.comment:first');
this.changeSelectedComment(firstSourcererLink);
};
/**
* Sets the target on all links nested inside comments to be _blank.
*/
CodewalkViewer.prototype.targetCommentLinksAtBlank = function() {
this.context.find('.comment a[href], #description a[href]').each(function() {
if (!this.target) this.target = '_blank';
});
};
/**
* Installs event handlers for all the events we care about.
*/
CodewalkViewer.prototype.installEventHandlers = function() {
var self = this;
this.context.find('.comment')
.click(function(event) {
if (jQuery(event.target).is('a[href]')) return true;
self.changeSelectedComment(jQuery(this));
return false;
});
this.context.find('#code-selector')
.change(function() {self.navigateToCode(jQuery(this).val());});
this.context.find('#description-table .quote-feet.setting')
.click(function() {self.toggleDescription(jQuery(this)); return false;});
this.sizer
.mousedown(function(ev) {self.startSizerDrag(ev); return false;});
this.overlay
.mouseup(function(ev) {self.endSizerDrag(ev); return false;})
.mousemove(function(ev) {self.handleSizerDrag(ev); return false;});
this.context.find('#prev-comment')
.click(function() {
self.changeSelectedComment(self.lastSelected.prev()); return false;
});
this.context.find('#next-comment')
.click(function() {
self.changeSelectedComment(self.lastSelected.next()); return false;
});
// Workaround for Firefox 2 and 3, which steal focus from the main document
// whenever the iframe content is (re)loaded. The input field is not shown,
// but is a way for us to bring focus back to a place where we can detect
// keypresses.
this.context.find('#code-display')
.load(function(ev) {self.shortcutInput.focus();});
jQuery(document).keypress(function(ev) {
switch(ev.which) {
case 110: // 'n'
self.changeSelectedComment(self.lastSelected.next());
return false;
case 112: // 'p'
self.changeSelectedComment(self.lastSelected.prev());
return false;
default: // ignore
}
});
window.onresize = function() {self.updateHeight();};
};
/**
* Starts dragging the pane sizer.
* @param {Object} ev The mousedown event that started us dragging.
*/
CodewalkViewer.prototype.startSizerDrag = function(ev) {
this.initialCodeWidth = this.codeColumn.width();
this.initialCommentsWidth = this.commentColumn.width();
this.initialMouseX = ev.pageX;
this.overlay.show();
};
/**
* Handles dragging the pane sizer.
* @param {Object} ev The mousemove event updating dragging position.
*/
CodewalkViewer.prototype.handleSizerDrag = function(ev) {
var delta = ev.pageX - this.initialMouseX;
if (this.codeColumn.is('.right')) delta = -delta;
var proposedCodeWidth = this.initialCodeWidth + delta;
var proposedCommentWidth = this.initialCommentsWidth - delta;
var mw = CodewalkViewer.MIN_PANE_WIDTH;
if (proposedCodeWidth < mw) delta = mw - this.initialCodeWidth;
if (proposedCommentWidth < mw) delta = this.initialCommentsWidth - mw;
proposedCodeWidth = this.initialCodeWidth + delta;
proposedCommentWidth = this.initialCommentsWidth - delta;
// If window is too small, don't even try to resize.
if (proposedCodeWidth < mw || proposedCommentWidth < mw) return;
this.codeColumn.width(proposedCodeWidth);
this.commentColumn.width(proposedCommentWidth);
this.options.codeWidth = parseInt(
this.codeColumn.width() /
(this.codeColumn.width() + this.commentColumn.width()) * 100);
this.context.find('#code-column-width').text(this.options.codeWidth + '%');
};
/**
* Ends dragging the pane sizer.
* @param {Object} ev The mouseup event that caused us to stop dragging.
*/
CodewalkViewer.prototype.endSizerDrag = function(ev) {
this.overlay.hide();
this.updateHeight();
};
/**
* Toggles the Codewalk description between being shown and hidden.
* @param {jQuery} target The target that was clicked to trigger this function.
*/
CodewalkViewer.prototype.toggleDescription = function(target) {
var description = this.context.find('#description');
description.toggle();
target.find('span').text(description.is(':hidden') ? 'show' : 'hide');
this.updateHeight();
};
/**
* Changes the side of the window on which the code is shown and saves the
* setting in a cookie.
* @param {string?} codeSide The side on which the code should be, either
* 'left' or 'right'.
*/
CodewalkViewer.prototype.changeCodeSide = function(codeSide) {
var commentSide = codeSide == 'left' ? 'right' : 'left';
this.context.find('#set-code-' + codeSide).addClass('selected');
this.context.find('#set-code-' + commentSide).removeClass('selected');
// Remove previous side class and add new one.
this.codeColumn.addClass(codeSide).removeClass(commentSide);
this.commentColumn.addClass(commentSide).removeClass(codeSide);
this.sizer.css(codeSide, 'auto').css(commentSide, 0);
this.options.codeSide = codeSide;
};
/**
* Adds selected class to newly selected comment, removes selected style from
* previously selected comment, changes drop down options so that the correct
* file is selected, and updates the code popout link.
* @param {jQuery} target The target that was clicked to trigger this function.
*/
CodewalkViewer.prototype.changeSelectedComment = function(target) {
var currentFile = target.find('.comment-link').attr('href');
if (!currentFile) return;
if (!(this.lastSelected && this.lastSelected.get(0) === target.get(0))) {
if (this.lastSelected) this.lastSelected.removeClass('selected');
target.addClass('selected');
this.lastSelected = target;
var targetTop = target.position().top;
var parentTop = target.parent().position().top;
if (targetTop + target.height() > parentTop + target.parent().height() ||
targetTop < parentTop) {
var delta = targetTop - parentTop;
target.parent().animate(
{'scrollTop': target.parent().scrollTop() + delta},
Math.max(delta / 2, 200), 'swing');
}
var fname = currentFile.match(/(?:select=|fileprint=)\/[^&]+/)[0];
fname = fname.slice(fname.indexOf('=')+2, fname.length);
this.context.find('#code-selector').val(fname);
this.context.find('#prev-comment').toggleClass(
'disabled', !target.prev().length);
this.context.find('#next-comment').toggleClass(
'disabled', !target.next().length);
}
// Force original file even if user hasn't changed comments since they may
// have navigated away from it within the iframe without us knowing.
this.navigateToCode(currentFile);
};
/**
* Updates the viewer by changing the height of the comments and code so that
* they fit within the height of the window. The function is typically called
* after the user changes the window size.
*/
CodewalkViewer.prototype.updateHeight = function() {
var windowHeight = jQuery(window).height() - 5 // GOK
var areaHeight = windowHeight - this.codeArea.offset().top
var footerHeight = this.context.find('#footer').outerHeight(true)
this.commentArea.height(areaHeight - footerHeight - this.context.find('#comment-options').outerHeight(true))
var codeHeight = areaHeight - footerHeight - 15 // GOK
this.codeArea.height(codeHeight)
this.codeDisplay.height(codeHeight - this.codeDisplay.offset().top + this.codeArea.offset().top);
this.sizer.height(codeHeight);
};
window.initFuncs.push(function() {
var viewer = new CodewalkViewer(jQuery('#codewalk-main'));
viewer.selectFirstComment();
viewer.targetCommentLinksAtBlank();
viewer.installEventHandlers();
viewer.updateHeight();
});

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<codewalk title="How to Write a Codewalk">
<step title="Introduction" src="doc/codewalk/codewalk.xml">
A codewalk is a guided tour through a piece of code.
It consists of a sequence of steps, each typically explaining
a highlighted section of code.
<br/><br/>
The <a href="/cmd/godoc">godoc</a> web server translates
an XML file like the one in the main window pane into the HTML
page that you're viewing now.
<br/><br/>
The codewalk with URL path <code>/doc/codewalk/</code><i>name</i>
is loaded from the input file <code>$GOROOT/doc/codewalk/</code><i>name</i><code>.xml</code>.
<br/><br/>
This codewalk explains how to write a codewalk by examining
its own source code,
<code><a href="/doc/codewalk/codewalk.xml">$GOROOT/doc/codewalk/codewalk.xml</a></code>,
shown in the main window pane to the left.
</step>
<step title="Title" src="doc/codewalk/codewalk.xml:/title=/">
The codewalk input file is an XML file containing a single
<code>&lt;codewalk&gt;</code> element.
That element's <code>title</code> attribute gives the title
that is used both on the codewalk page and in the codewalk list.
</step>
<step title="Steps" src="doc/codewalk/codewalk.xml:/&lt;step/,/step&gt;/">
Each step in the codewalk is a <code>&lt;step&gt;</code> element
nested inside the main <code>&lt;codewalk&gt;</code>.
The step element's <code>title</code> attribute gives the step's title,
which is shown in a shaded bar above the main step text.
The element's <code>src</code> attribute specifies the source
code to show in the main window pane and, optionally, a range of
lines to highlight.
<br/><br/>
The first step in this codewalk does not highlight any lines:
its <code>src</code> is just a file name.
</step>
<step title="Specifying a source line" src='doc/codewalk/codewalk.xml:/title="Title"/'>
The most complex part of the codewalk specification is
saying what lines to highlight.
Instead of ordinary line numbers,
the codewalk uses an address syntax that makes it possible
to describe the match by its content.
As the file gets edited, this descriptive address has a better
chance to continue to refer to the right section of the file.
<br/><br/>
To specify a source line, use a <code>src</code> attribute of the form
<i>filename</i><code>:</code><i>address</i>,
where <i>address</i> is an address in the syntax used by the text editors <i>sam</i> and <i>acme</i>.
<br/><br/>
The simplest address is a single regular expression.
The highlighted line in the main window pane shows that the
address for the &ldquo;Title&rdquo; step was <code>/title=/</code>,
which matches the first instance of that <a href="/pkg/regexp">regular expression</a> (<code>title=</code>) in the file.
</step>
<step title="Specifying a source range" src='doc/codewalk/codewalk.xml:/title="Steps"/'>
To highlight a range of source lines, the simplest address to use is
a pair of regular expressions
<code>/</code><i>regexp1</i><code>/,/</code><i>regexp2</i><code>/</code>.
The highlight begins with the line containing the first match for <i>regexp1</i>
and ends with the line containing the first match for <i>regexp2</i>
after the end of the match for <i>regexp1</i>.
Ignoring the HTML quoting,
The line containing the first match for <i>regexp1</i> will be the first one highlighted,
and the line containing the first match for <i>regexp2</i>.
<br/><br/>
The address <code>/&lt;step/,/step&gt;/</code> looks for the first instance of
<code>&lt;step</code> in the file, and then starting after that point,
looks for the first instance of <code>step&gt;</code>.
(Click on the &ldquo;Steps&rdquo; step above to see the highlight in action.)
Note that the <code>&lt;</code> and <code>&gt;</code> had to be written
using XML escapes in order to be valid XML.
</step>
<step title="Advanced addressing" src="doc/codewalk/codewalk.xml:/Advanced/,/step&gt;/">
The <code>/</code><i>regexp</i><code>/</code>
and <code>/</code><i>regexp1</i><code>/,/</code><i>regexp2</i><code>/</code>
forms suffice for most highlighting.
<br/><br/>
The full address syntax is summarized in this table
(an excerpt of Table II from
<a href="https://9p.io/sys/doc/sam/sam.html">The text editor <code>sam</code></a>):
<br/><br/>
<table>
<tr><td colspan="2"><b>Simple addresses</b></td></tr>
<tr><td><code>#</code><i>n</i></td>
<td>The empty string after character <i>n</i></td></tr>
<tr><td><i>n</i></td>
<td>Line <i>n</i></td></tr>
<tr><td><code>/</code><i>regexp</i><code>/</code></td>
<td>The first following match of the regular expression</td></tr>
<!-- not supported (yet?)
<tr><td><code>–/</code><i>regexp</i><code>/</code></td>
<td>The first previous match of the regular expression</td></tr>
-->
<tr><td><code>$</code></td>
<td>The null string at the end of the file</td></tr>
<tr><td colspan="2"><b>Compound addresses</b></td></tr>
<tr><td><i>a1</i><code>+</code><i>a2</i></td>
<td>The address <i>a2</i> evaluated starting at the right of <i>a1</i></td></tr>
<tr><td><i>a1</i><code>-</code><i>a2</i></td>
<td>The address <i>a2</i> evaluated in the reverse direction starting at the left of <i>a1</i></td></tr>
<tr><td><i>a1</i><code>,</code><i>a2</i></td>
<td>From the left of <i>a1</i> to the right of <i>a2</i> (default <code>0,$</code>).</td></tr>
</table>
</step>
</codewalk>

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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main_test
import (
"bytes"
"os"
"os/exec"
"strings"
"testing"
)
// TestMarkov tests the code dependency of markov.xml.
func TestMarkov(t *testing.T) {
cmd := exec.Command("go", "run", "markov.go")
cmd.Stdin = strings.NewReader("foo")
cmd.Stderr = bytes.NewBuffer(nil)
out, err := cmd.Output()
if err != nil {
t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, cmd.Stderr)
}
if !bytes.Equal(out, []byte("foo\n")) {
t.Fatalf(`%s with input "foo" did not output "foo":\n%s`, strings.Join(cmd.Args, " "), out)
}
}
// TestPig tests the code dependency of functions.xml.
func TestPig(t *testing.T) {
cmd := exec.Command("go", "run", "pig.go")
cmd.Stderr = bytes.NewBuffer(nil)
out, err := cmd.Output()
if err != nil {
t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, cmd.Stderr)
}
const want = "Wins, losses staying at k = 100: 210/990 (21.2%), 780/990 (78.8%)\n"
if !bytes.Contains(out, []byte(want)) {
t.Fatalf(`%s: unexpected output\ngot:\n%s\nwant output containing:\n%s`, strings.Join(cmd.Args, " "), out, want)
}
}
// TestURLPoll tests the code dependency of sharemem.xml.
func TestURLPoll(t *testing.T) {
cmd := exec.Command("go", "build", "-o", os.DevNull, "urlpoll.go")
out, err := cmd.CombinedOutput()
if err != nil {
t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, out)
}
}

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<codewalk title="First-Class Functions in Go">
<step title="Introduction" src="doc/codewalk/pig.go">
Go supports first class functions, higher-order functions, user-defined
function types, function literals, closures, and multiple return values.
<br/><br/>
This rich feature set supports a functional programming style in a strongly
typed language.
<br/><br/>
In this codewalk we will look at a simple program that simulates a dice game
called <a href="http://en.wikipedia.org/wiki/Pig_(dice)">Pig</a> and evaluates
basic strategies.
</step>
<step title="Game overview" src="doc/codewalk/pig.go:/\/\/ A score/,/thisTurn int\n}/">
Pig is a two-player game played with a 6-sided die. Each turn, you may roll or stay.
<ul>
<li> If you roll a 1, you lose all points for your turn and play passes to
your opponent. Any other roll adds its value to your turn score. </li>
<li> If you stay, your turn score is added to your total score, and play passes
to your opponent. </li>
</ul>
The first person to reach 100 total points wins.
<br/><br/>
The <code>score</code> type stores the scores of the current and opposing
players, in addition to the points accumulated during the current turn.
</step>
<step title="User-defined function types" src="doc/codewalk/pig.go:/\/\/ An action/,/bool\)/">
In Go, functions can be passed around just like any other value. A function's
type signature describes the types of its arguments and return values.
<br/><br/>
The <code>action</code> type is a function that takes a <code>score</code>
and returns the resulting <code>score</code> and whether the current turn is
over.
<br/><br/>
If the turn is over, the <code>player</code> and <code>opponent</code> fields
in the resulting <code>score</code> should be swapped, as it is now the other player's
turn.
</step>
<step title="Multiple return values" src="doc/codewalk/pig.go:/\/\/ roll returns/,/true\n}/">
Go functions can return multiple values.
<br/><br/>
The functions <code>roll</code> and <code>stay</code> each return a pair of
values. They also match the <code>action</code> type signature. These
<code>action</code> functions define the rules of Pig.
</step>
<step title="Higher-order functions" src="doc/codewalk/pig.go:/\/\/ A strategy/,/action\n/">
A function can use other functions as arguments and return values.
<br/><br/>
A <code>strategy</code> is a function that takes a <code>score</code> as input
and returns an <code>action</code> to perform. <br/>
(Remember, an <code>action</code> is itself a function.)
</step>
<step title="Function literals and closures" src="doc/codewalk/pig.go:/return func/,/return roll\n\t}/">
Anonymous functions can be declared in Go, as in this example. Function
literals are closures: they inherit the scope of the function in which they
are declared.
<br/><br/>
One basic strategy in Pig is to continue rolling until you have accumulated at
least k points in a turn, and then stay. The argument <code>k</code> is
enclosed by this function literal, which matches the <code>strategy</code> type
signature.
</step>
<step title="Simulating games" src="doc/codewalk/pig.go:/\/\/ play/,/currentPlayer\n}/">
We simulate a game of Pig by calling an <code>action</code> to update the
<code>score</code> until one player reaches 100 points. Each
<code>action</code> is selected by calling the <code>strategy</code> function
associated with the current player.
</step>
<step title="Simulating a tournament" src="doc/codewalk/pig.go:/\/\/ roundRobin/,/gamesPerStrategy\n}/">
The <code>roundRobin</code> function simulates a tournament and tallies wins.
Each strategy plays each other strategy <code>gamesPerSeries</code> times.
</step>
<step title="Variadic function declarations" src="doc/codewalk/pig.go:/\/\/ ratioS/,/string {/">
Variadic functions like <code>ratioString</code> take a variable number of
arguments. These arguments are available as a slice inside the function.
</step>
<step title="Simulation results" src="doc/codewalk/pig.go:/func main/,/\n}/">
The <code>main</code> function defines 100 basic strategies, simulates a round
robin tournament, and then prints the win/loss record of each strategy.
<br/><br/>
Among these strategies, staying at 25 is best, but the <a
href="http://www.google.com/search?q=optimal+play+pig">optimal strategy for
Pig</a> is much more complex.
</step>
</codewalk>

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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/*
Generating random text: a Markov chain algorithm
Based on the program presented in the "Design and Implementation" chapter
of The Practice of Programming (Kernighan and Pike, Addison-Wesley 1999).
See also Computer Recreations, Scientific American 260, 122 - 125 (1989).
A Markov chain algorithm generates text by creating a statistical model of
potential textual suffixes for a given prefix. Consider this text:
I am not a number! I am a free man!
Our Markov chain algorithm would arrange this text into this set of prefixes
and suffixes, or "chain": (This table assumes a prefix length of two words.)
Prefix Suffix
"" "" I
"" I am
I am a
I am not
a free man!
am a free
am not a
a number! I
number! I am
not a number!
To generate text using this table we select an initial prefix ("I am", for
example), choose one of the suffixes associated with that prefix at random
with probability determined by the input statistics ("a"),
and then create a new prefix by removing the first word from the prefix
and appending the suffix (making the new prefix is "am a"). Repeat this process
until we can't find any suffixes for the current prefix or we exceed the word
limit. (The word limit is necessary as the chain table may contain cycles.)
Our version of this program reads text from standard input, parsing it into a
Markov chain, and writes generated text to standard output.
The prefix and output lengths can be specified using the -prefix and -words
flags on the command-line.
*/
package main
import (
"bufio"
"flag"
"fmt"
"io"
"math/rand"
"os"
"strings"
"time"
)
// Prefix is a Markov chain prefix of one or more words.
type Prefix []string
// String returns the Prefix as a string (for use as a map key).
func (p Prefix) String() string {
return strings.Join(p, " ")
}
// Shift removes the first word from the Prefix and appends the given word.
func (p Prefix) Shift(word string) {
copy(p, p[1:])
p[len(p)-1] = word
}
// Chain contains a map ("chain") of prefixes to a list of suffixes.
// A prefix is a string of prefixLen words joined with spaces.
// A suffix is a single word. A prefix can have multiple suffixes.
type Chain struct {
chain map[string][]string
prefixLen int
}
// NewChain returns a new Chain with prefixes of prefixLen words.
func NewChain(prefixLen int) *Chain {
return &Chain{make(map[string][]string), prefixLen}
}
// Build reads text from the provided Reader and
// parses it into prefixes and suffixes that are stored in Chain.
func (c *Chain) Build(r io.Reader) {
br := bufio.NewReader(r)
p := make(Prefix, c.prefixLen)
for {
var s string
if _, err := fmt.Fscan(br, &s); err != nil {
break
}
key := p.String()
c.chain[key] = append(c.chain[key], s)
p.Shift(s)
}
}
// Generate returns a string of at most n words generated from Chain.
func (c *Chain) Generate(n int) string {
p := make(Prefix, c.prefixLen)
var words []string
for i := 0; i < n; i++ {
choices := c.chain[p.String()]
if len(choices) == 0 {
break
}
next := choices[rand.Intn(len(choices))]
words = append(words, next)
p.Shift(next)
}
return strings.Join(words, " ")
}
func main() {
// Register command-line flags.
numWords := flag.Int("words", 100, "maximum number of words to print")
prefixLen := flag.Int("prefix", 2, "prefix length in words")
flag.Parse() // Parse command-line flags.
rand.Seed(time.Now().UnixNano()) // Seed the random number generator.
c := NewChain(*prefixLen) // Initialize a new Chain.
c.Build(os.Stdin) // Build chains from standard input.
text := c.Generate(*numWords) // Generate text.
fmt.Println(text) // Write text to standard output.
}

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<!--
Copyright 2011 The Go Authors. All rights reserved.
Use of this source code is governed by a BSD-style
license that can be found in the LICENSE file.
-->
<codewalk title="Generating arbitrary text: a Markov chain algorithm">
<step title="Introduction" src="doc/codewalk/markov.go:/Generating/,/line\./">
This codewalk describes a program that generates random text using
a Markov chain algorithm. The package comment describes the algorithm
and the operation of the program. Please read it before continuing.
</step>
<step title="Modeling Markov chains" src="doc/codewalk/markov.go:/ chain/">
A chain consists of a prefix and a suffix. Each prefix is a set
number of words, while a suffix is a single word.
A prefix can have an arbitrary number of suffixes.
To model this data, we use a <code>map[string][]string</code>.
Each map key is a prefix (a <code>string</code>) and its values are
lists of suffixes (a slice of strings, <code>[]string</code>).
<br/><br/>
Here is the example table from the package comment
as modeled by this data structure:
<pre>
map[string][]string{
" ": {"I"},
" I": {"am"},
"I am": {"a", "not"},
"a free": {"man!"},
"am a": {"free"},
"am not": {"a"},
"a number!": {"I"},
"number! I": {"am"},
"not a": {"number!"},
}</pre>
While each prefix consists of multiple words, we
store prefixes in the map as a single <code>string</code>.
It would seem more natural to store the prefix as a
<code>[]string</code>, but we can't do this with a map because the
key type of a map must implement equality (and slices do not).
<br/><br/>
Therefore, in most of our code we will model prefixes as a
<code>[]string</code> and join the strings together with a space
to generate the map key:
<pre>
Prefix Map key
[]string{"", ""} " "
[]string{"", "I"} " I"
[]string{"I", "am"} "I am"
</pre>
</step>
<step title="The Chain struct" src="doc/codewalk/markov.go:/type Chain/,/}/">
The complete state of the chain table consists of the table itself and
the word length of the prefixes. The <code>Chain</code> struct stores
this data.
</step>
<step title="The NewChain constructor function" src="doc/codewalk/markov.go:/func New/,/\n}/">
The <code>Chain</code> struct has two unexported fields (those that
do not begin with an upper case character), and so we write a
<code>NewChain</code> constructor function that initializes the
<code>chain</code> map with <code>make</code> and sets the
<code>prefixLen</code> field.
<br/><br/>
This is constructor function is not strictly necessary as this entire
program is within a single package (<code>main</code>) and therefore
there is little practical difference between exported and unexported
fields. We could just as easily write out the contents of this function
when we want to construct a new Chain.
But using these unexported fields is good practice; it clearly denotes
that only methods of Chain and its constructor function should access
those fields. Also, structuring <code>Chain</code> like this means we
could easily move it into its own package at some later date.
</step>
<step title="The Prefix type" src="doc/codewalk/markov.go:/type Prefix/">
Since we'll be working with prefixes often, we define a
<code>Prefix</code> type with the concrete type <code>[]string</code>.
Defining a named type clearly allows us to be explicit when we are
working with a prefix instead of just a <code>[]string</code>.
Also, in Go we can define methods on any named type (not just structs),
so we can add methods that operate on <code>Prefix</code> if we need to.
</step>
<step title="The String method" src="doc/codewalk/markov.go:/func[^\n]+String/,/}/">
The first method we define on <code>Prefix</code> is
<code>String</code>. It returns a <code>string</code> representation
of a <code>Prefix</code> by joining the slice elements together with
spaces. We will use this method to generate keys when working with
the chain map.
</step>
<step title="Building the chain" src="doc/codewalk/markov.go:/func[^\n]+Build/,/\n}/">
The <code>Build</code> method reads text from an <code>io.Reader</code>
and parses it into prefixes and suffixes that are stored in the
<code>Chain</code>.
<br/><br/>
The <code><a href="/pkg/io/#Reader">io.Reader</a></code> is an
interface type that is widely used by the standard library and
other Go code. Our code uses the
<code><a href="/pkg/fmt/#Fscan">fmt.Fscan</a></code> function, which
reads space-separated values from an <code>io.Reader</code>.
<br/><br/>
The <code>Build</code> method returns once the <code>Reader</code>'s
<code>Read</code> method returns <code>io.EOF</code> (end of file)
or some other read error occurs.
</step>
<step title="Buffering the input" src="doc/codewalk/markov.go:/bufio\.NewReader/">
This function does many small reads, which can be inefficient for some
<code>Readers</code>. For efficiency we wrap the provided
<code>io.Reader</code> with
<code><a href="/pkg/bufio/">bufio.NewReader</a></code> to create a
new <code>io.Reader</code> that provides buffering.
</step>
<step title="The Prefix variable" src="doc/codewalk/markov.go:/make\(Prefix/">
At the top of the function we make a <code>Prefix</code> slice
<code>p</code> using the <code>Chain</code>'s <code>prefixLen</code>
field as its length.
We'll use this variable to hold the current prefix and mutate it with
each new word we encounter.
</step>
<step title="Scanning words" src="doc/codewalk/markov.go:/var s string/,/\n }/">
In our loop we read words from the <code>Reader</code> into a
<code>string</code> variable <code>s</code> using
<code>fmt.Fscan</code>. Since <code>Fscan</code> uses space to
separate each input value, each call will yield just one word
(including punctuation), which is exactly what we need.
<br/><br/>
<code>Fscan</code> returns an error if it encounters a read error
(<code>io.EOF</code>, for example) or if it can't scan the requested
value (in our case, a single string). In either case we just want to
stop scanning, so we <code>break</code> out of the loop.
</step>
<step title="Adding a prefix and suffix to the chain" src="doc/codewalk/markov.go:/ key/,/key\], s\)">
The word stored in <code>s</code> is a new suffix. We add the new
prefix/suffix combination to the <code>chain</code> map by computing
the map key with <code>p.String</code> and appending the suffix
to the slice stored under that key.
<br/><br/>
The built-in <code>append</code> function appends elements to a slice
and allocates new storage when necessary. When the provided slice is
<code>nil</code>, <code>append</code> allocates a new slice.
This behavior conveniently ties in with the semantics of our map:
retrieving an unset key returns the zero value of the value type and
the zero value of <code>[]string</code> is <code>nil</code>.
When our program encounters a new prefix (yielding a <code>nil</code>
value in the map) <code>append</code> will allocate a new slice.
<br/><br/>
For more information about the <code>append</code> function and slices
in general see the
<a href="/doc/articles/slices_usage_and_internals.html">Slices: usage and internals</a> article.
</step>
<step title="Pushing the suffix onto the prefix" src="doc/codewalk/markov.go:/p\.Shift/">
Before reading the next word our algorithm requires us to drop the
first word from the prefix and push the current suffix onto the prefix.
<br/><br/>
When in this state
<pre>
p == Prefix{"I", "am"}
s == "not" </pre>
the new value for <code>p</code> would be
<pre>
p == Prefix{"am", "not"}</pre>
This operation is also required during text generation so we put
the code to perform this mutation of the slice inside a method on
<code>Prefix</code> named <code>Shift</code>.
</step>
<step title="The Shift method" src="doc/codewalk/markov.go:/func[^\n]+Shift/,/\n}/">
The <code>Shift</code> method uses the built-in <code>copy</code>
function to copy the last len(p)-1 elements of <code>p</code> to
the start of the slice, effectively moving the elements
one index to the left (if you consider zero as the leftmost index).
<pre>
p := Prefix{"I", "am"}
copy(p, p[1:])
// p == Prefix{"am", "am"}</pre>
We then assign the provided <code>word</code> to the last index
of the slice:
<pre>
// suffix == "not"
p[len(p)-1] = suffix
// p == Prefix{"am", "not"}</pre>
</step>
<step title="Generating text" src="doc/codewalk/markov.go:/func[^\n]+Generate/,/\n}/">
The <code>Generate</code> method is similar to <code>Build</code>
except that instead of reading words from a <code>Reader</code>
and storing them in a map, it reads words from the map and
appends them to a slice (<code>words</code>).
<br/><br/>
<code>Generate</code> uses a conditional for loop to generate
up to <code>n</code> words.
</step>
<step title="Getting potential suffixes" src="doc/codewalk/markov.go:/choices/,/}\n/">
At each iteration of the loop we retrieve a list of potential suffixes
for the current prefix. We access the <code>chain</code> map at key
<code>p.String()</code> and assign its contents to <code>choices</code>.
<br/><br/>
If <code>len(choices)</code> is zero we break out of the loop as there
are no potential suffixes for that prefix.
This test also works if the key isn't present in the map at all:
in that case, <code>choices</code> will be <code>nil</code> and the
length of a <code>nil</code> slice is zero.
</step>
<step title="Choosing a suffix at random" src="doc/codewalk/markov.go:/next := choices/,/Shift/">
To choose a suffix we use the
<code><a href="/pkg/math/rand/#Intn">rand.Intn</a></code> function.
It returns a random integer up to (but not including) the provided
value. Passing in <code>len(choices)</code> gives us a random index
into the full length of the list.
<br/><br/>
We use that index to pick our new suffix, assign it to
<code>next</code> and append it to the <code>words</code> slice.
<br/><br/>
Next, we <code>Shift</code> the new suffix onto the prefix just as
we did in the <code>Build</code> method.
</step>
<step title="Returning the generated text" src="doc/codewalk/markov.go:/Join\(words/">
Before returning the generated text as a string, we use the
<code>strings.Join</code> function to join the elements of
the <code>words</code> slice together, separated by spaces.
</step>
<step title="Command-line flags" src="doc/codewalk/markov.go:/Register command-line flags/,/prefixLen/">
To make it easy to tweak the prefix and generated text lengths we
use the <code><a href="/pkg/flag/">flag</a></code> package to parse
command-line flags.
<br/><br/>
These calls to <code>flag.Int</code> register new flags with the
<code>flag</code> package. The arguments to <code>Int</code> are the
flag name, its default value, and a description. The <code>Int</code>
function returns a pointer to an integer that will contain the
user-supplied value (or the default value if the flag was omitted on
the command-line).
</step>
<step title="Program set up" src="doc/codewalk/markov.go:/flag.Parse/,/rand.Seed/">
The <code>main</code> function begins by parsing the command-line
flags with <code>flag.Parse</code> and seeding the <code>rand</code>
package's random number generator with the current time.
<br/><br/>
If the command-line flags provided by the user are invalid the
<code>flag.Parse</code> function will print an informative usage
message and terminate the program.
</step>
<step title="Creating and building a new Chain" src="doc/codewalk/markov.go:/c := NewChain/,/c\.Build/">
To create the new <code>Chain</code> we call <code>NewChain</code>
with the value of the <code>prefix</code> flag.
<br/><br/>
To build the chain we call <code>Build</code> with
<code>os.Stdin</code> (which implements <code>io.Reader</code>) so
that it will read its input from standard input.
</step>
<step title="Generating and printing text" src="doc/codewalk/markov.go:/c\.Generate/,/fmt.Println/">
Finally, to generate text we call <code>Generate</code> with
the value of the <code>words</code> flag and assigning the result
to the variable <code>text</code>.
<br/><br/>
Then we call <code>fmt.Println</code> to write the text to standard
output, followed by a carriage return.
</step>
<step title="Using this program" src="doc/codewalk/markov.go">
To use this program, first build it with the
<a href="/cmd/go/">go</a> command:
<pre>
$ go build markov.go</pre>
And then execute it while piping in some input text:
<pre>
$ echo "a man a plan a canal panama" \
| ./markov -prefix=1
a plan a man a plan a canal panama</pre>
Here's a transcript of generating some text using the Go distribution's
README file as source material:
<pre>
$ ./markov -words=10 &lt; $GOROOT/README
This is the source code repository for the Go source
$ ./markov -prefix=1 -words=10 &lt; $GOROOT/README
This is the go directory (the one containing this README).
$ ./markov -prefix=1 -words=10 &lt; $GOROOT/README
This is the variable if you have just untarred a</pre>
</step>
<step title="An exercise for the reader" src="doc/codewalk/markov.go">
The <code>Generate</code> function does a lot of allocations when it
builds the <code>words</code> slice. As an exercise, modify it to
take an <code>io.Writer</code> to which it incrementally writes the
generated text with <code>Fprint</code>.
Aside from being more efficient this makes <code>Generate</code>
more symmetrical to <code>Build</code>.
</step>
</codewalk>

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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"fmt"
"math/rand"
)
const (
win = 100 // The winning score in a game of Pig
gamesPerSeries = 10 // The number of games per series to simulate
)
// A score includes scores accumulated in previous turns for each player,
// as well as the points scored by the current player in this turn.
type score struct {
player, opponent, thisTurn int
}
// An action transitions stochastically to a resulting score.
type action func(current score) (result score, turnIsOver bool)
// roll returns the (result, turnIsOver) outcome of simulating a die roll.
// If the roll value is 1, then thisTurn score is abandoned, and the players'
// roles swap. Otherwise, the roll value is added to thisTurn.
func roll(s score) (score, bool) {
outcome := rand.Intn(6) + 1 // A random int in [1, 6]
if outcome == 1 {
return score{s.opponent, s.player, 0}, true
}
return score{s.player, s.opponent, outcome + s.thisTurn}, false
}
// stay returns the (result, turnIsOver) outcome of staying.
// thisTurn score is added to the player's score, and the players' roles swap.
func stay(s score) (score, bool) {
return score{s.opponent, s.player + s.thisTurn, 0}, true
}
// A strategy chooses an action for any given score.
type strategy func(score) action
// stayAtK returns a strategy that rolls until thisTurn is at least k, then stays.
func stayAtK(k int) strategy {
return func(s score) action {
if s.thisTurn >= k {
return stay
}
return roll
}
}
// play simulates a Pig game and returns the winner (0 or 1).
func play(strategy0, strategy1 strategy) int {
strategies := []strategy{strategy0, strategy1}
var s score
var turnIsOver bool
currentPlayer := rand.Intn(2) // Randomly decide who plays first
for s.player+s.thisTurn < win {
action := strategies[currentPlayer](s)
s, turnIsOver = action(s)
if turnIsOver {
currentPlayer = (currentPlayer + 1) % 2
}
}
return currentPlayer
}
// roundRobin simulates a series of games between every pair of strategies.
func roundRobin(strategies []strategy) ([]int, int) {
wins := make([]int, len(strategies))
for i := 0; i < len(strategies); i++ {
for j := i + 1; j < len(strategies); j++ {
for k := 0; k < gamesPerSeries; k++ {
winner := play(strategies[i], strategies[j])
if winner == 0 {
wins[i]++
} else {
wins[j]++
}
}
}
}
gamesPerStrategy := gamesPerSeries * (len(strategies) - 1) // no self play
return wins, gamesPerStrategy
}
// ratioString takes a list of integer values and returns a string that lists
// each value and its percentage of the sum of all values.
// e.g., ratios(1, 2, 3) = "1/6 (16.7%), 2/6 (33.3%), 3/6 (50.0%)"
func ratioString(vals ...int) string {
total := 0
for _, val := range vals {
total += val
}
s := ""
for _, val := range vals {
if s != "" {
s += ", "
}
pct := 100 * float64(val) / float64(total)
s += fmt.Sprintf("%d/%d (%0.1f%%)", val, total, pct)
}
return s
}
func main() {
strategies := make([]strategy, win)
for k := range strategies {
strategies[k] = stayAtK(k + 1)
}
wins, games := roundRobin(strategies)
for k := range strategies {
fmt.Printf("Wins, losses staying at k =% 4d: %s\n",
k+1, ratioString(wins[k], games-wins[k]))
}
}

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<codewalk title="Share Memory By Communicating">
<step title="Introduction" src="doc/codewalk/urlpoll.go">
Go's approach to concurrency differs from the traditional use of
threads and shared memory. Philosophically, it can be summarized:
<br/><br/>
<i>Don't communicate by sharing memory; share memory by communicating.</i>
<br/><br/>
Channels allow you to pass references to data structures between goroutines.
If you consider this as passing around ownership of the data (the ability to
read and write it), they become a powerful and expressive synchronization
mechanism.
<br/><br/>
In this codewalk we will look at a simple program that polls a list of
URLs, checking their HTTP response codes and periodically printing their state.
</step>
<step title="State type" src="doc/codewalk/urlpoll.go:/State/,/}/">
The State type represents the state of a URL.
<br/><br/>
The Pollers send State values to the StateMonitor,
which maintains a map of the current state of each URL.
</step>
<step title="Resource type" src="doc/codewalk/urlpoll.go:/Resource/,/}/">
A Resource represents the state of a URL to be polled: the URL itself
and the number of errors encountered since the last successful poll.
<br/><br/>
When the program starts, it allocates one Resource for each URL.
The main goroutine and the Poller goroutines send the Resources to
each other on channels.
</step>
<step title="Poller function" src="doc/codewalk/urlpoll.go:/func Poller/,/\n}/">
Each Poller receives Resource pointers from an input channel.
In this program, the convention is that sending a Resource pointer on
a channel passes ownership of the underlying data from the sender
to the receiver. Because of this convention, we know that
no two goroutines will access this Resource at the same time.
This means we don't have to worry about locking to prevent concurrent
access to these data structures.
<br/><br/>
The Poller processes the Resource by calling its Poll method.
<br/><br/>
It sends a State value to the status channel, to inform the StateMonitor
of the result of the Poll.
<br/><br/>
Finally, it sends the Resource pointer to the out channel. This can be
interpreted as the Poller saying &quot;I'm done with this Resource&quot; and
returning ownership of it to the main goroutine.
<br/><br/>
Several goroutines run Pollers, processing Resources in parallel.
</step>
<step title="The Poll method" src="doc/codewalk/urlpoll.go:/Poll executes/,/\n}/">
The Poll method (of the Resource type) performs an HTTP HEAD request
for the Resource's URL and returns the HTTP response's status code.
If an error occurs, Poll logs the message to standard error and returns the
error string instead.
</step>
<step title="main function" src="doc/codewalk/urlpoll.go:/func main/,/\n}/">
The main function starts the Poller and StateMonitor goroutines
and then loops passing completed Resources back to the pending
channel after appropriate delays.
</step>
<step title="Creating channels" src="doc/codewalk/urlpoll.go:/Create our/,/complete/">
First, main makes two channels of *Resource, pending and complete.
<br/><br/>
Inside main, a new goroutine sends one Resource per URL to pending
and the main goroutine receives completed Resources from complete.
<br/><br/>
The pending and complete channels are passed to each of the Poller
goroutines, within which they are known as in and out.
</step>
<step title="Initializing StateMonitor" src="doc/codewalk/urlpoll.go:/Launch the StateMonitor/,/statusInterval/">
StateMonitor will initialize and launch a goroutine that stores the state
of each Resource. We will look at this function in detail later.
<br/><br/>
For now, the important thing to note is that it returns a channel of State,
which is saved as status and passed to the Poller goroutines.
</step>
<step title="Launching Poller goroutines" src="doc/codewalk/urlpoll.go:/Launch some Poller/,/}/">
Now that it has the necessary channels, main launches a number of
Poller goroutines, passing the channels as arguments.
The channels provide the means of communication between the main, Poller, and
StateMonitor goroutines.
</step>
<step title="Send Resources to pending" src="doc/codewalk/urlpoll.go:/Send some Resources/,/}\(\)/">
To add the initial work to the system, main starts a new goroutine
that allocates and sends one Resource per URL to pending.
<br/><br/>
The new goroutine is necessary because unbuffered channel sends and
receives are synchronous. That means these channel sends will block until
the Pollers are ready to read from pending.
<br/><br/>
Were these sends performed in the main goroutine with fewer Pollers than
channel sends, the program would reach a deadlock situation, because
main would not yet be receiving from complete.
<br/><br/>
Exercise for the reader: modify this part of the program to read a list of
URLs from a file. (You may want to move this goroutine into its own
named function.)
</step>
<step title="Main Event Loop" src="doc/codewalk/urlpoll.go:/range complete/,/\n }/">
When a Poller is done with a Resource, it sends it on the complete channel.
This loop receives those Resource pointers from complete.
For each received Resource, it starts a new goroutine calling
the Resource's Sleep method. Using a new goroutine for each
ensures that the sleeps can happen in parallel.
<br/><br/>
Note that any single Resource pointer may only be sent on either pending or
complete at any one time. This ensures that a Resource is either being
handled by a Poller goroutine or sleeping, but never both simultaneously.
In this way, we share our Resource data by communicating.
</step>
<step title="The Sleep method" src="doc/codewalk/urlpoll.go:/Sleep/,/\n}/">
Sleep calls time.Sleep to pause before sending the Resource to done.
The pause will either be of a fixed length (pollInterval) plus an
additional delay proportional to the number of sequential errors (r.errCount).
<br/><br/>
This is an example of a typical Go idiom: a function intended to run inside
a goroutine takes a channel, upon which it sends its return value
(or other indication of completed state).
</step>
<step title="StateMonitor" src="doc/codewalk/urlpoll.go:/StateMonitor/,/\n}/">
The StateMonitor receives State values on a channel and periodically
outputs the state of all Resources being polled by the program.
</step>
<step title="The updates channel" src="doc/codewalk/urlpoll.go:/updates :=/">
The variable updates is a channel of State, on which the Poller goroutines
send State values.
<br/><br/>
This channel is returned by the function.
</step>
<step title="The urlStatus map" src="doc/codewalk/urlpoll.go:/urlStatus/">
The variable urlStatus is a map of URLs to their most recent status.
</step>
<step title="The Ticker object" src="doc/codewalk/urlpoll.go:/ticker/">
A time.Ticker is an object that repeatedly sends a value on a channel at a
specified interval.
<br/><br/>
In this case, ticker triggers the printing of the current state to
standard output every updateInterval nanoseconds.
</step>
<step title="The StateMonitor goroutine" src="doc/codewalk/urlpoll.go:/go func/,/}\(\)/">
StateMonitor will loop forever, selecting on two channels:
ticker.C and update. The select statement blocks until one of its
communications is ready to proceed.
<br/><br/>
When StateMonitor receives a tick from ticker.C, it calls logState to
print the current state. When it receives a State update from updates,
it records the new status in the urlStatus map.
<br/><br/>
Notice that this goroutine owns the urlStatus data structure,
ensuring that it can only be accessed sequentially.
This prevents memory corruption issues that might arise from parallel reads
and/or writes to a shared map.
</step>
<step title="Conclusion" src="doc/codewalk/urlpoll.go">
In this codewalk we have explored a simple example of using Go's concurrency
primitives to share memory through communication.
<br/><br/>
This should provide a starting point from which to explore the ways in which
goroutines and channels can be used to write expressive and concise concurrent
programs.
</step>
</codewalk>

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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"log"
"net/http"
"time"
)
const (
numPollers = 2 // number of Poller goroutines to launch
pollInterval = 60 * time.Second // how often to poll each URL
statusInterval = 10 * time.Second // how often to log status to stdout
errTimeout = 10 * time.Second // back-off timeout on error
)
var urls = []string{
"http://www.google.com/",
"http://golang.org/",
"http://blog.golang.org/",
}
// State represents the last-known state of a URL.
type State struct {
url string
status string
}
// StateMonitor maintains a map that stores the state of the URLs being
// polled, and prints the current state every updateInterval nanoseconds.
// It returns a chan State to which resource state should be sent.
func StateMonitor(updateInterval time.Duration) chan<- State {
updates := make(chan State)
urlStatus := make(map[string]string)
ticker := time.NewTicker(updateInterval)
go func() {
for {
select {
case <-ticker.C:
logState(urlStatus)
case s := <-updates:
urlStatus[s.url] = s.status
}
}
}()
return updates
}
// logState prints a state map.
func logState(s map[string]string) {
log.Println("Current state:")
for k, v := range s {
log.Printf(" %s %s", k, v)
}
}
// Resource represents an HTTP URL to be polled by this program.
type Resource struct {
url string
errCount int
}
// Poll executes an HTTP HEAD request for url
// and returns the HTTP status string or an error string.
func (r *Resource) Poll() string {
resp, err := http.Head(r.url)
if err != nil {
log.Println("Error", r.url, err)
r.errCount++
return err.Error()
}
r.errCount = 0
return resp.Status
}
// Sleep sleeps for an appropriate interval (dependent on error state)
// before sending the Resource to done.
func (r *Resource) Sleep(done chan<- *Resource) {
time.Sleep(pollInterval + errTimeout*time.Duration(r.errCount))
done <- r
}
func Poller(in <-chan *Resource, out chan<- *Resource, status chan<- State) {
for r := range in {
s := r.Poll()
status <- State{r.url, s}
out <- r
}
}
func main() {
// Create our input and output channels.
pending, complete := make(chan *Resource), make(chan *Resource)
// Launch the StateMonitor.
status := StateMonitor(statusInterval)
// Launch some Poller goroutines.
for i := 0; i < numPollers; i++ {
go Poller(pending, complete, status)
}
// Send some Resources to the pending queue.
go func() {
for _, url := range urls {
pending <- &Resource{url: url}
}
}()
for r := range complete {
go r.Sleep(pending)
}
}

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<!--{
"Title": "Debugging Go Code with GDB",
"Path": "/doc/gdb"
}-->
<!--
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
-->
<i>
<p>
The following instructions apply to the standard toolchain
(the <code>gc</code> Go compiler and tools).
Gccgo has native gdb support.
</p>
<p>
Note that
<a href="https://github.com/go-delve/delve">Delve</a> is a better
alternative to GDB when debugging Go programs built with the standard
toolchain. It understands the Go runtime, data structures, and
expressions better than GDB. Delve currently supports Linux, OSX,
and Windows on <code>amd64</code>.
For the most up-to-date list of supported platforms, please see
<a href="https://github.com/go-delve/delve/tree/master/Documentation/installation">
the Delve documentation</a>.
</p>
</i>
<p>
GDB does not understand Go programs well.
The stack management, threading, and runtime contain aspects that differ
enough from the execution model GDB expects that they can confuse
the debugger and cause incorrect results even when the program is
compiled with gccgo.
As a consequence, although GDB can be useful in some situations (e.g.,
debugging Cgo code, or debugging the runtime itself), it is not
a reliable debugger for Go programs, particularly heavily concurrent
ones. Moreover, it is not a priority for the Go project to address
these issues, which are difficult.
</p>
<p>
In short, the instructions below should be taken only as a guide to how
to use GDB when it works, not as a guarantee of success.
Besides this overview you might want to consult the
<a href="https://sourceware.org/gdb/current/onlinedocs/gdb/">GDB manual</a>.
</p>
<p>
</p>
<h2 id="Introduction">Introduction</h2>
<p>
When you compile and link your Go programs with the <code>gc</code> toolchain
on Linux, macOS, FreeBSD or NetBSD, the resulting binaries contain DWARFv4
debugging information that recent versions (&ge;7.5) of the GDB debugger can
use to inspect a live process or a core dump.
</p>
<p>
Pass the <code>'-w'</code> flag to the linker to omit the debug information
(for example, <code>go</code> <code>build</code> <code>-ldflags=-w</code> <code>prog.go</code>).
</p>
<p>
The code generated by the <code>gc</code> compiler includes inlining of
function invocations and registerization of variables. These optimizations
can sometimes make debugging with <code>gdb</code> harder.
If you find that you need to disable these optimizations,
build your program using <code>go</code> <code>build</code> <code>-gcflags=all="-N -l"</code>.
</p>
<p>
If you want to use gdb to inspect a core dump, you can trigger a dump
on a program crash, on systems that permit it, by setting
<code>GOTRACEBACK=crash</code> in the environment (see the
<a href="/pkg/runtime/#hdr-Environment_Variables"> runtime package
documentation</a> for more info).
</p>
<h3 id="Common_Operations">Common Operations</h3>
<ul>
<li>
Show file and line number for code, set breakpoints and disassemble:
<pre>(gdb) <b>list</b>
(gdb) <b>list <i>line</i></b>
(gdb) <b>list <i>file.go</i>:<i>line</i></b>
(gdb) <b>break <i>line</i></b>
(gdb) <b>break <i>file.go</i>:<i>line</i></b>
(gdb) <b>disas</b></pre>
</li>
<li>
Show backtraces and unwind stack frames:
<pre>(gdb) <b>bt</b>
(gdb) <b>frame <i>n</i></b></pre>
</li>
<li>
Show the name, type and location on the stack frame of local variables,
arguments and return values:
<pre>(gdb) <b>info locals</b>
(gdb) <b>info args</b>
(gdb) <b>p variable</b>
(gdb) <b>whatis variable</b></pre>
</li>
<li>
Show the name, type and location of global variables:
<pre>(gdb) <b>info variables <i>regexp</i></b></pre>
</li>
</ul>
<h3 id="Go_Extensions">Go Extensions</h3>
<p>
A recent extension mechanism to GDB allows it to load extension scripts for a
given binary. The toolchain uses this to extend GDB with a handful of
commands to inspect internals of the runtime code (such as goroutines) and to
pretty print the built-in map, slice and channel types.
</p>
<ul>
<li>
Pretty printing a string, slice, map, channel or interface:
<pre>(gdb) <b>p <i>var</i></b></pre>
</li>
<li>
A $len() and $cap() function for strings, slices and maps:
<pre>(gdb) <b>p $len(<i>var</i>)</b></pre>
</li>
<li>
A function to cast interfaces to their dynamic types:
<pre>(gdb) <b>p $dtype(<i>var</i>)</b>
(gdb) <b>iface <i>var</i></b></pre>
<p class="detail"><b>Known issue:</b> GDB can’t automatically find the dynamic
type of an interface value if its long name differs from its short name
(annoying when printing stacktraces, the pretty printer falls back to printing
the short type name and a pointer).</p>
</li>
<li>
Inspecting goroutines:
<pre>(gdb) <b>info goroutines</b>
(gdb) <b>goroutine <i>n</i> <i>cmd</i></b>
(gdb) <b>help goroutine</b></pre>
For example:
<pre>(gdb) <b>goroutine 12 bt</b></pre>
You can inspect all goroutines by passing <code>all</code> instead of a specific goroutine's ID.
For example:
<pre>(gdb) <b>goroutine all bt</b></pre>
</li>
</ul>
<p>
If you'd like to see how this works, or want to extend it, take a look at <a
href="/src/runtime/runtime-gdb.py">src/runtime/runtime-gdb.py</a> in
the Go source distribution. It depends on some special magic types
(<code>hash&lt;T,U&gt;</code>) and variables (<code>runtime.m</code> and
<code>runtime.g</code>) that the linker
(<a href="/src/cmd/link/internal/ld/dwarf.go">src/cmd/link/internal/ld/dwarf.go</a>) ensures are described in
the DWARF code.
</p>
<p>
If you're interested in what the debugging information looks like, run
<code>objdump</code> <code>-W</code> <code>a.out</code> and browse through the <code>.debug_*</code>
sections.
</p>
<h3 id="Known_Issues">Known Issues</h3>
<ol>
<li>String pretty printing only triggers for type string, not for types derived
from it.</li>
<li>Type information is missing for the C parts of the runtime library.</li>
<li>GDB does not understand Go’s name qualifications and treats
<code>"fmt.Print"</code> as an unstructured literal with a <code>"."</code>
that needs to be quoted. It objects even more strongly to method names of
the form <code>pkg.(*MyType).Meth</code>.
<li>As of Go 1.11, debug information is compressed by default.
Older versions of gdb, such as the one available by default on MacOS,
do not understand the compression.
You can generate uncompressed debug information by using <code>go
build -ldflags=-compressdwarf=false</code>.
(For convenience you can put the <code>-ldflags</code> option in
the <a href="/cmd/go/#hdr-Environment_variables"><code>GOFLAGS</code>
environment variable</a> so that you don't have to specify it each time.)
</li>
</ol>
<h2 id="Tutorial">Tutorial</h2>
<p>
In this tutorial we will inspect the binary of the
<a href="/pkg/regexp/">regexp</a> package's unit tests. To build the binary,
change to <code>$GOROOT/src/regexp</code> and run <code>go</code> <code>test</code> <code>-c</code>.
This should produce an executable file named <code>regexp.test</code>.
</p>
<h3 id="Getting_Started">Getting Started</h3>
<p>
Launch GDB, debugging <code>regexp.test</code>:
</p>
<pre>
$ <b>gdb regexp.test</b>
GNU gdb (GDB) 7.2-gg8
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv 3+: GNU GPL version 3 or later &lt;http://gnu.org/licenses/gpl.html&gt;
Type "show copying" and "show warranty" for licensing/warranty details.
This GDB was configured as "x86_64-linux".
Reading symbols from /home/user/go/src/regexp/regexp.test...
done.
Loading Go Runtime support.
(gdb)
</pre>
<p>
The message "Loading Go Runtime support" means that GDB loaded the
extension from <code>$GOROOT/src/runtime/runtime-gdb.py</code>.
</p>
<p>
To help GDB find the Go runtime sources and the accompanying support script,
pass your <code>$GOROOT</code> with the <code>'-d'</code> flag:
</p>
<pre>
$ <b>gdb regexp.test -d $GOROOT</b>
</pre>
<p>
If for some reason GDB still can't find that directory or that script, you can load
it by hand by telling gdb (assuming you have the go sources in
<code>~/go/</code>):
</p>
<pre>
(gdb) <b>source ~/go/src/runtime/runtime-gdb.py</b>
Loading Go Runtime support.
</pre>
<h3 id="Inspecting_the_source">Inspecting the source</h3>
<p>
Use the <code>"l"</code> or <code>"list"</code> command to inspect source code.
</p>
<pre>
(gdb) <b>l</b>
</pre>
<p>
List a specific part of the source parameterizing <code>"list"</code> with a
function name (it must be qualified with its package name).
</p>
<pre>
(gdb) <b>l main.main</b>
</pre>
<p>
List a specific file and line number:
</p>
<pre>
(gdb) <b>l regexp.go:1</b>
(gdb) <i># Hit enter to repeat last command. Here, this lists next 10 lines.</i>
</pre>
<h3 id="Naming">Naming</h3>
<p>
Variable and function names must be qualified with the name of the packages
they belong to. The <code>Compile</code> function from the <code>regexp</code>
package is known to GDB as <code>'regexp.Compile'</code>.
</p>
<p>
Methods must be qualified with the name of their receiver types. For example,
the <code>*Regexp</code> type’s <code>String</code> method is known as
<code>'regexp.(*Regexp).String'</code>.
</p>
<p>
Variables that shadow other variables are magically suffixed with a number in the debug info.
Variables referenced by closures will appear as pointers magically prefixed with '&amp;'.
</p>
<h3 id="Setting_breakpoints">Setting breakpoints</h3>
<p>
Set a breakpoint at the <code>TestFind</code> function:
</p>
<pre>
(gdb) <b>b 'regexp.TestFind'</b>
Breakpoint 1 at 0x424908: file /home/user/go/src/regexp/find_test.go, line 148.
</pre>
<p>
Run the program:
</p>
<pre>
(gdb) <b>run</b>
Starting program: /home/user/go/src/regexp/regexp.test
Breakpoint 1, regexp.TestFind (t=0xf8404a89c0) at /home/user/go/src/regexp/find_test.go:148
148 func TestFind(t *testing.T) {
</pre>
<p>
Execution has paused at the breakpoint.
See which goroutines are running, and what they're doing:
</p>
<pre>
(gdb) <b>info goroutines</b>
1 waiting runtime.gosched
* 13 running runtime.goexit
</pre>
<p>
the one marked with the <code>*</code> is the current goroutine.
</p>
<h3 id="Inspecting_the_stack">Inspecting the stack</h3>
<p>
Look at the stack trace for where we’ve paused the program:
</p>
<pre>
(gdb) <b>bt</b> <i># backtrace</i>
#0 regexp.TestFind (t=0xf8404a89c0) at /home/user/go/src/regexp/find_test.go:148
#1 0x000000000042f60b in testing.tRunner (t=0xf8404a89c0, test=0x573720) at /home/user/go/src/testing/testing.go:156
#2 0x000000000040df64 in runtime.initdone () at /home/user/go/src/runtime/proc.c:242
#3 0x000000f8404a89c0 in ?? ()
#4 0x0000000000573720 in ?? ()
#5 0x0000000000000000 in ?? ()
</pre>
<p>
The other goroutine, number 1, is stuck in <code>runtime.gosched</code>, blocked on a channel receive:
</p>
<pre>
(gdb) <b>goroutine 1 bt</b>
#0 0x000000000040facb in runtime.gosched () at /home/user/go/src/runtime/proc.c:873
#1 0x00000000004031c9 in runtime.chanrecv (c=void, ep=void, selected=void, received=void)
at /home/user/go/src/runtime/chan.c:342
#2 0x0000000000403299 in runtime.chanrecv1 (t=void, c=void) at/home/user/go/src/runtime/chan.c:423
#3 0x000000000043075b in testing.RunTests (matchString={void (struct string, struct string, bool *, error *)}
0x7ffff7f9ef60, tests= []testing.InternalTest = {...}) at /home/user/go/src/testing/testing.go:201
#4 0x00000000004302b1 in testing.Main (matchString={void (struct string, struct string, bool *, error *)}
0x7ffff7f9ef80, tests= []testing.InternalTest = {...}, benchmarks= []testing.InternalBenchmark = {...})
at /home/user/go/src/testing/testing.go:168
#5 0x0000000000400dc1 in main.main () at /home/user/go/src/regexp/_testmain.go:98
#6 0x00000000004022e7 in runtime.mainstart () at /home/user/go/src/runtime/amd64/asm.s:78
#7 0x000000000040ea6f in runtime.initdone () at /home/user/go/src/runtime/proc.c:243
#8 0x0000000000000000 in ?? ()
</pre>
<p>
The stack frame shows we’re currently executing the <code>regexp.TestFind</code> function, as expected.
</p>
<pre>
(gdb) <b>info frame</b>
Stack level 0, frame at 0x7ffff7f9ff88:
rip = 0x425530 in regexp.TestFind (/home/user/go/src/regexp/find_test.go:148);
saved rip 0x430233
called by frame at 0x7ffff7f9ffa8
source language minimal.
Arglist at 0x7ffff7f9ff78, args: t=0xf840688b60
Locals at 0x7ffff7f9ff78, Previous frame's sp is 0x7ffff7f9ff88
Saved registers:
rip at 0x7ffff7f9ff80
</pre>
<p>
The command <code>info</code> <code>locals</code> lists all variables local to the function and their values, but is a bit
dangerous to use, since it will also try to print uninitialized variables. Uninitialized slices may cause gdb to try
to print arbitrary large arrays.
</p>
<p>
The function’s arguments:
</p>
<pre>
(gdb) <b>info args</b>
t = 0xf840688b60
</pre>
<p>
When printing the argument, notice that it’s a pointer to a
<code>Regexp</code> value. Note that GDB has incorrectly put the <code>*</code>
on the right-hand side of the type name and made up a 'struct' keyword, in traditional C style.
</p>
<pre>
(gdb) <b>p re</b>
(gdb) p t
$1 = (struct testing.T *) 0xf840688b60
(gdb) p t
$1 = (struct testing.T *) 0xf840688b60
(gdb) p *t
$2 = {errors = "", failed = false, ch = 0xf8406f5690}
(gdb) p *t-&gt;ch
$3 = struct hchan&lt;*testing.T&gt;
</pre>
<p>
That <code>struct</code> <code>hchan&lt;*testing.T&gt;</code> is the
runtime-internal representation of a channel. It is currently empty,
or gdb would have pretty-printed its contents.
</p>
<p>
Stepping forward:
</p>
<pre>
(gdb) <b>n</b> <i># execute next line</i>
149 for _, test := range findTests {
(gdb) <i># enter is repeat</i>
150 re := MustCompile(test.pat)
(gdb) <b>p test.pat</b>
$4 = ""
(gdb) <b>p re</b>
$5 = (struct regexp.Regexp *) 0xf84068d070
(gdb) <b>p *re</b>
$6 = {expr = "", prog = 0xf840688b80, prefix = "", prefixBytes = []uint8, prefixComplete = true,
prefixRune = 0, cond = 0 '\000', numSubexp = 0, longest = false, mu = {state = 0, sema = 0},
machine = []*regexp.machine}
(gdb) <b>p *re->prog</b>
$7 = {Inst = []regexp/syntax.Inst = {{Op = 5 '\005', Out = 0, Arg = 0, Rune = []int}, {Op =
6 '\006', Out = 2, Arg = 0, Rune = []int}, {Op = 4 '\004', Out = 0, Arg = 0, Rune = []int}},
Start = 1, NumCap = 2}
</pre>
<p>
We can step into the <code>String</code>function call with <code>"s"</code>:
</p>
<pre>
(gdb) <b>s</b>
regexp.(*Regexp).String (re=0xf84068d070, noname=void) at /home/user/go/src/regexp/regexp.go:97
97 func (re *Regexp) String() string {
</pre>
<p>
Get a stack trace to see where we are:
</p>
<pre>
(gdb) <b>bt</b>
#0 regexp.(*Regexp).String (re=0xf84068d070, noname=void)
at /home/user/go/src/regexp/regexp.go:97
#1 0x0000000000425615 in regexp.TestFind (t=0xf840688b60)
at /home/user/go/src/regexp/find_test.go:151
#2 0x0000000000430233 in testing.tRunner (t=0xf840688b60, test=0x5747b8)
at /home/user/go/src/testing/testing.go:156
#3 0x000000000040ea6f in runtime.initdone () at /home/user/go/src/runtime/proc.c:243
....
</pre>
<p>
Look at the source code:
</p>
<pre>
(gdb) <b>l</b>
92 mu sync.Mutex
93 machine []*machine
94 }
95
96 // String returns the source text used to compile the regular expression.
97 func (re *Regexp) String() string {
98 return re.expr
99 }
100
101 // Compile parses a regular expression and returns, if successful,
</pre>
<h3 id="Pretty_Printing">Pretty Printing</h3>
<p>
GDB's pretty printing mechanism is triggered by regexp matches on type names. An example for slices:
</p>
<pre>
(gdb) <b>p utf</b>
$22 = []uint8 = {0 '\000', 0 '\000', 0 '\000', 0 '\000'}
</pre>
<p>
Since slices, arrays and strings are not C pointers, GDB can't interpret the subscripting operation for you, but
you can look inside the runtime representation to do that (tab completion helps here):
</p>
<pre>
(gdb) <b>p slc</b>
$11 = []int = {0, 0}
(gdb) <b>p slc-&gt;</b><i>&lt;TAB&gt;</i>
array slc len
(gdb) <b>p slc->array</b>
$12 = (int *) 0xf84057af00
(gdb) <b>p slc->array[1]</b>
$13 = 0</pre>
<p>
The extension functions $len and $cap work on strings, arrays and slices:
</p>
<pre>
(gdb) <b>p $len(utf)</b>
$23 = 4
(gdb) <b>p $cap(utf)</b>
$24 = 4
</pre>
<p>
Channels and maps are 'reference' types, which gdb shows as pointers to C++-like types <code>hash&lt;int,string&gt;*</code>. Dereferencing will trigger prettyprinting
</p>
<p>
Interfaces are represented in the runtime as a pointer to a type descriptor and a pointer to a value. The Go GDB runtime extension decodes this and automatically triggers pretty printing for the runtime type. The extension function <code>$dtype</code> decodes the dynamic type for you (examples are taken from a breakpoint at <code>regexp.go</code> line 293.)
</p>
<pre>
(gdb) <b>p i</b>
$4 = {str = "cbb"}
(gdb) <b>whatis i</b>
type = regexp.input
(gdb) <b>p $dtype(i)</b>
$26 = (struct regexp.inputBytes *) 0xf8400b4930
(gdb) <b>iface i</b>
regexp.input: struct regexp.inputBytes *
</pre>

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<!--{
"Title": "Diagnostics",
"Template": true
}-->
<!--
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
-->
<h2 id="introduction">Introduction</h2>
<p>
The Go ecosystem provides a large suite of APIs and tools to
diagnose logic and performance problems in Go programs. This page
summarizes the available tools and helps Go users pick the right one
for their specific problem.
</p>
<p>
Diagnostics solutions can be categorized into the following groups:
</p>
<ul>
<li><strong>Profiling</strong>: Profiling tools analyze the complexity and costs of a
Go program such as its memory usage and frequently called
functions to identify the expensive sections of a Go program.</li>
<li><strong>Tracing</strong>: Tracing is a way to instrument code to analyze latency
throughout the lifecycle of a call or user request. Traces provide an
overview of how much latency each component contributes to the overall
latency in a system. Traces can span multiple Go processes.</li>
<li><strong>Debugging</strong>: Debugging allows us to pause a Go program and examine
its execution. Program state and flow can be verified with debugging.</li>
<li><strong>Runtime statistics and events</strong>: Collection and analysis of runtime stats and events
provides a high-level overview of the health of Go programs. Spikes/dips of metrics
helps us to identify changes in throughput, utilization, and performance.</li>
</ul>
<p>
Note: Some diagnostics tools may interfere with each other. For example, precise
memory profiling skews CPU profiles and goroutine blocking profiling affects scheduler
trace. Use tools in isolation to get more precise info.
</p>
<h2 id="profiling">Profiling</h2>
<p>
Profiling is useful for identifying expensive or frequently called sections
of code. The Go runtime provides <a href="https://golang.org/pkg/runtime/pprof/">
profiling data</a> in the format expected by the
<a href="https://github.com/google/pprof/blob/master/doc/README.md">pprof visualization tool</a>.
The profiling data can be collected during testing
via <code>go</code> <code>test</code> or endpoints made available from the <a href="/pkg/net/http/pprof/">
net/http/pprof</a> package. Users need to collect the profiling data and use pprof tools to filter
and visualize the top code paths.
</p>
<p>Predefined profiles provided by the <a href="/pkg/runtime/pprof">runtime/pprof</a> package:</p>
<ul>
<li>
<strong>cpu</strong>: CPU profile determines where a program spends
its time while actively consuming CPU cycles (as opposed to while sleeping or waiting for I/O).
</li>
<li>
<strong>heap</strong>: Heap profile reports memory allocation samples;
used to monitor current and historical memory usage, and to check for memory leaks.
</li>
<li>
<strong>threadcreate</strong>: Thread creation profile reports the sections
of the program that lead the creation of new OS threads.
</li>
<li>
<strong>goroutine</strong>: Goroutine profile reports the stack traces of all current goroutines.
</li>
<li>
<strong>block</strong>: Block profile shows where goroutines block waiting on synchronization
primitives (including timer channels). Block profile is not enabled by default;
use <code>runtime.SetBlockProfileRate</code> to enable it.
</li>
<li>
<strong>mutex</strong>: Mutex profile reports the lock contentions. When you think your
CPU is not fully utilized due to a mutex contention, use this profile. Mutex profile
is not enabled by default, see <code>runtime.SetMutexProfileFraction</code> to enable it.
</li>
</ul>
<p><strong>What other profilers can I use to profile Go programs?</strong></p>
<p>
On Linux, <a href="https://perf.wiki.kernel.org/index.php/Tutorial">perf tools</a>
can be used for profiling Go programs. Perf can profile
and unwind cgo/SWIG code and kernel, so it can be useful to get insights into
native/kernel performance bottlenecks. On macOS,
<a href="https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/InstrumentsUserGuide/">Instruments</a>
suite can be used profile Go programs.
</p>
<p><strong>Can I profile my production services?</strong></p>
<p>Yes. It is safe to profile programs in production, but enabling
some profiles (e.g. the CPU profile) adds cost. You should expect to
see performance downgrade. The performance penalty can be estimated
by measuring the overhead of the profiler before turning it on in
production.
</p>
<p>
You may want to periodically profile your production services.
Especially in a system with many replicas of a single process, selecting
a random replica periodically is a safe option.
Select a production process, profile it for
X seconds for every Y seconds and save the results for visualization and
analysis; then repeat periodically. Results may be manually and/or automatically
reviewed to find problems.
Collection of profiles can interfere with each other,
so it is recommended to collect only a single profile at a time.
</p>
<p>
<strong>What are the best ways to visualize the profiling data?</strong>
</p>
<p>
The Go tools provide text, graph, and <a href="http://valgrind.org/docs/manual/cl-manual.html">callgrind</a>
visualization of the profile data using
<code><a href="https://github.com/google/pprof/blob/master/doc/README.md">go tool pprof</a></code>.
Read <a href="https://blog.golang.org/profiling-go-programs">Profiling Go programs</a>
to see them in action.
</p>
<p>
<img width="800" src="https://storage.googleapis.com/golangorg-assets/pprof-text.png">
<br>
<small>Listing of the most expensive calls as text.</small>
</p>
<p>
<img width="800" src="https://storage.googleapis.com/golangorg-assets/pprof-dot.png">
<br>
<small>Visualization of the most expensive calls as a graph.</small>
</p>
<p>Weblist view displays the expensive parts of the source line by line in
an HTML page. In the following example, 530ms is spent in the
<code>runtime.concatstrings</code> and cost of each line is presented
in the listing.</p>
<p>
<img width="800" src="https://storage.googleapis.com/golangorg-assets/pprof-weblist.png">
<br>
<small>Visualization of the most expensive calls as weblist.</small>
</p>
<p>
Another way to visualize profile data is a <a href="http://www.brendangregg.com/flamegraphs.html">flame graph</a>.
Flame graphs allow you to move in a specific ancestry path, so you can zoom
in/out of specific sections of code.
The <a href="https://github.com/google/pprof">upstream pprof</a>
has support for flame graphs.
</p>
<p>
<img width="800" src="https://storage.googleapis.com/golangorg-assets/flame.png">
<br>
<small>Flame graphs offers visualization to spot the most expensive code-paths.</small>
</p>
<p><strong>Am I restricted to the built-in profiles?</strong></p>
<p>
Additionally to what is provided by the runtime, Go users can create
their custom profiles via <a href="/pkg/runtime/pprof/#Profile">pprof.Profile</a>
and use the existing tools to examine them.
</p>
<p><strong>Can I serve the profiler handlers (/debug/pprof/...) on a different path and port?</strong></p>
<p>
Yes. The <code>net/http/pprof</code> package registers its handlers to the default
mux by default, but you can also register them yourself by using the handlers
exported from the package.
</p>
<p>
For example, the following example will serve the pprof.Profile
handler on :7777 at /custom_debug_path/profile:
</p>
<p>
<pre>
package main
import (
"log"
"net/http"
"net/http/pprof"
)
func main() {
mux := http.NewServeMux()
mux.HandleFunc("/custom_debug_path/profile", pprof.Profile)
log.Fatal(http.ListenAndServe(":7777", mux))
}
</pre>
</p>
<h2 id="tracing">Tracing</h2>
<p>
Tracing is a way to instrument code to analyze latency throughout the
lifecycle of a chain of calls. Go provides
<a href="https://godoc.org/golang.org/x/net/trace">golang.org/x/net/trace</a>
package as a minimal tracing backend per Go node and provides a minimal
instrumentation library with a simple dashboard. Go also provides
an execution tracer to trace the runtime events within an interval.
</p>
<p>Tracing enables us to:</p>
<ul>
<li>Instrument and analyze application latency in a Go process.</li>
<li>Measure the cost of specific calls in a long chain of calls.</li>
<li>Figure out the utilization and performance improvements.
Bottlenecks are not always obvious without tracing data.</li>
</ul>
<p>
In monolithic systems, it's relatively easy to collect diagnostic data
from the building blocks of a program. All modules live within one
process and share common resources to report logs, errors, and other
diagnostic information. Once your system grows beyond a single process and
starts to become distributed, it becomes harder to follow a call starting
from the front-end web server to all of its back-ends until a response is
returned back to the user. This is where distributed tracing plays a big
role to instrument and analyze your production systems.
</p>
<p>
Distributed tracing is a way to instrument code to analyze latency throughout
the lifecycle of a user request. When a system is distributed and when
conventional profiling and debugging tools don’t scale, you might want
to use distributed tracing tools to analyze the performance of your user
requests and RPCs.
</p>
<p>Distributed tracing enables us to:</p>
<ul>
<li>Instrument and profile application latency in a large system.</li>
<li>Track all RPCs within the lifecycle of a user request and see integration issues
that are only visible in production.</li>
<li>Figure out performance improvements that can be applied to our systems.
Many bottlenecks are not obvious before the collection of tracing data.</li>
</ul>
<p>The Go ecosystem provides various distributed tracing libraries per tracing system
and backend-agnostic ones.</p>
<p><strong>Is there a way to automatically intercept each function call and create traces?</strong></p>
<p>
Go doesn’t provide a way to automatically intercept every function call and create
trace spans. You need to manually instrument your code to create, end, and annotate spans.
</p>
<p><strong>How should I propagate trace headers in Go libraries?</strong></p>
<p>
You can propagate trace identifiers and tags in the
<a href="/pkg/context#Context"><code>context.Context</code></a>.
There is no canonical trace key or common representation of trace headers
in the industry yet. Each tracing provider is responsible for providing propagation
utilities in their Go libraries.
</p>
<p>
<strong>What other low-level events from the standard library or
runtime can be included in a trace?</strong>
</p>
<p>
The standard library and runtime are trying to expose several additional APIs
to notify on low level internal events. For example,
<a href="/pkg/net/http/httptrace#ClientTrace"><code>httptrace.ClientTrace</code></a>
provides APIs to follow low-level events in the life cycle of an outgoing request.
There is an ongoing effort to retrieve low-level runtime events from
the runtime execution tracer and allow users to define and record their user events.
</p>
<h2 id="debugging">Debugging</h2>
<p>
Debugging is the process of identifying why a program misbehaves.
Debuggers allow us to understand a program’s execution flow and current state.
There are several styles of debugging; this section will only focus on attaching
a debugger to a program and core dump debugging.
</p>
<p>Go users mostly use the following debuggers:</p>
<ul>
<li>
<a href="https://github.com/derekparker/delve">Delve</a>:
Delve is a debugger for the Go programming language. It has
support for Go’s runtime concepts and built-in types. Delve is
trying to be a fully featured reliable debugger for Go programs.
</li>
<li>
<a href="https://golang.org/doc/gdb">GDB</a>:
Go provides GDB support via the standard Go compiler and Gccgo.
The stack management, threading, and runtime contain aspects that differ
enough from the execution model GDB expects that they can confuse the
debugger, even when the program is compiled with gccgo. Even though
GDB can be used to debug Go programs, it is not ideal and may
create confusion.
</li>
</ul>
<p><strong>How well do debuggers work with Go programs?</strong></p>
<p>
The <code>gc</code> compiler performs optimizations such as
function inlining and variable registerization. These optimizations
sometimes make debugging with debuggers harder. There is an ongoing
effort to improve the quality of the DWARF information generated for
optimized binaries. Until those improvements are available, we recommend
disabling optimizations when building the code being debugged. The following
command builds a package with no compiler optimizations:
<p>
<pre>
$ go build -gcflags=all="-N -l"
</pre>
</p>
As part of the improvement effort, Go 1.10 introduced a new compiler
flag <code>-dwarflocationlists</code>. The flag causes the compiler to
add location lists that helps debuggers work with optimized binaries.
The following command builds a package with optimizations but with
the DWARF location lists:
<p>
<pre>
$ go build -gcflags="-dwarflocationlists=true"
</pre>
</p>
<p><strong>What’s the recommended debugger user interface?</strong></p>
<p>
Even though both delve and gdb provides CLIs, most editor integrations
and IDEs provides debugging-specific user interfaces.
</p>
<p><strong>Is it possible to do postmortem debugging with Go programs?</strong></p>
<p>
A core dump file is a file that contains the memory dump of a running
process and its process status. It is primarily used for post-mortem
debugging of a program and to understand its state
while it is still running. These two cases make debugging of core
dumps a good diagnostic aid to postmortem and analyze production
services. It is possible to obtain core files from Go programs and
use delve or gdb to debug, see the
<a href="https://golang.org/wiki/CoreDumpDebugging">core dump debugging</a>
page for a step-by-step guide.
</p>
<h2 id="runtime">Runtime statistics and events</h2>
<p>
The runtime provides stats and reporting of internal events for
users to diagnose performance and utilization problems at the
runtime level.
</p>
<p>
Users can monitor these stats to better understand the overall
health and performance of Go programs.
Some frequently monitored stats and states:
</p>
<ul>
<li><code><a href="/pkg/runtime/#ReadMemStats">runtime.ReadMemStats</a></code>
reports the metrics related to heap
allocation and garbage collection. Memory stats are useful for
monitoring how much memory resources a process is consuming,
whether the process can utilize memory well, and to catch
memory leaks.</li>
<li><code><a href="/pkg/runtime/debug/#ReadGCStats">debug.ReadGCStats</a></code>
reads statistics about garbage collection.
It is useful to see how much of the resources are spent on GC pauses.
It also reports a timeline of garbage collector pauses and pause time percentiles.</li>
<li><code><a href="/pkg/runtime/debug/#Stack">debug.Stack</a></code>
returns the current stack trace. Stack trace
is useful to see how many goroutines are currently running,
what they are doing, and whether they are blocked or not.</li>
<li><code><a href="/pkg/runtime/debug/#WriteHeapDump">debug.WriteHeapDump</a></code>
suspends the execution of all goroutines
and allows you to dump the heap to a file. A heap dump is a
snapshot of a Go process' memory at a given time. It contains all
allocated objects as well as goroutines, finalizers, and more.</li>
<li><code><a href="/pkg/runtime#NumGoroutine">runtime.NumGoroutine</a></code>
returns the number of current goroutines.
The value can be monitored to see whether enough goroutines are
utilized, or to detect goroutine leaks.</li>
</ul>
<h3 id="execution-tracer">Execution tracer</h3>
<p>Go comes with a runtime execution tracer to capture a wide range
of runtime events. Scheduling, syscall, garbage collections,
heap size, and other events are collected by runtime and available
for visualization by the go tool trace. Execution tracer is a tool
to detect latency and utilization problems. You can examine how well
the CPU is utilized, and when networking or syscalls are a cause of
preemption for the goroutines.</p>
<p>Tracer is useful to:</p>
<ul>
<li>Understand how your goroutines execute.</li>
<li>Understand some of the core runtime events such as GC runs.</li>
<li>Identify poorly parallelized execution.</li>
</ul>
<p>However, it is not great for identifying hot spots such as
analyzing the cause of excessive memory or CPU usage.
Use profiling tools instead first to address them.</p>
<p>
<img width="800" src="https://storage.googleapis.com/golangorg-assets/tracer-lock.png">
</p>
<p>Above, the go tool trace visualization shows the execution started
fine, and then it became serialized. It suggests that there might
be lock contention for a shared resource that creates a bottleneck.</p>
<p>See <a href="https://golang.org/cmd/trace/"><code>go</code> <code>tool</code> <code>trace</code></a>
to collect and analyze runtime traces.
</p>
<h3 id="godebug">GODEBUG</h3>
<p>Runtime also emits events and information if
<a href="https://golang.org/pkg/runtime/#hdr-Environment_Variables">GODEBUG</a>
environmental variable is set accordingly.</p>
<ul>
<li>GODEBUG=gctrace=1 prints garbage collector events at
each collection, summarizing the amount of memory collected
and the length of the pause.</li>
<li>GODEBUG=inittrace=1 prints a summary of execution time and memory allocation
information for completed package initialization work.</li>
<li>GODEBUG=schedtrace=X prints scheduling events every X milliseconds.</li>
</ul>
<p>The GODEBUG environmental variable can be used to disable use of
instruction set extensions in the standard library and runtime.</p>
<ul>
<li>GODEBUG=cpu.all=off disables the use of all optional
instruction set extensions.</li>
<li>GODEBUG=cpu.<em>extension</em>=off disables use of instructions from the
specified instruction set extension.<br>
<em>extension</em> is the lower case name for the instruction set extension
such as <em>sse41</em> or <em>avx</em>.</li>
</ul>

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<!--{
"Title": "Editor plugins and IDEs",
"Template": true
}-->
<h2 id="introduction">Introduction</h2>
<p>
This document lists commonly used editor plugins and IDEs from the Go ecosystem
that make Go development more productive and seamless.
A comprehensive list of editor support and IDEs for Go development is available at
<a href="https://golang.org/wiki/IDEsAndTextEditorPlugins">the wiki</a>.
</p>
<h2 id="options">Options</h2>
<p>
The Go ecosystem provides a variety of editor plugins and IDEs to enhance your day-to-day
editing, navigation, testing, and debugging experience.
</p>
<ul>
<li><a href="https://marketplace.visualstudio.com/items?itemName=golang.go">Visual Studio Code</a>:
Go extension provides support for the Go programming language</li>
<li><a href="https://www.jetbrains.com/go">GoLand</a>: GoLand is distributed either as a standalone IDE
or as a plugin for IntelliJ IDEA Ultimate</li>
<li><a href="https://github.com/fatih/vim-go">vim</a>: vim-go plugin provides Go programming language support</li>
<p>
Note that these are only a few top solutions; a more comprehensive
community-maintained list of
<a href="https://github.com/golang/go/wiki/IDEsAndTextEditorPlugins">IDEs and text editor plugins</a>
is available at the Wiki.
</p>

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<!--{
"Title": "Contributing to the gccgo frontend"
}-->
<h2>Introduction</h2>
<p>
These are some notes on contributing to the gccgo frontend for GCC.
For information on contributing to parts of Go other than gccgo,
see <a href="/doc/contribute.html">Contributing to the Go project</a>. For
information on building gccgo for yourself,
see <a href="/doc/gccgo_install.html">Setting up and using gccgo</a>.
For more of the gritty details on the process of doing development
with the gccgo frontend,
see <a href="https://go.googlesource.com/gofrontend/+/master/HACKING">the
file HACKING</a> in the gofrontend repository.
</p>
<h2>Legal Prerequisites</h2>
<p>
You must follow the <a href="/doc/contribute.html#copyright">Go copyright
rules</a> for all changes to the gccgo frontend and the associated
libgo library. Code that is part of GCC rather than gccgo must follow
the general <a href="https://gcc.gnu.org/contribute.html">GCC
contribution rules</a>.
</p>
<h2>Code</h2>
<p>
The master sources for the gccgo frontend may be found at
<a href="https://go.googlesource.com/gofrontend">https://go.googlesource.com/gofrontend</a>.
They are mirrored
at <a href="https://github.com/golang/gofrontend">https://github.com/golang/gofrontend</a>.
The master sources are not buildable by themselves, but only in
conjunction with GCC (in the future, other compilers may be
supported). Changes made to the gccgo frontend are also applied to
the GCC source code repository hosted at <code>gcc.gnu.org</code>. In
the <code>gofrontend</code> repository, the <code>go</code> directory
is mirrored to the <code>gcc/go/gofrontend</code> directory in the GCC
repository, and the <code>gofrontend</code> <code>libgo</code>
directory is mirrored to the GCC <code>libgo</code> directory. In
addition, the <code>test</code> directory
from <a href="//go.googlesource.com/go">the main Go repository</a>
is mirrored to the <code>gcc/testsuite/go.test/test</code> directory
in the GCC repository.
</p>
<p>
Changes to these directories always flow from the master sources to
the GCC repository. The files should never be changed in the GCC
repository except by changing them in the master sources and mirroring
them.
</p>
<p>
The gccgo frontend is written in C++.
It follows the GNU and GCC coding standards for C++.
In writing code for the frontend, follow the formatting of the
surrounding code.
Almost all GCC-specific code is not in the frontend proper and is
instead in the GCC sources in the <code>gcc/go</code> directory.
</p>
<p>
The run-time library for gccgo is mostly the same as the library
in <a href="//go.googlesource.com/go">the main Go repository</a>.
The library code in the Go repository is periodically merged into
the <code>libgo/go</code> directory of the <code>gofrontend</code> and
then the GCC repositories, using the shell
script <code>libgo/merge.sh</code>. Accordingly, most library changes
should be made in the main Go repository. The files outside
of <code>libgo/go</code> are gccgo-specific; that said, some of the
files in <code>libgo/runtime</code> are based on files
in <code>src/runtime</code> in the main Go repository.
</p>
<h2>Testing</h2>
<p>
All patches must be tested. A patch that introduces new failures is
not acceptable.
</p>
<p>
To run the gccgo test suite, run <code>make check-go</code> in your
build directory. This will run various tests
under <code>gcc/testsuite/go.*</code> and will also run
the <code>libgo</code> testsuite. This copy of the tests from the
main Go repository is run using the DejaGNU script found
in <code>gcc/testsuite/go.test/go-test.exp</code>.
</p>
<p>
Most new tests should be submitted to the main Go repository for later
mirroring into the GCC repository. If there is a need for specific
tests for gccgo, they should go in
the <code>gcc/testsuite/go.go-torture</code>
or <code>gcc/testsuite/go.dg</code> directories in the GCC repository.
</p>
<h2>Submitting Changes</h2>
<p>
Changes to the Go frontend should follow the same process as for the
main Go repository, only for the <code>gofrontend</code> project and
the <code>gofrontend-dev@googlegroups.com</code> mailing list
rather than the <code>go</code> project and the
<code>golang-dev@googlegroups.com</code> mailing list. Those changes
will then be merged into the GCC sources.
</p>

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<!--{
"Title": "Setting up and using gccgo",
"Path": "/doc/install/gccgo"
}-->
<p>
This document explains how to use gccgo, a compiler for
the Go language. The gccgo compiler is a new frontend
for GCC, the widely used GNU compiler. Although the
frontend itself is under a BSD-style license, gccgo is
normally used as part of GCC and is then covered by
the <a href="https://www.gnu.org/licenses/gpl.html">GNU General Public
License</a> (the license covers gccgo itself as part of GCC; it
does not cover code generated by gccgo).
</p>
<p>
Note that gccgo is not the <code>gc</code> compiler; see
the <a href="/doc/install.html">Installing Go</a> instructions for that
compiler.
</p>
<h2 id="Releases">Releases</h2>
<p>
The simplest way to install gccgo is to install a GCC binary release
built to include Go support. GCC binary releases are available from
<a href="https://gcc.gnu.org/install/binaries.html">various
websites</a> and are typically included as part of GNU/Linux
distributions. We expect that most people who build these binaries
will include Go support.
</p>
<p>
The GCC 4.7.1 release and all later 4.7 releases include a complete
<a href="/doc/go1.html">Go 1</a> compiler and libraries.
</p>
<p>
Due to timing, the GCC 4.8.0 and 4.8.1 releases are close to but not
identical to Go 1.1. The GCC 4.8.2 release includes a complete Go
1.1.2 implementation.
</p>
<p>
The GCC 4.9 releases include a complete Go 1.2 implementation.
</p>
<p>
The GCC 5 releases include a complete implementation of the Go 1.4
user libraries. The Go 1.4 runtime is not fully merged, but that
should not be visible to Go programs.
</p>
<p>
The GCC 6 releases include a complete implementation of the Go 1.6.1
user libraries. The Go 1.6 runtime is not fully merged, but that
should not be visible to Go programs.
</p>
<p>
The GCC 7 releases include a complete implementation of the Go 1.8.1
user libraries. As with earlier releases, the Go 1.8 runtime is not
fully merged, but that should not be visible to Go programs.
</p>
<p>
The GCC 8 releases include a complete implementation of the Go 1.10.1
release. The Go 1.10 runtime has now been fully merged into the GCC
development sources, and concurrent garbage collection is fully
supported.
</p>
<p>
The GCC 9 releases include a complete implementation of the Go 1.12.2
release.
</p>
<h2 id="Source_code">Source code</h2>
<p>
If you cannot use a release, or prefer to build gccgo for yourself, the
gccgo source code is accessible via Git. The GCC web site has
<a href="https://gcc.gnu.org/git.html">instructions for getting the GCC
source code</a>. The gccgo source code is included. As a convenience, a
stable version of the Go support is available in the
<code>devel/gccgo</code> branch of the main GCC code repository:
<code>git://gcc.gnu.org/git/gcc.git</code>.
This branch is periodically updated with stable Go compiler sources.
</p>
<p>
Note that although <code>gcc.gnu.org</code> is the most convenient way
to get the source code for the Go frontend, it is not where the master
sources live. If you want to contribute changes to the Go frontend
compiler, see <a href="/doc/gccgo_contribute.html">Contributing to
gccgo</a>.
</p>
<h2 id="Building">Building</h2>
<p>
Building gccgo is just like building GCC
with one or two additional options. See
the <a href="https://gcc.gnu.org/install/">instructions on the gcc web
site</a>. When you run <code>configure</code>, add the
option <code>--enable-languages=c,c++,go</code> (along with other
languages you may want to build). If you are targeting a 32-bit x86,
then you will want to build gccgo to default to
supporting locked compare and exchange instructions; do this by also
using the <code>configure</code> option <code>--with-arch=i586</code>
(or a newer architecture, depending on where you need your programs to
run). If you are targeting a 64-bit x86, but sometimes want to use
the <code>-m32</code> option, then use the <code>configure</code>
option <code>--with-arch-32=i586</code>.
</p>
<h3 id="Gold">Gold</h3>
<p>
On x86 GNU/Linux systems the gccgo compiler is able to
use a small discontiguous stack for goroutines. This permits programs
to run many more goroutines, since each goroutine can use a relatively
small stack. Doing this requires using the gold linker version 2.22
or later. You can either install GNU binutils 2.22 or later, or you
can build gold yourself.
</p>
<p>
To build gold yourself, build the GNU binutils,
using <code>--enable-gold=default</code> when you run
the <code>configure</code> script. Before building, you must install
the flex and bison packages. A typical sequence would look like
this (you can replace <code>/opt/gold</code> with any directory to
which you have write access):
</p>
<pre>
git clone git://sourceware.org/git/binutils-gdb.git
mkdir binutils-objdir
cd binutils-objdir
../binutils-gdb/configure --enable-gold=default --prefix=/opt/gold
make
make install
</pre>
<p>
However you install gold, when you configure gccgo, use the
option <code>--with-ld=<var>GOLD_BINARY</var></code>.
</p>
<h3 id="Prerequisites">Prerequisites</h3>
<p>
A number of prerequisites are required to build GCC, as
described on
the <a href="https://gcc.gnu.org/install/prerequisites.html">gcc web
site</a>. It is important to install all the prerequisites before
running the gcc <code>configure</code> script.
The prerequisite libraries can be conveniently downloaded using the
script <code>contrib/download_prerequisites</code> in the GCC sources.
<h3 id="Build_commands">Build commands</h3>
<p>
Once all the prerequisites are installed, then a typical build and
install sequence would look like this (only use
the <code>--with-ld</code> option if you are using the gold linker as
described above):
</p>
<pre>
git clone --branch devel/gccgo git://gcc.gnu.org/git/gcc.git gccgo
mkdir objdir
cd objdir
../gccgo/configure --prefix=/opt/gccgo --enable-languages=c,c++,go --with-ld=/opt/gold/bin/ld
make
make install
</pre>
<h2 id="Using_gccgo">Using gccgo</h2>
<p>
The gccgo compiler works like other gcc frontends. As of GCC 5 the gccgo
installation also includes a version of the <code>go</code> command,
which may be used to build Go programs as described at
<a href="https://golang.org/cmd/go">https://golang.org/cmd/go</a>.
</p>
<p>
To compile a file without using the <code>go</code> command:
</p>
<pre>
gccgo -c file.go
</pre>
<p>
That produces <code>file.o</code>. To link files together to form an
executable:
</p>
<pre>
gccgo -o file file.o
</pre>
<p>
To run the resulting file, you will need to tell the program where to
find the compiled Go packages. There are a few ways to do this:
</p>
<ul>
<li>
<p>
Set the <code>LD_LIBRARY_PATH</code> environment variable:
</p>
<pre>
LD_LIBRARY_PATH=${prefix}/lib/gcc/MACHINE/VERSION
[or]
LD_LIBRARY_PATH=${prefix}/lib64/gcc/MACHINE/VERSION
export LD_LIBRARY_PATH
</pre>
<p>
Here <code>${prefix}</code> is the <code>--prefix</code> option used
when building gccgo. For a binary install this is
normally <code>/usr</code>. Whether to use <code>lib</code>
or <code>lib64</code> depends on the target.
Typically <code>lib64</code> is correct for x86_64 systems,
and <code>lib</code> is correct for other systems. The idea is to
name the directory where <code>libgo.so</code> is found.
</p>
</li>
<li>
<p>
Passing a <code>-Wl,-R</code> option when you link (replace lib with
lib64 if appropriate for your system):
</p>
<pre>
go build -gccgoflags -Wl,-R,${prefix}/lib/gcc/MACHINE/VERSION
[or]
gccgo -o file file.o -Wl,-R,${prefix}/lib/gcc/MACHINE/VERSION
</pre>
</li>
<li>
<p>
Use the <code>-static-libgo</code> option to link statically against
the compiled packages.
</p>
</li>
<li>
<p>
Use the <code>-static</code> option to do a fully static link (the
default for the <code>gc</code> compiler).
</p>
</li>
</ul>
<h2 id="Options">Options</h2>
<p>
The gccgo compiler supports all GCC options
that are language independent, notably the <code>-O</code>
and <code>-g</code> options.
</p>
<p>
The <code>-fgo-pkgpath=PKGPATH</code> option may be used to set a
unique prefix for the package being compiled.
This option is automatically used by the go command, but you may want
to use it if you invoke gccgo directly.
This option is intended for use with large
programs that contain many packages, in order to allow multiple
packages to use the same identifier as the package name.
The <code>PKGPATH</code> may be any string; a good choice for the
string is the path used to import the package.
</p>
<p>
The <code>-I</code> and <code>-L</code> options, which are synonyms
for the compiler, may be used to set the search path for finding
imports.
These options are not needed if you build with the go command.
</p>
<h2 id="Imports">Imports</h2>
<p>
When you compile a file that exports something, the export
information will be stored directly in the object file.
If you build with gccgo directly, rather than with the go command,
then when you import a package, you must tell gccgo how to find the
file.
</p>
<p>
When you import the package <var>FILE</var> with gccgo,
it will look for the import data in the following files, and use the
first one that it finds.
<ul>
<li><code><var>FILE</var>.gox</code>
<li><code>lib<var>FILE</var>.so</code>
<li><code>lib<var>FILE</var>.a</code>
<li><code><var>FILE</var>.o</code>
</ul>
<p>
<code><var>FILE</var>.gox</code>, when used, will typically contain
nothing but export data. This can be generated from
<code><var>FILE</var>.o</code> via
</p>
<pre>
objcopy -j .go_export FILE.o FILE.gox
</pre>
<p>
The gccgo compiler will look in the current
directory for import files. In more complex scenarios you
may pass the <code>-I</code> or <code>-L</code> option to
gccgo. Both options take directories to search. The
<code>-L</code> option is also passed to the linker.
</p>
<p>
The gccgo compiler does not currently (2015-06-15) record
the file name of imported packages in the object file. You must
arrange for the imported data to be linked into the program.
Again, this is not necessary when building with the go command.
</p>
<pre>
gccgo -c mypackage.go # Exports mypackage
gccgo -c main.go # Imports mypackage
gccgo -o main main.o mypackage.o # Explicitly links with mypackage.o
</pre>
<h2 id="Debugging">Debugging</h2>
<p>
If you use the <code>-g</code> option when you compile, you can run
<code>gdb</code> on your executable. The debugger has only limited
knowledge about Go. You can set breakpoints, single-step,
etc. You can print variables, but they will be printed as though they
had C/C++ types. For numeric types this doesn't matter. Go strings
and interfaces will show up as two-element structures. Go
maps and channels are always represented as C pointers to run-time
structures.
</p>
<h2 id="C_Interoperability">C Interoperability</h2>
<p>
When using gccgo there is limited interoperability with C,
or with C++ code compiled using <code>extern "C"</code>.
</p>
<h3 id="Types">Types</h3>
<p>
Basic types map directly: an <code>int32</code> in Go is
an <code>int32_t</code> in C, an <code>int64</code> is
an <code>int64_t</code>, etc.
The Go type <code>int</code> is an integer that is the same size as a
pointer, and as such corresponds to the C type <code>intptr_t</code>.
Go <code>byte</code> is equivalent to C <code>unsigned char</code>.
Pointers in Go are pointers in C.
A Go <code>struct</code> is the same as C <code>struct</code> with the
same fields and types.
</p>
<p>
The Go <code>string</code> type is currently defined as a two-element
structure (this is <b style="color: red;">subject to change</b>):
</p>
<pre>
struct __go_string {
const unsigned char *__data;
intptr_t __length;
};
</pre>
<p>
You can't pass arrays between C and Go. However, a pointer to an
array in Go is equivalent to a C pointer to the
equivalent of the element type.
For example, Go <code>*[10]int</code> is equivalent to C <code>int*</code>,
assuming that the C pointer does point to 10 elements.
</p>
<p>
A slice in Go is a structure. The current definition is
(this is <b style="color: red;">subject to change</b>):
</p>
<pre>
struct __go_slice {
void *__values;
intptr_t __count;
intptr_t __capacity;
};
</pre>
<p>
The type of a Go function is a pointer to a struct (this is
<b style="color: red;">subject to change</b>). The first field in the
struct points to the code of the function, which will be equivalent to
a pointer to a C function whose parameter types are equivalent, with
an additional trailing parameter. The trailing parameter is the
closure, and the argument to pass is a pointer to the Go function
struct.
When a Go function returns more than one value, the C function returns
a struct. For example, these functions are roughly equivalent:
</p>
<pre>
func GoFunction(int) (int, float64)
struct { int i; float64 f; } CFunction(int, void*)
</pre>
<p>
Go <code>interface</code>, <code>channel</code>, and <code>map</code>
types have no corresponding C type (<code>interface</code> is a
two-element struct and <code>channel</code> and <code>map</code> are
pointers to structs in C, but the structs are deliberately undocumented). C
<code>enum</code> types correspond to some integer type, but precisely
which one is difficult to predict in general; use a cast. C <code>union</code>
types have no corresponding Go type. C <code>struct</code> types containing
bitfields have no corresponding Go type. C++ <code>class</code> types have
no corresponding Go type.
</p>
<p>
Memory allocation is completely different between C and Go, as Go uses
garbage collection. The exact guidelines in this area are undetermined,
but it is likely that it will be permitted to pass a pointer to allocated
memory from C to Go. The responsibility of eventually freeing the pointer
will remain with C side, and of course if the C side frees the pointer
while the Go side still has a copy the program will fail. When passing a
pointer from Go to C, the Go function must retain a visible copy of it in
some Go variable. Otherwise the Go garbage collector may delete the
pointer while the C function is still using it.
</p>
<h3 id="Function_names">Function names</h3>
<p>
Go code can call C functions directly using a Go extension implemented
in gccgo: a function declaration may be preceded by
<code>//extern NAME</code>. For example, here is how the C function
<code>open</code> can be declared in Go:
</p>
<pre>
//extern open
func c_open(name *byte, mode int, perm int) int
</pre>
<p>
The C function naturally expects a NUL-terminated string, which in
Go is equivalent to a pointer to an array (not a slice!) of
<code>byte</code> with a terminating zero byte. So a sample call
from Go would look like (after importing the <code>syscall</code> package):
</p>
<pre>
var name = [4]byte{'f', 'o', 'o', 0};
i := c_open(&amp;name[0], syscall.O_RDONLY, 0);
</pre>
<p>
(this serves as an example only, to open a file in Go please use Go's
<code>os.Open</code> function instead).
</p>
<p>
Note that if the C function can block, such as in a call
to <code>read</code>, calling the C function may block the Go program.
Unless you have a clear understanding of what you are doing, all calls
between C and Go should be implemented through cgo or SWIG, as for
the <code>gc</code> compiler.
</p>
<p>
The name of Go functions accessed from C is subject to change. At present
the name of a Go function that does not have a receiver is
<code>prefix.package.Functionname</code>. The prefix is set by
the <code>-fgo-prefix</code> option used when the package is compiled;
if the option is not used, the default is <code>go</code>.
To call the function from C you must set the name using
a GCC extension.
</p>
<pre>
extern int go_function(int) __asm__ ("myprefix.mypackage.Function");
</pre>
<h3 id="Automatic_generation_of_Go_declarations_from_C_source_code">
Automatic generation of Go declarations from C source code</h3>
<p>
The Go version of GCC supports automatically generating
Go declarations from C code. The facility is rather awkward, and most
users should use the <a href="/cmd/cgo">cgo</a> program with
the <code>-gccgo</code> option instead.
</p>
<p>
Compile your C code as usual, and add the option
<code>-fdump-go-spec=<var>FILENAME</var></code>. This will create the
file <code><var>FILENAME</var></code> as a side effect of the
compilation. This file will contain Go declarations for the types,
variables and functions declared in the C code. C types that can not
be represented in Go will be recorded as comments in the Go code. The
generated file will not have a <code>package</code> declaration, but
can otherwise be compiled directly by gccgo.
</p>
<p>
This procedure is full of unstated caveats and restrictions and we make no
guarantee that it will not change in the future. It is more useful as a
starting point for real Go code than as a regular procedure.
</p>

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<!--{
"Title": "Go 1.11 Release Notes",
"Path": "/doc/go1.11",
"Template": true
}-->
<!--
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
-->
<style>
main ul li { margin: 0.5em 0; }
</style>
<h2 id="introduction">Introduction to Go 1.11</h2>
<p>
The latest Go release, version 1.11, arrives six months after <a href="go1.10">Go 1.10</a>.
Most of its changes are in the implementation of the toolchain, runtime, and libraries.
As always, the release maintains the Go 1 <a href="/doc/go1compat.html">promise of compatibility</a>.
We expect almost all Go programs to continue to compile and run as before.
</p>
<h2 id="language">Changes to the language</h2>
<p>
There are no changes to the language specification.
</p>
<h2 id="ports">Ports</h2>
<p> <!-- CL 94255, CL 115038, etc -->
As <a href="go1.10#ports">announced in the Go 1.10 release notes</a>, Go 1.11 now requires
OpenBSD 6.2 or later, macOS 10.10 Yosemite or later, or Windows 7 or later;
support for previous versions of these operating systems has been removed.
</p>
<p> <!-- CL 121657 -->
Go 1.11 supports the upcoming OpenBSD 6.4 release. Due to changes in
the OpenBSD kernel, older versions of Go will not work on OpenBSD 6.4.
</p>
<p>
There are <a href="https://golang.org/issue/25206">known issues</a> with NetBSD on i386 hardware.
</p>
<p><!-- CL 107935 -->
The race detector is now supported on <code>linux/ppc64le</code>
and, to a lesser extent, on <code>netbsd/amd64</code>. The NetBSD race detector support
has <a href="https://golang.org/issue/26403">known issues</a>.
</p>
<p><!-- CL 109255 -->
The memory sanitizer (<code>-msan</code>) is now supported on <code>linux/arm64</code>.
</p>
<p><!-- CL 93875 -->
The build modes <code>c-shared</code> and <code>c-archive</code> are now supported on
<code>freebsd/amd64</code>.
</p>
<p id="mips"><!-- CL 108475 -->
On 64-bit MIPS systems, the new environment variable settings
<code>GOMIPS64=hardfloat</code> (the default) and
<code>GOMIPS64=softfloat</code> select whether to use
hardware instructions or software emulation for floating-point computations.
For 32-bit systems, the environment variable is still <code>GOMIPS</code>,
as <a href="go1.10#mips">added in Go 1.10</a>.
</p>
<p><!-- CL 107475 -->
On soft-float ARM systems (<code>GOARM=5</code>), Go now uses a more
efficient software floating point interface. This is transparent to
Go code, but ARM assembly that uses floating-point instructions not
guarded on GOARM will break and must be ported to
the <a href="https://golang.org/cl/107475">new interface</a>.
</p>
<p><!-- CL 94076 -->
Go 1.11 on ARMv7 no longer requires a Linux kernel configured
with <code>KUSER_HELPERS</code>. This setting is enabled in default
kernel configurations, but is sometimes disabled in stripped-down
configurations.
</p>
<h3 id="wasm">WebAssembly</h3>
<p>
Go 1.11 adds an experimental port to <a href="https://webassembly.org">WebAssembly</a>
(<code>js/wasm</code>).
</p>
<p>
Go programs currently compile to one WebAssembly module that
includes the Go runtime for goroutine scheduling, garbage
collection, maps, etc.
As a result, the resulting size is at minimum around
2 MB, or 500 KB compressed. Go programs can call into JavaScript
using the new experimental
<a href="/pkg/syscall/js/"><code>syscall/js</code></a> package.
Binary size and interop with other languages has not yet been a
priority but may be addressed in future releases.
</p>
<p>
As a result of the addition of the new <code>GOOS</code> value
"<code>js</code>" and <code>GOARCH</code> value "<code>wasm</code>",
Go files named <code>*_js.go</code> or <code>*_wasm.go</code> will
now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored by Go
tools</a> except when those GOOS/GOARCH values are being used.
If you have existing filenames matching those patterns, you will need to rename them.
</p>
<p>
More information can be found on the
<a href="https://golang.org/wiki/WebAssembly">WebAssembly wiki page</a>.
</p>
<h3 id="riscv">RISC-V GOARCH values reserved</h3>
<p><!-- CL 106256 -->
The main Go compiler does not yet support the RISC-V architecture <!-- is gonna change everything -->
but we've reserved the <code>GOARCH</code> values
"<code>riscv</code>" and "<code>riscv64</code>", as used by Gccgo,
which does support RISC-V. This means that Go files
named <code>*_riscv.go</code> will now also
be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored by Go
tools</a> except when those GOOS/GOARCH values are being used.
</p>
<h2 id="tools">Tools</h2>
<h3 id="modules">Modules, package versioning, and dependency management</h3>
<p>
Go 1.11 adds preliminary support for a <a href="/cmd/go/#hdr-Modules__module_versions__and_more">new concept called “modules,”</a>
an alternative to GOPATH with integrated support for versioning and
package distribution.
Using modules, developers are no longer confined to working inside GOPATH,
version dependency information is explicit yet lightweight,
and builds are more reliable and reproducible.
</p>
<p>
Module support is considered experimental.
Details are likely to change in response to feedback from Go 1.11 users,
and we have more tools planned.
Although the details of module support may change, projects that convert
to modules using Go 1.11 will continue to work with Go 1.12 and later.
If you encounter bugs using modules,
please <a href="https://golang.org/issue/new">file issues</a>
so we can fix them. For more information, see the
<a href="/cmd/go#hdr-Modules__module_versions__and_more"><code>go</code> command documentation</a>.
</p>
<h3 id="importpath">Import path restriction</h3>
<p>
Because Go module support assigns special meaning to the
<code>@</code> symbol in command line operations,
the <code>go</code> command now disallows the use of
import paths containing <code>@</code> symbols.
Such import paths were never allowed by <code>go</code> <code>get</code>,
so this restriction can only affect users building
custom GOPATH trees by other means.
</p>
<h3 id="gopackages">Package loading</h3>
<p>
The new package
<a href="https://godoc.org/golang.org/x/tools/go/packages"><code>golang.org/x/tools/go/packages</code></a>
provides a simple API for locating and loading packages of Go source code.
Although not yet part of the standard library, for many tasks it
effectively replaces the <a href="/pkg/go/build"><code>go/build</code></a>
package, whose API is unable to fully support modules.
Because it runs an external query command such as
<a href="/cmd/go/#hdr-List_packages"><code>go list</code></a>
to obtain information about Go packages, it enables the construction of
analysis tools that work equally well with alternative build systems
such as <a href="https://bazel.build">Bazel</a>
and <a href="https://buckbuild.com">Buck</a>.
</p>
<h3 id="gocache">Build cache requirement</h3>
<p>
Go 1.11 will be the last release to support setting the environment
variable <code>GOCACHE=off</code> to disable the
<a href="/cmd/go/#hdr-Build_and_test_caching">build cache</a>,
introduced in Go 1.10.
Starting in Go 1.12, the build cache will be required,
as a step toward eliminating <code>$GOPATH/pkg</code>.
The module and package loading support described above
already require that the build cache be enabled.
If you have disabled the build cache to avoid problems you encountered,
please <a href="https://golang.org/issue/new">file an issue</a> to let us know about them.
</p>
<h3 id="compiler">Compiler toolchain</h3>
<p><!-- CL 109918 -->
More functions are now eligible for inlining by default, including
functions that call <code>panic</code>.
</p>
<p><!-- CL 97375 -->
The compiler toolchain now supports column information
in <a href="/cmd/compile/#hdr-Compiler_Directives">line
directives</a>.
</p>
<p><!-- CL 106797 -->
A new package export data format has been introduced.
This should be transparent to end users, except for speeding up
build times for large Go projects.
If it does cause problems, it can be turned off again by
passing <code>-gcflags=all=-iexport=false</code> to
the <code>go</code> tool when building a binary.
</p>
<p><!-- CL 100459 -->
The compiler now rejects unused variables declared in a type switch
guard, such as <code>x</code> in the following example:
</p>
<pre>
func f(v interface{}) {
switch x := v.(type) {
}
}
</pre>
<p>
This was already rejected by both <code>gccgo</code>
and <a href="/pkg/go/types/">go/types</a>.
</p>
<h3 id="assembler">Assembler</h3>
<p><!-- CL 113315 -->
The assembler for <code>amd64</code> now accepts AVX512 instructions.
</p>
<h3 id="debugging">Debugging</h3>
<p><!-- CL 100738, CL 93664 -->
The compiler now produces significantly more accurate debug
information for optimized binaries, including variable location
information, line numbers, and breakpoint locations.
This should make it possible to debug binaries
compiled <em>without</em> <code>-N</code>&nbsp;<code>-l</code>.
There are still limitations to the quality of the debug information,
some of which are fundamental, and some of which will continue to
improve with future releases.
</p>
<p><!-- CL 118276 -->
DWARF sections are now compressed by default because of the expanded
and more accurate debug information produced by the compiler.
This is transparent to most ELF tools (such as debuggers on Linux
and *BSD) and is supported by the Delve debugger on all platforms,
but has limited support in the native tools on macOS and Windows.
To disable DWARF compression,
pass <code>-ldflags=-compressdwarf=false</code> to
the <code>go</code> tool when building a binary.
</p>
<p><!-- CL 109699 -->
Go 1.11 adds experimental support for calling Go functions from
within a debugger.
This is useful, for example, to call <code>String</code> methods
when paused at a breakpoint.
This is currently only supported by Delve (version 1.1.0 and up).
</p>
<h3 id="test">Test</h3>
<p>
Since Go 1.10, the <code>go</code>&nbsp;<code>test</code> command runs
<code>go</code>&nbsp;<code>vet</code> on the package being tested,
to identify problems before running the test. Since <code>vet</code>
typechecks the code with <a href="/pkg/go/types/">go/types</a>
before running, tests that do not typecheck will now fail.
In particular, tests that contain an unused variable inside a
closure compiled with Go 1.10, because the Go compiler incorrectly
accepted them (<a href="https://golang.org/issues/3059">Issue #3059</a>),
but will now fail, since <code>go/types</code> correctly reports an
"unused variable" error in this case.
</p>
<p><!-- CL 102696 -->
The <code>-memprofile</code> flag
to <code>go</code>&nbsp;<code>test</code> now defaults to the
"allocs" profile, which records the total bytes allocated since the
test began (including garbage-collected bytes).
</p>
<h3 id="vet">Vet</h3>
<p><!-- CL 108555 -->
The <a href="/cmd/vet/"><code>go</code>&nbsp;<code>vet</code></a>
command now reports a fatal error when the package under analysis
does not typecheck. Previously, a type checking error simply caused
a warning to be printed, and <code>vet</code> to exit with status 1.
</p>
<p><!-- CL 108559 -->
Additionally, <a href="/cmd/vet"><code>go</code>&nbsp;<code>vet</code></a>
has become more robust when format-checking <code>printf</code> wrappers.
Vet now detects the mistake in this example:
</p>
<pre>
func wrapper(s string, args ...interface{}) {
fmt.Printf(s, args...)
}
func main() {
wrapper("%s", 42)
}
</pre>
<h3 id="trace">Trace</h3>
<p><!-- CL 63274 -->
With the new <code>runtime/trace</code>
package's <a href="/pkg/runtime/trace/#hdr-User_annotation">user
annotation API</a>, users can record application-level information
in execution traces and create groups of related goroutines.
The <code>go</code>&nbsp;<code>tool</code>&nbsp;<code>trace</code>
command visualizes this information in the trace view and the new
user task/region analysis page.
</p>
<h3 id="cgo">Cgo</h3>
<p>
Since Go 1.10, cgo has translated some C pointer types to the Go
type <code>uintptr</code>. These types include
the <code>CFTypeRef</code> hierarchy in Darwin's CoreFoundation
framework and the <code>jobject</code> hierarchy in Java's JNI
interface. In Go 1.11, several improvements have been made to the code
that detects these types. Code that uses these types may need some
updating. See the <a href="go1.10.html#cgo">Go 1.10 release notes</a> for
details. <!-- CL 126275, CL 127156, CL 122217, CL 122575, CL 123177 -->
</p>
<h3 id="go_command">Go command</h3>
<p><!-- CL 126656 -->
The environment variable <code>GOFLAGS</code> may now be used
to set default flags for the <code>go</code> command.
This is useful in certain situations.
Linking can be noticeably slower on underpowered systems due to DWARF,
and users may want to set <code>-ldflags=-w</code> by default.
For modules, some users and CI systems will want vendoring always,
so they should set <code>-mod=vendor</code> by default.
For more information, see the <a href="/cmd/go/#hdr-Environment_variables"><code>go</code>
command documentation</a>.
</p>
<h3 id="godoc">Godoc</h3>
<p>
Go 1.11 will be the last release to support <code>godoc</code>'s command-line interface.
In future releases, <code>godoc</code> will only be a web server. Users should use
<code>go</code> <code>doc</code> for command-line help output instead.
</p>
<p><!-- CL 85396, CL 124495 -->
The <code>godoc</code> web server now shows which version of Go introduced
new API features. The initial Go version of types, funcs, and methods are shown
right-aligned. For example, see <a href="/pkg/os/#UserCacheDir"><code>UserCacheDir</code></a>, with "1.11"
on the right side. For struct fields, inline comments are added when the struct field was
added in a Go version other than when the type itself was introduced.
For a struct field example, see
<a href="/pkg/net/http/httptrace/#ClientTrace.Got1xxResponse"><code>ClientTrace.Got1xxResponse</code></a>.
</p>
<h3 id="gofmt">Gofmt</h3>
<p>
One minor detail of the default formatting of Go source code has changed.
When formatting expression lists with inline comments, the comments were
aligned according to a heuristic.
However, in some cases the alignment would be split up too easily, or
introduce too much whitespace.
The heuristic has been changed to behave better for human-written code.
</p>
<p>
Note that these kinds of minor updates to gofmt are expected from time to
time.
In general, systems that need consistent formatting of Go source code should
use a specific version of the <code>gofmt</code> binary.
See the <a href="/pkg/go/format/">go/format</a> package documentation for more
information.
</p>
<h3 id="run">Run</h3>
<p>
<!-- CL 109341 -->
The <a href="/cmd/go/"><code>go</code>&nbsp;<code>run</code></a>
command now allows a single import path, a directory name or a
pattern matching a single package.
This allows <code>go</code>&nbsp;<code>run</code>&nbsp;<code>pkg</code> or <code>go</code>&nbsp;<code>run</code>&nbsp;<code>dir</code>, most importantly <code>go</code>&nbsp;<code>run</code>&nbsp;<code>.</code>
</p>
<h2 id="runtime">Runtime</h2>
<p><!-- CL 85887 -->
The runtime now uses a sparse heap layout so there is no longer a
limit to the size of the Go heap (previously, the limit was 512GiB).
This also fixes rare "address space conflict" failures in mixed Go/C
binaries or binaries compiled with <code>-race</code>.
</p>
<p><!-- CL 108679, CL 106156 -->
On macOS and iOS, the runtime now uses <code>libSystem.dylib</code> instead of
calling the kernel directly. This should make Go binaries more
compatible with future versions of macOS and iOS.
The <a href="/pkg/syscall">syscall</a> package still makes direct
system calls; fixing this is planned for a future release.
</p>
<h2 id="performance">Performance</h2>
<p>
As always, the changes are so general and varied that precise
statements about performance are difficult to make. Most programs
should run a bit faster, due to better generated code and
optimizations in the core library.
</p>
<p><!-- CL 74851 -->
There were multiple performance changes to the <code>math/big</code>
package as well as many changes across the tree specific to <code>GOARCH=arm64</code>.
</p>
<h3 id="performance-compiler">Compiler toolchain</h3>
<p><!-- CL 110055 -->
The compiler now optimizes map clearing operations of the form:
</p>
<pre>
for k := range m {
delete(m, k)
}
</pre>
<p><!-- CL 109517 -->
The compiler now optimizes slice extension of the form
<code>append(s,</code>&nbsp;<code>make([]T,</code>&nbsp;<code>n)...)</code>.
</p>
<p><!-- CL 100277, CL 105635, CL 109776 -->
The compiler now performs significantly more aggressive bounds-check
and branch elimination. Notably, it now recognizes transitive
relations, so if <code>i&lt;j</code> and <code>j&lt;len(s)</code>,
it can use these facts to eliminate the bounds check
for <code>s[i]</code>. It also understands simple arithmetic such
as <code>s[i-10]</code> and can recognize more inductive cases in
loops. Furthermore, the compiler now uses bounds information to more
aggressively optimize shift operations.
</p>
<h2 id="library">Core library</h2>
<p>
All of the changes to the standard library are minor.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
As always, there are various minor changes and updates to the library,
made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
in mind.
</p>
<!-- CL 115095: https://golang.org/cl/115095: yes (`go test pkg` now always builds pkg even if there are no test files): cmd/go: output coverage report even if there are no test files -->
<!-- CL 110395: https://golang.org/cl/110395: cmd/go, cmd/compile: use Windows response files to avoid arg length limits -->
<!-- CL 112436: https://golang.org/cl/112436: cmd/pprof: add readline support similar to upstream -->
<dl id="crypto"><dt><a href="/pkg/crypto/">crypto</a></dt>
<dd>
<p><!-- CL 64451 -->
Certain crypto operations, including
<a href="/pkg/crypto/ecdsa/#Sign"><code>ecdsa.Sign</code></a>,
<a href="/pkg/crypto/rsa/#EncryptPKCS1v15"><code>rsa.EncryptPKCS1v15</code></a> and
<a href="/pkg/crypto/rsa/#GenerateKey"><code>rsa.GenerateKey</code></a>,
now randomly read an extra byte of randomness to ensure tests don't rely on internal behavior.
</p>
</dl><!-- crypto -->
<dl id="crypto/cipher"><dt><a href="/pkg/crypto/cipher/">crypto/cipher</a></dt>
<dd>
<p><!-- CL 48510, CL 116435 -->
The new function <a href="/pkg/crypto/cipher/#NewGCMWithTagSize"><code>NewGCMWithTagSize</code></a>
implements Galois Counter Mode with non-standard tag lengths for compatibility with existing cryptosystems.
</p>
</dl><!-- crypto/cipher -->
<dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
<dd>
<p><!-- CL 103876 -->
<a href="/pkg/crypto/rsa/#PublicKey"><code>PublicKey</code></a> now implements a
<a href="/pkg/crypto/rsa/#PublicKey.Size"><code>Size</code></a> method that
returns the modulus size in bytes.
</p>
</dl><!-- crypto/rsa -->
<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
<dd>
<p><!-- CL 85115 -->
<a href="/pkg/crypto/tls/#ConnectionState"><code>ConnectionState</code></a>'s new
<a href="/pkg/crypto/tls/#ConnectionState.ExportKeyingMaterial"><code>ExportKeyingMaterial</code></a>
method allows exporting keying material bound to the
connection according to RFC 5705.
</p>
</dl><!-- crypto/tls -->
<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
<dd>
<p><!-- CL 123355, CL 123695 -->
The deprecated, legacy behavior of treating the <code>CommonName</code> field as
a hostname when no Subject Alternative Names are present is now disabled when the CN is not a
valid hostname.
The <code>CommonName</code> can be completely ignored by adding the experimental value
<code>x509ignoreCN=1</code> to the <code>GODEBUG</code> environment variable.
When the CN is ignored, certificates without SANs validate under chains with name constraints
instead of returning <code>NameConstraintsWithoutSANs</code>.
</p>
<p><!-- CL 113475 -->
Extended key usage restrictions are again checked only if they appear in the <code>KeyUsages</code>
field of <a href="/pkg/crypto/x509/#VerifyOptions"><code>VerifyOptions</code></a>, instead of always being checked.
This matches the behavior of Go 1.9 and earlier.
</p>
<p><!-- CL 102699 -->
The value returned by <a href="/pkg/crypto/x509/#SystemCertPool"><code>SystemCertPool</code></a>
is now cached and might not reflect system changes between invocations.
</p>
</dl><!-- crypto/x509 -->
<dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
<dd>
<p><!-- CL 112115 -->
More <a href="/pkg/debug/elf/#ELFOSABI_NONE"><code>ELFOSABI</code></a>
and <a href="/pkg/debug/elf/#EM_NONE"><code>EM</code></a>
constants have been added.
</p>
</dl><!-- debug/elf -->
<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>
<dd>
<p><!-- CL 110561 -->
<code>Marshal</code> and <code><a href="/pkg/encoding/asn1/#Unmarshal">Unmarshal</a></code>
now support "private" class annotations for fields.
</p>
</dl><!-- encoding/asn1 -->
<dl id="encoding/base32"><dt><a href="/pkg/encoding/base32/">encoding/base32</a></dt>
<dd>
<p><!-- CL 112516 -->
The decoder now consistently
returns <code>io.ErrUnexpectedEOF</code> for an incomplete
chunk. Previously it would return <code>io.EOF</code> in some
cases.
</p>
</dl><!-- encoding/base32 -->
<dl id="encoding/csv"><dt><a href="/pkg/encoding/csv/">encoding/csv</a></dt>
<dd>
<p><!-- CL 99696 -->
The <code>Reader</code> now rejects attempts to set
the <a href="/pkg/encoding/csv/#Reader.Comma"><code>Comma</code></a>
field to a double-quote character, as double-quote characters
already have a special meaning in CSV.
</p>
</dl><!-- encoding/csv -->
<!-- CL 100235 was reverted -->
<dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
<dd>
<p><!-- CL 121815 -->
The package has changed its behavior when a typed interface
value is passed to an implicit escaper function. Previously such
a value was written out as (an escaped form)
of <code>&lt;nil&gt;</code>. Now such values are ignored, just
as an untyped <code>nil</code> value is (and always has been)
ignored.
</p>
</dl><!-- html/template -->
<dl id="image/gif"><dt><a href="/pkg/image/gif/">image/gif</a></dt>
<dd>
<p><!-- CL 93076 -->
Non-looping animated GIFs are now supported. They are denoted by having a
<code><a href="/pkg/image/gif/#GIF.LoopCount">LoopCount</a></code> of -1.
</p>
</dl><!-- image/gif -->
<dl id="io/ioutil"><dt><a href="/pkg/io/ioutil/">io/ioutil</a></dt>
<dd>
<p><!-- CL 105675 -->
The <code><a href="/pkg/io/ioutil/#TempFile">TempFile</a></code>
function now supports specifying where the random characters in
the filename are placed. If the <code>prefix</code> argument
includes a "<code>*</code>", the random string replaces the
"<code>*</code>". For example, a <code>prefix</code> argument of "<code>myname.*.bat</code>" will
result in a random filename such as
"<code>myname.123456.bat</code>". If no "<code>*</code>" is
included the old behavior is retained, and the random digits are
appended to the end.
</p>
</dl><!-- io/ioutil -->
<dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
<dd>
<p><!-- CL 108996 -->
<a href="/pkg/math/big/#Int.ModInverse"><code>ModInverse</code></a> now returns nil when g and n are not relatively prime. The result was previously undefined.
</p>
</dl><!-- math/big -->
<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
<dd>
<p><!-- CL 121055 -->
The handling of form-data with missing/empty file names has been
restored to the behavior in Go 1.9: in the
<a href="/pkg/mime/multipart/#Form"><code>Form</code></a> for
the form-data part the value is available in
the <code>Value</code> field rather than the <code>File</code>
field. In Go releases 1.10 through 1.10.3 a form-data part with
a missing/empty file name and a non-empty "Content-Type" field
was stored in the <code>File</code> field. This change was a
mistake in 1.10 and has been reverted to the 1.9 behavior.
</p>
</dl><!-- mime/multipart -->
<dl id="mime/quotedprintable"><dt><a href="/pkg/mime/quotedprintable/">mime/quotedprintable</a></dt>
<dd>
<p><!-- CL 121095 -->
To support invalid input found in the wild, the package now
permits non-ASCII bytes but does not validate their encoding.
</p>
</dl><!-- mime/quotedprintable -->
<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
<dd>
<p><!-- CL 72810 -->
The new <a href="/pkg/net/#ListenConfig"><code>ListenConfig</code></a> type and the new
<a href="/pkg/net/#Dialer.Control"><code>Dialer.Control</code></a> field permit
setting socket options before accepting and creating connections, respectively.
</p>
<p><!-- CL 76391 -->
The <a href="/pkg/syscall/#RawConn"><code>syscall.RawConn</code></a> <code>Read</code>
and <code>Write</code> methods now work correctly on Windows.
</p>
<p><!-- CL 107715 -->
The <code>net</code> package now automatically uses the
<a href="http://man7.org/linux/man-pages/man2/splice.2.html"><code>splice</code> system call</a>
on Linux when copying data between TCP connections in
<a href="/pkg/net/#TCPConn.ReadFrom"><code>TCPConn.ReadFrom</code></a>, as called by
<a href="/pkg/io/#Copy"><code>io.Copy</code></a>. The result is faster, more efficient TCP proxying.
</p>
<p><!-- CL 108297 -->
The <a href="/pkg/net/#TCPConn.File"><code>TCPConn.File</code></a>,
<a href="/pkg/net/#UDPConn.File"><code>UDPConn.File</code></a>,
<a href="/pkg/net/#UnixCOnn.File"><code>UnixConn.File</code></a>,
and <a href="/pkg/net/#IPConn.File"><code>IPConn.File</code></a>
methods no longer put the returned <code>*os.File</code> into
blocking mode.
</p>
</dl><!-- net -->
<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
<dd>
<p><!-- CL 71272 -->
The <a href="/pkg/net/http/#Transport"><code>Transport</code></a> type has a
new <a href="/pkg/net/http/#Transport.MaxConnsPerHost"><code>MaxConnsPerHost</code></a>
option that permits limiting the maximum number of connections
per host.
</p>
<p><!-- CL 79919 -->
The <a href="/pkg/net/http/#Cookie"><code>Cookie</code></a> type has a new
<a href="/pkg/net/http/#Cookie.SameSite"><code>SameSite</code></a> field
(of new type also named
<a href="/pkg/net/http/#SameSite"><code>SameSite</code></a>) to represent the new cookie attribute recently supported by most browsers.
The <code>net/http</code>'s <code>Transport</code> does not use the <code>SameSite</code>
attribute itself, but the package supports parsing and serializing the
attribute for browsers to use.
</p>
<p><!-- CL 81778 -->
It is no longer allowed to reuse a <a href="/pkg/net/http/#Server"><code>Server</code></a>
after a call to
<a href="/pkg/net/http/#Server.Shutdown"><code>Shutdown</code></a> or
<a href="/pkg/net/http/#Server.Close"><code>Close</code></a>. It was never officially supported
in the past and had often surprising behavior. Now, all future calls to the server's <code>Serve</code>
methods will return errors after a shutdown or close.
</p>
<!-- CL 89275 was reverted before Go 1.11 -->
<p><!-- CL 93296 -->
The constant <code>StatusMisdirectedRequest</code> is now defined for HTTP status code 421.
</p>
<p><!-- CL 123875 -->
The HTTP server will no longer cancel contexts or send on
<a href="/pkg/net/http/#CloseNotifier"><code>CloseNotifier</code></a>
channels upon receiving pipelined HTTP/1.1 requests. Browsers do
not use HTTP pipelining, but some clients (such as
Debian's <code>apt</code>) may be configured to do so.
</p>
<p><!-- CL 115255 -->
<a href="/pkg/net/http/#ProxyFromEnvironment"><code>ProxyFromEnvironment</code></a>, which is used by the
<a href="/pkg/net/http/#DefaultTransport"><code>DefaultTransport</code></a>, now
supports CIDR notation and ports in the <code>NO_PROXY</code> environment variable.
</p>
</dl><!-- net/http -->
<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
<dd>
<p><!-- CL 77410 -->
The
<a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a>
has a new
<a href="/pkg/net/http/httputil/#ReverseProxy.ErrorHandler"><code>ErrorHandler</code></a>
option to permit changing how errors are handled.
</p>
<p><!-- CL 115135 -->
The <code>ReverseProxy</code> now also passes
"<code>TE:</code>&nbsp;<code>trailers</code>" request headers
through to the backend, as required by the gRPC protocol.
</p>
</dl><!-- net/http/httputil -->
<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
<dd>
<p><!-- CL 78835 -->
The new <a href="/pkg/os/#UserCacheDir"><code>UserCacheDir</code></a> function
returns the default root directory to use for user-specific cached data.
</p>
<p><!-- CL 94856 -->
The new <a href="/pkg/os/#ModeIrregular"><code>ModeIrregular</code></a>
is a <a href="/pkg/os/#FileMode"><code>FileMode</code></a> bit to represent
that a file is not a regular file, but nothing else is known about it, or that
it's not a socket, device, named pipe, symlink, or other file type for which
Go has a defined mode bit.
</p>
<p><!-- CL 99337 -->
<a href="/pkg/os/#Symlink"><code>Symlink</code></a> now works
for unprivileged users on Windows 10 on machines with Developer
Mode enabled.
</p>
<p><!-- CL 100077 -->
When a non-blocking descriptor is passed
to <a href="/pkg/os#NewFile"><code>NewFile</code></a>, the
resulting <code>*File</code> will be kept in non-blocking
mode. This means that I/O for that <code>*File</code> will use
the runtime poller rather than a separate thread, and that
the <a href="/pkg/os/#File.SetDeadline"><code>SetDeadline</code></a>
methods will work.
</p>
</dl><!-- os -->
<dl id="os/signal"><dt><a href="/pkg/os/signal/">os/signal</a></dt>
<dd>
<p><!-- CL 108376 -->
The new <a href="/pkg/os/signal/#Ignored"><code>Ignored</code></a> function reports
whether a signal is currently ignored.
</p>
</dl><!-- os/signal -->
<dl id="os/user"><dt><a href="/pkg/os/user/">os/user</a></dt>
<dd>
<p><!-- CL 92456 -->
The <code>os/user</code> package can now be built in pure Go
mode using the build tag "<code>osusergo</code>",
independent of the use of the environment
variable <code>CGO_ENABLED=0</code>. Previously the only way to use
the package's pure Go implementation was to disable <code>cgo</code>
support across the entire program.
</p>
</dl><!-- os/user -->
<!-- CL 101715 was reverted -->
<dl id="pkg-runtime"><dt id="runtime-again"><a href="/pkg/runtime/">runtime</a></dt>
<dd>
<p><!-- CL 70993 -->
Setting the <code>GODEBUG=tracebackancestors=<em>N</em></code>
environment variable now extends tracebacks with the stacks at
which goroutines were created, where <em>N</em> limits the
number of ancestor goroutines to report.
</p>
</dl><!-- runtime -->
<dl id="runtime/pprof"><dt><a href="/pkg/runtime/pprof/">runtime/pprof</a></dt>
<dd>
<p><!-- CL 102696 -->
This release adds a new "allocs" profile type that profiles
total number of bytes allocated since the program began
(including garbage-collected bytes). This is identical to the
existing "heap" profile viewed in <code>-alloc_space</code> mode.
Now <code>go test -memprofile=...</code> reports an "allocs" profile
instead of "heap" profile.
</p>
</dl><!-- runtime/pprof -->
<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
<dd>
<p><!-- CL 87095 -->
The mutex profile now includes reader/writer contention
for <a href="/pkg/sync/#RWMutex"><code>RWMutex</code></a>.
Writer/writer contention was already included in the mutex
profile.
</p>
</dl><!-- sync -->
<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
<dd>
<p><!-- CL 106275 -->
On Windows, several fields were changed from <code>uintptr</code> to a new
<a href="/pkg/syscall/?GOOS=windows&GOARCH=amd64#Pointer"><code>Pointer</code></a>
type to avoid problems with Go's garbage collector. The same change was made
to the <a href="https://godoc.org/golang.org/x/sys/windows"><code>golang.org/x/sys/windows</code></a>
package. For any code affected, users should first migrate away from the <code>syscall</code>
package to the <code>golang.org/x/sys/windows</code> package, and then change
to using the <code>Pointer</code>, while obeying the
<a href="/pkg/unsafe/#Pointer"><code>unsafe.Pointer</code> conversion rules</a>.
</p>
<p><!-- CL 118658 -->
On Linux, the <code>flags</code> parameter to
<a href="/pkg/syscall/?GOOS=linux&GOARCH=amd64#Faccessat"><code>Faccessat</code></a>
is now implemented just as in glibc. In earlier Go releases the
flags parameter was ignored.
</p>
<p><!-- CL 118658 -->
On Linux, the <code>flags</code> parameter to
<a href="/pkg/syscall/?GOOS=linux&GOARCH=amd64#Fchmodat"><code>Fchmodat</code></a>
is now validated. Linux's <code>fchmodat</code> doesn't support the <code>flags</code> parameter
so we now mimic glibc's behavior and return an error if it's non-zero.
</p>
</dl><!-- syscall -->
<dl id="text/scanner"><dt><a href="/pkg/text/scanner/">text/scanner</a></dt>
<dd>
<p><!-- CL 112037 -->
The <a href="/pkg/text/scanner/#Scanner.Scan"><code>Scanner.Scan</code></a> method now returns
the <a href="/pkg/text/scanner/#RawString"><code>RawString</code></a> token
instead of <a href="/pkg/text/scanner/#String"><code>String</code></a>
for raw string literals.
</p>
</dl><!-- text/scanner -->
<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
<dd>
<p><!-- CL 84480 -->
Modifying template variables via assignments is now permitted via the <code>=</code> token:
</p>
<pre>
{{"{{"}} $v := "init" {{"}}"}}
{{"{{"}} if true {{"}}"}}
{{"{{"}} $v = "changed" {{"}}"}}
{{"{{"}} end {{"}}"}}
v: {{"{{"}} $v {{"}}"}} {{"{{"}}/* "changed" */{{"}}"}}</pre>
<p><!-- CL 95215 -->
In previous versions untyped <code>nil</code> values passed to
template functions were ignored. They are now passed as normal
arguments.
</p>
</dl><!-- text/template -->
<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
<dd>
<p><!-- CL 98157 -->
Parsing of timezones denoted by sign and offset is now
supported. In previous versions, numeric timezone names
(such as <code>+03</code>) were not considered valid, and only
three-letter abbreviations (such as <code>MST</code>) were accepted
when expecting a timezone name.
</p>
</dl><!-- time -->

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<!--{
"Title": "Go 1.12 Release Notes",
"Path": "/doc/go1.12",
"Template": true
}-->
<!--
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
-->
<style>
main ul li { margin: 0.5em 0; }
</style>
<h2 id="introduction">Introduction to Go 1.12</h2>
<p>
The latest Go release, version 1.12, arrives six months after <a href="go1.11">Go 1.11</a>.
Most of its changes are in the implementation of the toolchain, runtime, and libraries.
As always, the release maintains the Go 1 <a href="/doc/go1compat">promise of compatibility</a>.
We expect almost all Go programs to continue to compile and run as before.
</p>
<h2 id="language">Changes to the language</h2>
<p>
There are no changes to the language specification.
</p>
<h2 id="ports">Ports</h2>
<p><!-- CL 138675 -->
The race detector is now supported on <code>linux/arm64</code>.
</p>
<p id="freebsd">
Go 1.12 is the last release that is supported on FreeBSD 10.x, which has
already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD
12.0+.
FreeBSD 12.0+ requires a kernel with the COMPAT_FREEBSD11 option set (this is the default).
</p>
<p><!-- CL 146898 -->
cgo is now supported on <code>linux/ppc64</code>.
</p>
<p id="hurd"><!-- CL 146023 -->
<code>hurd</code> is now a recognized value for <code>GOOS</code>, reserved
for the GNU/Hurd system for use with <code>gccgo</code>.
</p>
<h3 id="windows">Windows</h3>
<p>
Go's new <code>windows/arm</code> port supports running Go on Windows 10
IoT Core on 32-bit ARM chips such as the Raspberry Pi 3.
</p>
<h3 id="aix">AIX</h3>
<p>
Go now supports AIX 7.2 and later on POWER8 architectures (<code>aix/ppc64</code>). External linking, cgo, pprof and the race detector aren't yet supported.
</p>
<h3 id="darwin">Darwin</h3>
<p>
Go 1.12 is the last release that will run on macOS 10.10 Yosemite.
Go 1.13 will require macOS 10.11 El Capitan or later.
</p>
<p><!-- CL 141639 -->
<code>libSystem</code> is now used when making syscalls on Darwin,
ensuring forward-compatibility with future versions of macOS and iOS.
<!-- CL 153338 -->
The switch to <code>libSystem</code> triggered additional App Store
checks for private API usage. Since it is considered private,
<code>syscall.Getdirentries</code> now always fails with
<code>ENOSYS</code> on iOS.
Additionally, <a href="/pkg/syscall/#Setrlimit"><code>syscall.Setrlimit</code></a>
reports <code>invalid</code> <code>argument</code> in places where it historically
succeeded. These consequences are not specific to Go and users should expect
behavioral parity with <code>libSystem</code>'s implementation going forward.
</p>
<h2 id="tools">Tools</h2>
<h3 id="vet"><code>go tool vet</code> no longer supported</h3>
<p>
The <code>go vet</code> command has been rewritten to serve as the
base for a range of different source code analysis tools. See
the <a href="https://godoc.org/golang.org/x/tools/go/analysis">golang.org/x/tools/go/analysis</a>
package for details. A side-effect is that <code>go tool vet</code>
is no longer supported. External tools that use <code>go tool
vet</code> must be changed to use <code>go
vet</code>. Using <code>go vet</code> instead of <code>go tool
vet</code> should work with all supported versions of Go.
</p>
<p>
As part of this change, the experimental <code>-shadow</code> option
is no longer available with <code>go vet</code>. Checking for
variable shadowing may now be done using
<pre>
go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
go vet -vettool=$(which shadow)
</pre>
</p>
<h3 id="tour">Tour</h3>
<p> <!-- CL 152657 -->
The Go tour is no longer included in the main binary distribution. To
run the tour locally, instead of running <code>go</code> <code>tool</code> <code>tour</code>,
manually install it:
<pre>
go get -u golang.org/x/tour
tour
</pre>
</p>
<h3 id="gocache">Build cache requirement</h3>
<p>
The <a href="/cmd/go/#hdr-Build_and_test_caching">build cache</a> is now
required as a step toward eliminating
<code>$GOPATH/pkg</code>. Setting the environment variable
<code>GOCACHE=off</code> will cause <code>go</code> commands that write to the
cache to fail.
</p>
<h3 id="binary-only">Binary-only packages</h3>
<p>
Go 1.12 is the last release that will support binary-only packages.
</p>
<h3 id="cgo">Cgo</h3>
<p>
Go 1.12 will translate the C type <code>EGLDisplay</code> to the Go type <code>uintptr</code>.
This change is similar to how Go 1.10 and newer treats Darwin's CoreFoundation
and Java's JNI types. See the
<a href="/cmd/cgo/#hdr-Special_cases">cgo documentation</a>
for more information.
</p>
<p><!-- CL 152657 -->
Mangled C names are no longer accepted in packages that use Cgo. Use the Cgo
names instead. For example, use the documented cgo name <code>C.char</code>
rather than the mangled name <code>_Ctype_char</code> that cgo generates.
</p>
<h3 id="modules">Modules</h3>
<p><!-- CL 148517 -->
When <code>GO111MODULE</code> is set to <code>on</code>, the <code>go</code>
command now supports module-aware operations outside of a module directory,
provided that those operations do not need to resolve import paths relative to
the current directory or explicitly edit the <code>go.mod</code> file.
Commands such as <code>go</code> <code>get</code>,
<code>go</code> <code>list</code>, and
<code>go</code> <code>mod</code> <code>download</code> behave as if in a
module with initially-empty requirements.
In this mode, <code>go</code> <code>env</code> <code>GOMOD</code> reports
the system's null device (<code>/dev/null</code> or <code>NUL</code>).
</p>
<p><!-- CL 146382 -->
<code>go</code> commands that download and extract modules are now safe to
invoke concurrently.
The module cache (<code>GOPATH/pkg/mod</code>) must reside in a filesystem that
supports file locking.
</p>
<p><!-- CL 147282, 147281 -->
The <code>go</code> directive in a <code>go.mod</code> file now indicates the
version of the language used by the files within that module.
It will be set to the current release
(<code>go</code> <code>1.12</code>) if no existing version is
present.
If the <code>go</code> directive for a module specifies a
version <em>newer</em> than the toolchain in use, the <code>go</code> command
will attempt to build the packages regardless, and will note the mismatch only if
that build fails.
</p>
<p><!-- CL 147282, 147281 -->
This changed use of the <code>go</code> directive means that if you
use Go 1.12 to build a module, thus recording <code>go 1.12</code>
in the <code>go.mod</code> file, you will get an error when
attempting to build the same module with Go 1.11 through Go 1.11.3.
Go 1.11.4 or later will work fine, as will releases older than Go 1.11.
If you must use Go 1.11 through 1.11.3, you can avoid the problem by
setting the language version to 1.11, using the Go 1.12 go tool,
via <code>go mod edit -go=1.11</code>.
</p>
<p><!-- CL 152739 -->
When an import cannot be resolved using the active modules,
the <code>go</code> command will now try to use the modules mentioned in the
main module's <code>replace</code> directives before consulting the module
cache and the usual network sources.
If a matching replacement is found but the <code>replace</code> directive does
not specify a version, the <code>go</code> command uses a pseudo-version
derived from the zero <code>time.Time</code> (such
as <code>v0.0.0-00010101000000-000000000000</code>).
</p>
<h3 id="compiler">Compiler toolchain</h3>
<p><!-- CL 134155, 134156 -->
The compiler's live variable analysis has improved. This may mean that
finalizers will be executed sooner in this release than in previous
releases. If that is a problem, consider the appropriate addition of a
<a href="/pkg/runtime/#KeepAlive"><code>runtime.KeepAlive</code></a> call.
</p>
<p><!-- CL 147361 -->
More functions are now eligible for inlining by default, including
functions that do nothing but call another function.
This extra inlining makes it additionally important to use
<a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>
instead of iterating over the result of
<a href="/pkg/runtime/#Callers"><code>runtime.Callers</code></a> directly.
<pre>
// Old code which no longer works correctly (it will miss inlined call frames).
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
for _, pc := range pcs[:n] {
f := runtime.FuncForPC(pc)
if f != nil {
fmt.Println(f.Name())
}
}
</pre>
<pre>
// New code which will work correctly.
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
frames := runtime.CallersFrames(pcs[:n])
for {
frame, more := frames.Next()
fmt.Println(frame.Function)
if !more {
break
}
}
</pre>
</p>
<p><!-- CL 153477 -->
Wrappers generated by the compiler to implement method expressions
are no longer reported
by <a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>
and <a href="/pkg/runtime/#Stack"><code>runtime.Stack</code></a>. They
are also not printed in panic stack traces.
This change aligns the <code>gc</code> toolchain to match
the <code>gccgo</code> toolchain, which already elided such wrappers
from stack traces.
Clients of these APIs might need to adjust for the missing
frames. For code that must interoperate between 1.11 and 1.12
releases, you can replace the method expression <code>x.M</code>
with the function literal <code>func (...) { x.M(...) } </code>.
</p>
<p><!-- CL 144340 -->
The compiler now accepts a <code>-lang</code> flag to set the Go language
version to use. For example, <code>-lang=go1.8</code> causes the compiler to
emit an error if the program uses type aliases, which were added in Go 1.9.
Language changes made before Go 1.12 are not consistently enforced.
</p>
<p><!-- CL 147160 -->
The compiler toolchain now uses different conventions to call Go
functions and assembly functions. This should be invisible to users,
except for calls that simultaneously cross between Go and
assembly <em>and</em> cross a package boundary. If linking results
in an error like "relocation target not defined for ABIInternal (but
is defined for ABI0)", please refer to the
<a href="https://github.com/golang/proposal/blob/master/design/27539-internal-abi.md#compatibility">compatibility section</a>
of the ABI design document.
</p>
<p><!-- CL 145179 -->
There have been many improvements to the DWARF debug information
produced by the compiler, including improvements to argument
printing and variable location information.
</p>
<p><!-- CL 61511 -->
Go programs now also maintain stack frame pointers on <code>linux/arm64</code>
for the benefit of profiling tools like <code>perf</code>. The frame pointer
maintenance has a small run-time overhead that varies but averages around 3%.
To build a toolchain that does not use frame pointers, set
<code>GOEXPERIMENT=noframepointer</code> when running <code>make.bash</code>.
</p>
<p><!-- CL 142717 -->
The obsolete "safe" compiler mode (enabled by the <code>-u</code> gcflag) has been removed.
</p>
<h3 id="godoc"><code>godoc</code> and <code>go</code> <code>doc</code></h3>
<p>
In Go 1.12, <code>godoc</code> no longer has a command-line interface and
is only a web server. Users should use <code>go</code> <code>doc</code>
for command-line help output instead. Go 1.12 is the last release that will
include the <code>godoc</code> webserver; in Go 1.13 it will be available
via <code>go</code> <code>get</code>.
</p>
<p><!-- CL 141977 -->
<code>go</code> <code>doc</code> now supports the <code>-all</code> flag,
which will cause it to print all exported APIs and their documentation,
as the <code>godoc</code> command line used to do.
</p>
<p><!-- CL 140959 -->
<code>go</code> <code>doc</code> also now includes the <code>-src</code> flag,
which will show the target's source code.
</p>
<h3 id="trace">Trace</h3>
<p><!-- CL 60790 -->
The trace tool now supports plotting mutator utilization curves,
including cross-references to the execution trace. These are useful
for analyzing the impact of the garbage collector on application
latency and throughput.
</p>
<h3 id="assembler">Assembler</h3>
<p><!-- CL 147218 -->
On <code>arm64</code>, the platform register was renamed from
<code>R18</code> to <code>R18_PLATFORM</code> to prevent accidental
use, as the OS could choose to reserve this register.
</p>
<h2 id="runtime">Runtime</h2>
<p><!-- CL 138959 -->
Go 1.12 significantly improves the performance of sweeping when a
large fraction of the heap remains live. This reduces allocation
latency immediately following a garbage collection.
</p>
<p><!-- CL 139719 -->
The Go runtime now releases memory back to the operating system more
aggressively, particularly in response to large allocations that
can't reuse existing heap space.
</p>
<p><!-- CL 146342, CL 146340, CL 146345, CL 146339, CL 146343, CL 146337, CL 146341, CL 146338 -->
The Go runtime's timer and deadline code is faster and scales better
with higher numbers of CPUs. In particular, this improves the
performance of manipulating network connection deadlines.
</p>
<p><!-- CL 135395 -->
On Linux, the runtime now uses <code>MADV_FREE</code> to release unused
memory. This is more efficient but may result in higher reported
RSS. The kernel will reclaim the unused data when it is needed.
To revert to the Go 1.11 behavior (<code>MADV_DONTNEED</code>), set the
environment variable <code>GODEBUG=madvdontneed=1</code>.
</p>
<p><!-- CL 149578 -->
Adding cpu.<em>extension</em>=off to the
<a href="/doc/diagnostics.html#godebug">GODEBUG</a> environment
variable now disables the use of optional CPU instruction
set extensions in the standard library and runtime. This is not
yet supported on Windows.
</p>
<p><!-- CL 158337 -->
Go 1.12 improves the accuracy of memory profiles by fixing
overcounting of large heap allocations.
</p>
<p><!-- CL 159717 -->
Tracebacks, <code>runtime.Caller</code>,
and <code>runtime.Callers</code> no longer include
compiler-generated initialization functions. Doing a traceback
during the initialization of a global variable will now show a
function named <code>PKG.init.ializers</code>.
</p>
<h2 id="library">Core library</h2>
<h3 id="tls_1_3">TLS 1.3</h3>
<p>
Go 1.12 adds opt-in support for TLS 1.3 in the <code>crypto/tls</code> package as
specified by <a href="https://www.rfc-editor.org/info/rfc8446">RFC 8446</a>. It can
be enabled by adding the value <code>tls13=1</code> to the <code>GODEBUG</code>
environment variable. It will be enabled by default in Go 1.13.
</p>
<p>
To negotiate TLS 1.3, make sure you do not set an explicit <code>MaxVersion</code> in
<a href="/pkg/crypto/tls/#Config"><code>Config</code></a> and run your program with
the environment variable <code>GODEBUG=tls13=1</code> set.
</p>
<p>
All TLS 1.2 features except <code>TLSUnique</code> in
<a href="/pkg/crypto/tls/#ConnectionState"><code>ConnectionState</code></a>
and renegotiation are available in TLS 1.3 and provide equivalent or
better security and performance. Note that even though TLS 1.3 is backwards
compatible with previous versions, certain legacy systems might not work
correctly when attempting to negotiate it. RSA certificate keys too small
to be secure (including 512-bit keys) will not work with TLS 1.3.
</p>
<p>
TLS 1.3 cipher suites are not configurable. All supported cipher suites are
safe, and if <code>PreferServerCipherSuites</code> is set in
<a href="/pkg/crypto/tls/#Config"><code>Config</code></a> the preference order
is based on the available hardware.
</p>
<p>
Early data (also called "0-RTT mode") is not currently supported as a
client or server. Additionally, a Go 1.12 server does not support skipping
unexpected early data if a client sends it. Since TLS 1.3 0-RTT mode
involves clients keeping state regarding which servers support 0-RTT,
a Go 1.12 server cannot be part of a load-balancing pool where some other
servers do support 0-RTT. If switching a domain from a server that supported
0-RTT to a Go 1.12 server, 0-RTT would have to be disabled for at least the
lifetime of the issued session tickets before the switch to ensure
uninterrupted operation.
</p>
<p>
In TLS 1.3 the client is the last one to speak in the handshake, so if it causes
an error to occur on the server, it will be returned on the client by the first
<a href="/pkg/crypto/tls/#Conn.Read"><code>Read</code></a>, not by
<a href="/pkg/crypto/tls/#Conn.Handshake"><code>Handshake</code></a>. For
example, that will be the case if the server rejects the client certificate.
Similarly, session tickets are now post-handshake messages, so are only
received by the client upon its first
<a href="/pkg/crypto/tls/#Conn.Read"><code>Read</code></a>.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
As always, there are various minor changes and updates to the library,
made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
in mind.
</p>
<!-- TODO: CL 115677: https://golang.org/cl/115677: cmd/vet: check embedded field tags too -->
<dl id="bufio"><dt><a href="/pkg/bufio/">bufio</a></dt>
<dd>
<p><!-- CL 149297 -->
<code>Reader</code>'s <a href="/pkg/bufio/#Reader.UnreadRune"><code>UnreadRune</code></a> and
<a href="/pkg/bufio/#Reader.UnreadByte"><code>UnreadByte</code></a> methods will now return an error
if they are called after <a href="/pkg/bufio/#Reader.Peek"><code>Peek</code></a>.
</p>
</dl><!-- bufio -->
<dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
<dd>
<p><!-- CL 137855 -->
The new function <a href="/pkg/bytes/#ReplaceAll"><code>ReplaceAll</code></a> returns a copy of
a byte slice with all non-overlapping instances of a value replaced by another.
</p>
<p><!-- CL 145098 -->
A pointer to a zero-value <a href="/pkg/bytes/#Reader"><code>Reader</code></a> is now
functionally equivalent to <a href="/pkg/bytes/#NewReader"><code>NewReader</code></a><code>(nil)</code>.
Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
</p>
</dl><!-- bytes -->
<dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
<dd>
<p><!-- CL 139419 -->
A warning will now be printed to standard error the first time
<code>Reader.Read</code> is blocked for more than 60 seconds waiting
to read entropy from the kernel.
</p>
<p><!-- CL 120055 -->
On FreeBSD, <code>Reader</code> now uses the <code>getrandom</code>
system call if available, <code>/dev/urandom</code> otherwise.
</p>
</dl><!-- crypto/rand -->
<dl id="crypto/rc4"><dt><a href="/pkg/crypto/rc4/">crypto/rc4</a></dt>
<dd>
<p><!-- CL 130397 -->
This release removes the assembly implementations, leaving only
the pure Go version. The Go compiler generates code that is
either slightly better or slightly worse, depending on the exact
CPU. RC4 is insecure and should only be used for compatibility
with legacy systems.
</p>
</dl><!-- crypto/rc4 -->
<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
<dd>
<p><!-- CL 143177 -->
If a client sends an initial message that does not look like TLS, the server
will no longer reply with an alert, and it will expose the underlying
<code>net.Conn</code> in the new field <code>Conn</code> of
<a href="/pkg/crypto/tls/#RecordHeaderError"><code>RecordHeaderError</code></a>.
</p>
</dl><!-- crypto/tls -->
<dl id="database/sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>
<dd>
<p><!-- CL 145738 -->
A query cursor can now be obtained by passing a
<a href="/pkg/database/sql/#Rows"><code>*Rows</code></a>
value to the <a href="/pkg/database/sql/#Row.Scan"><code>Row.Scan</code></a> method.
</p>
</dl><!-- database/sql -->
<dl id="expvar"><dt><a href="/pkg/expvar/">expvar</a></dt>
<dd>
<p><!-- CL 139537 -->
The new <a href="/pkg/expvar/#Map.Delete"><code>Delete</code></a> method allows
for deletion of key/value pairs from a <a href="/pkg/expvar/#Map"><code>Map</code></a>.
</p>
</dl><!-- expvar -->
<dl id="fmt"><dt><a href="/pkg/fmt/">fmt</a></dt>
<dd>
<p><!-- CL 142737 -->
Maps are now printed in key-sorted order to ease testing. The ordering rules are:
<ul>
<li>When applicable, nil compares low
<li>ints, floats, and strings order by <
<li>NaN compares less than non-NaN floats
<li>bool compares false before true
<li>Complex compares real, then imaginary
<li>Pointers compare by machine address
<li>Channel values compare by machine address
<li>Structs compare each field in turn
<li>Arrays compare each element in turn
<li>Interface values compare first by <code>reflect.Type</code> describing the concrete type
and then by concrete value as described in the previous rules.
</ul>
</p>
<p><!-- CL 129777 -->
When printing maps, non-reflexive key values like <code>NaN</code> were previously
displayed as <code>&lt;nil&gt;</code>. As of this release, the correct values are printed.
</p>
</dl><!-- fmt -->
<dl id="go/doc"><dt><a href="/pkg/go/doc/">go/doc</a></dt>
<dd>
<p><!-- CL 140958 -->
To address some outstanding issues in <a href="/cmd/doc/"><code>cmd/doc</code></a>,
this package has a new <a href="/pkg/go/doc/#Mode"><code>Mode</code></a> bit,
<code>PreserveAST</code>, which controls whether AST data is cleared.
</p>
</dl><!-- go/doc -->
<dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
<dd>
<p><!-- CL 134075 -->
The <a href="/pkg/go/token#File"><code>File</code></a> type has a new
<a href="/pkg/go/token#File.LineStart"><code>LineStart</code></a> field,
which returns the position of the start of a given line. This is especially useful
in programs that occasionally handle non-Go files, such as assembly, but wish to use
the <code>token.Pos</code> mechanism to identify file positions.
</p>
</dl><!-- go/token -->
<dl id="image"><dt><a href="/pkg/image/">image</a></dt>
<dd>
<p><!-- CL 118755 -->
The <a href="/pkg/image/#RegisterFormat"><code>RegisterFormat</code></a> function is now safe for concurrent use.
</p>
</dl><!-- image -->
<dl id="image/png"><dt><a href="/pkg/image/png/">image/png</a></dt>
<dd>
<p><!-- CL 134235 -->
Paletted images with fewer than 16 colors now encode to smaller outputs.
</p>
</dl><!-- image/png -->
<dl id="io"><dt><a href="/pkg/io/">io</a></dt>
<dd>
<p><!-- CL 139457 -->
The new <a href="/pkg/io#StringWriter"><code>StringWriter</code></a> interface wraps the
<a href="/pkg/io/#WriteString"><code>WriteString</code></a> function.
</p>
</dl><!-- io -->
<dl id="math"><dt><a href="/pkg/math/">math</a></dt>
<dd>
<p><!-- CL 153059 -->
The functions
<a href="/pkg/math/#Sin"><code>Sin</code></a>,
<a href="/pkg/math/#Cos"><code>Cos</code></a>,
<a href="/pkg/math/#Tan"><code>Tan</code></a>,
and <a href="/pkg/math/#Sincos"><code>Sincos</code></a> now
apply Payne-Hanek range reduction to huge arguments. This
produces more accurate answers, but they will not be bit-for-bit
identical with the results in earlier releases.
</p>
</dl><!-- math -->
<dl id="math/bits"><dt><a href="/pkg/math/bits/">math/bits</a></dt>
<dd>
<p><!-- CL 123157 -->
New extended precision operations <a href="/pkg/math/bits/#Add"><code>Add</code></a>, <a href="/pkg/math/bits/#Sub"><code>Sub</code></a>, <a href="/pkg/math/bits/#Mul"><code>Mul</code></a>, and <a href="/pkg/math/bits/#Div"><code>Div</code></a> are available in <code>uint</code>, <code>uint32</code>, and <code>uint64</code> versions.
</p>
</dl><!-- math/bits -->
<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
<dd>
<p><!-- CL 146659 -->
The
<a href="/pkg/net/#Dialer.DualStack"><code>Dialer.DualStack</code></a> setting is now ignored and deprecated;
RFC 6555 Fast Fallback ("Happy Eyeballs") is now enabled by default. To disable, set
<a href="/pkg/net/#Dialer.FallbackDelay"><code>Dialer.FallbackDelay</code></a> to a negative value.
</p>
<p><!-- CL 107196 -->
Similarly, TCP keep-alives are now enabled by default if
<a href="/pkg/net/#Dialer.KeepAlive"><code>Dialer.KeepAlive</code></a> is zero.
To disable, set it to a negative value.
</p>
<p><!-- CL 113997 -->
On Linux, the <a href="http://man7.org/linux/man-pages/man2/splice.2.html"><code>splice</code> system call</a> is now used when copying from a
<a href="/pkg/net/#UnixConn"><code>UnixConn</code></a> to a
<a href="/pkg/net/#TCPConn"><code>TCPConn</code></a>.
</p>
</dl><!-- net -->
<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
<dd>
<p><!-- CL 143177 -->
The HTTP server now rejects misdirected HTTP requests to HTTPS servers with a plaintext "400 Bad Request" response.
</p>
<p><!-- CL 130115 -->
The new <a href="/pkg/net/http/#Client.CloseIdleConnections"><code>Client.CloseIdleConnections</code></a>
method calls the <code>Client</code>'s underlying <code>Transport</code>'s <code>CloseIdleConnections</code>
if it has one.
</p>
<p><!-- CL 145398 -->
The <a href="/pkg/net/http/#Transport"><code>Transport</code></a> no longer rejects HTTP responses which declare
HTTP Trailers but don't use chunked encoding. Instead, the declared trailers are now just ignored.
</p>
<p><!-- CL 152080 --> <!-- CL 151857 -->
The <a href="/pkg/net/http/#Transport"><code>Transport</code></a> no longer handles <code>MAX_CONCURRENT_STREAMS</code> values
advertised from HTTP/2 servers as strictly as it did during Go 1.10 and Go 1.11. The default behavior is now back
to how it was in Go 1.9: each connection to a server can have up to <code>MAX_CONCURRENT_STREAMS</code> requests
active and then new TCP connections are created as needed. In Go 1.10 and Go 1.11 the <code>http2</code> package
would block and wait for requests to finish instead of creating new connections.
To get the stricter behavior back, import the
<a href="https://godoc.org/golang.org/x/net/http2"><code>golang.org/x/net/http2</code></a> package
directly and set
<a href="https://godoc.org/golang.org/x/net/http2#Transport.StrictMaxConcurrentStreams"><code>Transport.StrictMaxConcurrentStreams</code></a> to
<code>true</code>.
</p>
</dl><!-- net/http -->
<dl id="net/url"><dt><a href="/pkg/net/url/">net/url</a></dt>
<dd>
<p><!-- CL 159157, CL 160178 -->
<a href="/pkg/net/url/#Parse"><code>Parse</code></a>,
<a href="/pkg/net/url/#ParseRequestURI"><code>ParseRequestURI</code></a>,
and
<a href="/pkg/net/url/#URL.Parse"><code>URL.Parse</code></a>
now return an
error for URLs containing ASCII control characters, which includes NULL,
tab, and newlines.
</p>
</dl><!-- net/url -->
<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
<dd>
<p><!-- CL 146437 -->
The <a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a> now automatically
proxies WebSocket requests.
</p>
</dl><!-- net/http/httputil -->
<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
<dd>
<p><!-- CL 125443 -->
The new <a href="/pkg/os/#ProcessState.ExitCode"><code>ProcessState.ExitCode</code></a> method
returns the process's exit code.
</p>
<p><!-- CL 135075 -->
<code>ModeCharDevice</code> has been added to the <code>ModeType</code> bitmask, allowing for
<code>ModeDevice | ModeCharDevice</code> to be recovered when masking a
<a href="/pkg/os/#FileMode"><code>FileMode</code></a> with <code>ModeType</code>.
</p>
<p><!-- CL 139418 -->
The new function <a href="/pkg/os/#UserHomeDir"><code>UserHomeDir</code></a> returns the
current user's home directory.
</p>
<p><!-- CL 146020 -->
<a href="/pkg/os/#RemoveAll"><code>RemoveAll</code></a> now supports paths longer than 4096 characters
on most Unix systems.
</p>
<p><!-- CL 130676 -->
<a href="/pkg/os/#File.Sync"><code>File.Sync</code></a> now uses <code>F_FULLFSYNC</code> on macOS
to correctly flush the file contents to permanent storage.
This may cause the method to run more slowly than in previous releases.
</p>
<p><!--CL 155517 -->
<a href="/pkg/os/#File"><code>File</code></a> now supports
a <a href="/pkg/os/#File.SyscallConn"><code>SyscallConn</code></a>
method returning
a <a href="/pkg/syscall/#RawConn"><code>syscall.RawConn</code></a>
interface value. This may be used to invoke system-specific
operations on the underlying file descriptor.
</p>
</dl><!-- os -->
<dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
<dd>
<p><!-- CL 145220 -->
The <a href="/pkg/path/filepath/#IsAbs"><code>IsAbs</code></a> function now returns true when passed
a reserved filename on Windows such as <code>NUL</code>.
<a href="https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file#naming-conventions">List of reserved names.</a>
</p>
</dl><!-- path/filepath -->
<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
<dd>
<p><!-- CL 33572 -->
A new <a href="/pkg/reflect#MapIter"><code>MapIter</code></a> type is
an iterator for ranging over a map. This type is exposed through the
<a href="/pkg/reflect#Value"><code>Value</code></a> type's new
<a href="/pkg/reflect#Value.MapRange"><code>MapRange</code></a> method.
This follows the same iteration semantics as a range statement, with <code>Next</code>
to advance the iterator, and <code>Key</code>/<code>Value</code> to access each entry.
</p>
</dl><!-- reflect -->
<dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
<dd>
<p><!-- CL 139784 -->
<a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a> is no longer necessary
to avoid lock contention, so it has been given a partial deprecation comment.
<a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a>
may still be appropriate if the reason for its use is to make two copies with
different <a href="/pkg/regexp/#Regexp.Longest"><code>Longest</code></a> settings.
</p>
</dl><!-- regexp -->
<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>
<dd>
<p><!-- CL 144220 -->
A new <a href="/pkg/runtime/debug/#BuildInfo"><code>BuildInfo</code></a> type
exposes the build information read from the running binary, available only in
binaries built with module support. This includes the main package path, main
module information, and the module dependencies. This type is given through the
<a href="/pkg/runtime/debug/#ReadBuildInfo"><code>ReadBuildInfo</code></a> function
on <a href="/pkg/runtime/debug/#BuildInfo"><code>BuildInfo</code></a>.
</p>
</dl><!-- runtime/debug -->
<dl id="strings"><dt><a href="/pkg/strings/">strings</a></dt>
<dd>
<p><!-- CL 137855 -->
The new function <a href="/pkg/strings/#ReplaceAll"><code>ReplaceAll</code></a> returns a copy of
a string with all non-overlapping instances of a value replaced by another.
</p>
<p><!-- CL 145098 -->
A pointer to a zero-value <a href="/pkg/strings/#Reader"><code>Reader</code></a> is now
functionally equivalent to <a href="/pkg/strings/#NewReader"><code>NewReader</code></a><code>(nil)</code>.
Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
</p>
<p><!-- CL 122835 -->
The new <a href="/pkg/strings/#Builder.Cap"><code>Builder.Cap</code></a> method returns the capacity of the builder's underlying byte slice.
</p>
<p><!-- CL 131495 -->
The character mapping functions <a href="/pkg/strings/#Map"><code>Map</code></a>,
<a href="/pkg/strings/#Title"><code>Title</code></a>,
<a href="/pkg/strings/#ToLower"><code>ToLower</code></a>,
<a href="/pkg/strings/#ToLowerSpecial"><code>ToLowerSpecial</code></a>,
<a href="/pkg/strings/#ToTitle"><code>ToTitle</code></a>,
<a href="/pkg/strings/#ToTitleSpecial"><code>ToTitleSpecial</code></a>,
<a href="/pkg/strings/#ToUpper"><code>ToUpper</code></a>, and
<a href="/pkg/strings/#ToUpperSpecial"><code>ToUpperSpecial</code></a>
now always guarantee to return valid UTF-8. In earlier releases, if the input was invalid UTF-8 but no character replacements
needed to be applied, these routines incorrectly returned the invalid UTF-8 unmodified.
</p>
</dl><!-- strings -->
<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
<dd>
<p><!-- CL 138595 -->
64-bit inodes are now supported on FreeBSD 12. Some types have been adjusted accordingly.
</p>
<p><!-- CL 125456 -->
The Unix socket
(<a href="https://blogs.msdn.microsoft.com/commandline/2017/12/19/af_unix-comes-to-windows/"><code>AF_UNIX</code></a>)
address family is now supported for compatible versions of Windows.
</p>
<p><!-- CL 147117 -->
The new function <a href="/pkg/syscall/?GOOS=windows&GOARCH=amd64#Syscall18"><code>Syscall18</code></a>
has been introduced for Windows, allowing for calls with up to 18 arguments.
</p>
</dl><!-- syscall -->
<dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>
<dd>
<p><!-- CL 153559 -->
<p>
The <code>Callback</code> type and <code>NewCallback</code> function have been renamed;
they are now called
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Func"><code>Func</code></a> and
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#FuncOf"><code>FuncOf</code></a>, respectively.
This is a breaking change, but WebAssembly support is still experimental
and not yet subject to the
<a href="/doc/go1compat">Go 1 compatibility promise</a>. Any code using the
old names will need to be updated.
</p>
<p><!-- CL 141644 -->
If a type implements the new
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Wrapper"><code>Wrapper</code></a>
interface,
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#ValueOf"><code>ValueOf</code></a>
will use it to return the JavaScript value for that type.
</p>
<p><!-- CL 143137 -->
The meaning of the zero
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Value"><code>Value</code></a>
has changed. It now represents the JavaScript <code>undefined</code> value
instead of the number zero.
This is a breaking change, but WebAssembly support is still experimental
and not yet subject to the
<a href="/doc/go1compat">Go 1 compatibility promise</a>. Any code relying on
the zero <a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Value"><code>Value</code></a>
to mean the number zero will need to be updated.
</p>
<p><!-- CL 144384 -->
The new
<a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Value.Truthy"><code>Value.Truthy</code></a>
method reports the
<a href="https://developer.mozilla.org/en-US/docs/Glossary/Truthy">JavaScript "truthiness"</a>
of a given value.
</p>
</dl><!-- syscall/js -->
<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
<dd>
<p><!-- CL 139258 -->
The <a href="/cmd/go/#hdr-Testing_flags"><code>-benchtime</code></a> flag now supports setting an explicit iteration count instead of a time when the value ends with an "<code>x</code>". For example, <code>-benchtime=100x</code> runs the benchmark 100 times.
</p>
</dl><!-- testing -->
<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
<dd>
<p><!-- CL 142217 -->
When executing a template, long context values are no longer truncated in errors.
</p>
<p>
<code>executing "tmpl" at <.very.deep.context.v...>: map has no entry for key "notpresent"</code>
</p>
<p>
is now
</p>
<p>
<code>executing "tmpl" at <.very.deep.context.value.notpresent>: map has no entry for key "notpresent"</code>
</p>
<dd>
<p><!-- CL 143097 -->
If a user-defined function called by a template panics, the
panic is now caught and returned as an error by
the <code>Execute</code> or <code>ExecuteTemplate</code> method.
</p>
</dl><!-- text/template -->
<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
<dd>
<p><!-- CL 151299 -->
The time zone database in <code>$GOROOT/lib/time/zoneinfo.zip</code>
has been updated to version 2018i. Note that this ZIP file is
only used if a time zone database is not provided by the operating
system.
</p>
</dl><!-- time -->
<dl id="unsafe"><dt><a href="/pkg/unsafe/">unsafe</a></dt>
<dd>
<p><!-- CL 146058 -->
It is invalid to convert a nil <code>unsafe.Pointer</code> to <code>uintptr</code> and back with arithmetic.
(This was already invalid, but will now cause the compiler to misbehave.)
</p>
</dl><!-- unsafe -->

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<!--{
"Title": "Go 1.14 Release Notes",
"Path": "/doc/go1.14"
}-->
<!--
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
-->
<style>
main ul li { margin: 0.5em 0; }
</style>
<h2 id="introduction">Introduction to Go 1.14</h2>
<p>
The latest Go release, version 1.14, arrives six months after <a href="go1.13">Go 1.13</a>.
Most of its changes are in the implementation of the toolchain, runtime, and libraries.
As always, the release maintains the Go 1 <a href="/doc/go1compat.html">promise of compatibility</a>.
We expect almost all Go programs to continue to compile and run as before.
</p>
<p>
Module support in the <code>go</code> command is now ready for production use,
and we encourage all users to <a href="https://blog.golang.org/migrating-to-go-modules">migrate to Go
modules for dependency management</a>. If you are unable to migrate due to a problem in the Go
toolchain, please ensure that the problem has an
<a href="https://golang.org/issue?q=is%3Aissue+is%3Aopen+label%3Amodules">open issue</a>
filed. (If the issue is not on the <code>Go1.15</code> milestone, please let us
know why it prevents you from migrating so that we can prioritize it
appropriately.)
</p>
<h2 id="language">Changes to the language</h2>
<p>
Per the <a href="https://github.com/golang/proposal/blob/master/design/6977-overlapping-interfaces.md">overlapping interfaces proposal</a>,
Go 1.14 now permits embedding of interfaces with overlapping method sets:
methods from an embedded interface may have the same names and identical signatures
as methods already present in the (embedding) interface. This solves problems that typically
(but not exclusively) occur with diamond-shaped embedding graphs.
Explicitly declared methods in an interface must remain
<a href="https://tip.golang.org/ref/spec#Uniqueness_of_identifiers">unique</a>, as before.
</p>
<h2 id="ports">Ports</h2>
<h3 id="darwin">Darwin</h3>
<p>
Go 1.14 is the last release that will run on macOS 10.11 El Capitan.
Go 1.15 will require macOS 10.12 Sierra or later.
</p>
<p><!-- golang.org/issue/34749 -->
Go 1.14 is the last Go release to support 32-bit binaries on
macOS (the <code>darwin/386</code> port). They are no longer
supported by macOS, starting with macOS 10.15 (Catalina).
Go continues to support the 64-bit <code>darwin/amd64</code> port.
</p>
<p><!-- golang.org/issue/34751 -->
Go 1.14 will likely be the last Go release to support 32-bit
binaries on iOS, iPadOS, watchOS, and tvOS
(the <code>darwin/arm</code> port). Go continues to support the
64-bit <code>darwin/arm64</code> port.
</p>
<h3 id="windows">Windows</h3>
<p><!-- CL 203601 -->
Go binaries on Windows now
have <a href="https://docs.microsoft.com/en-us/windows/win32/memory/data-execution-prevention">DEP
(Data Execution Prevention)</a> enabled.
</p>
<p><!-- CL 202439 -->
On Windows, creating a file
via <a href="/pkg/os#CreateFile"><code>os.OpenFile</code></a> with
the <a href="/pkg/os/#O_CREATE"><code>os.O_CREATE</code></a> flag, or
via <a href="/pkg/syscall#Open"><code>syscall.Open</code></a> with
the <a href="/pkg/syscall#O_CREAT"><code>syscall.O_CREAT</code></a>
flag, will now create the file as read-only if the
bit <code>0o200</code> (owner write permission) is not set in the
permission argument. This makes the behavior on Windows more like
that on Unix systems.
</p>
<h3 id="wasm">WebAssembly</h3>
<p><!-- CL 203600 -->
JavaScript values referenced from Go via <code>js.Value</code>
objects can now be garbage collected.
</p>
<p><!-- CL 203600 -->
<code>js.Value</code> values can no longer be compared using
the <code>==</code> operator, and instead must be compared using
their <code>Equal</code> method.
</p>
<p><!-- CL 203600 -->
<code>js.Value</code> now
has <code>IsUndefined</code>, <code>IsNull</code>,
and <code>IsNaN</code> methods.
</p>
<h3 id="riscv">RISC-V</h3>
<p><!-- Issue 27532 -->
Go 1.14 contains experimental support for 64-bit RISC-V on Linux
(<code>GOOS=linux</code>, <code>GOARCH=riscv64</code>). Be aware
that performance, assembly syntax stability, and possibly
correctness are a work in progress.
</p>
<h3 id="freebsd">FreeBSD</h3>
<p><!-- CL 199919 -->
Go now supports the 64-bit ARM architecture on FreeBSD 12.0 or later (the
<code>freebsd/arm64</code> port).
</p>
<h3 id="nacl">Native Client (NaCl)</h3>
<p><!-- golang.org/issue/30439 -->
As <a href="go1.13#ports">announced</a> in the Go 1.13 release notes,
Go 1.14 drops support for the Native Client platform (<code>GOOS=nacl</code>).
</p>
<h3 id="illumos">Illumos</h3>
<p><!-- CL 203758 -->
The runtime now respects zone CPU caps
(the <code>zone.cpu-cap</code> resource control)
for <code>runtime.NumCPU</code> and the default value
of <code>GOMAXPROCS</code>.
</p>
<h2 id="tools">Tools</h2>
<h3 id="go-command">Go command</h3>
<h4 id="vendor">Vendoring</h4>
<!-- golang.org/issue/33848 -->
<p>
When the main module contains a top-level <code>vendor</code> directory and
its <code>go.mod</code> file specifies <code>go</code> <code>1.14</code> or
higher, the <code>go</code> command now defaults to <code>-mod=vendor</code>
for operations that accept that flag. A new value for that flag,
<code>-mod=mod</code>, causes the <code>go</code> command to instead load
modules from the module cache (as when no <code>vendor</code> directory is
present).
</p>
<p>
When <code>-mod=vendor</code> is set (explicitly or by default), the
<code>go</code> command now verifies that the main module's
<code>vendor/modules.txt</code> file is consistent with its
<code>go.mod</code> file.
</p>
<p>
<code>go</code> <code>list</code> <code>-m</code> no longer silently omits
transitive dependencies that do not provide packages in
the <code>vendor</code> directory. It now fails explicitly if
<code>-mod=vendor</code> is set and information is requested for a module not
mentioned in <code>vendor/modules.txt</code>.
</p>
<h4 id="go-flags">Flags</h4>
<p><!-- golang.org/issue/32502, golang.org/issue/30345 -->
The <code>go</code> <code>get</code> command no longer accepts
the <code>-mod</code> flag. Previously, the flag's setting either
<a href="https://golang.org/issue/30345">was ignored</a> or
<a href="https://golang.org/issue/32502">caused the build to fail</a>.
</p>
<p><!-- golang.org/issue/33326 -->
<code>-mod=readonly</code> is now set by default when the <code>go.mod</code>
file is read-only and no top-level <code>vendor</code> directory is present.
</p>
<p><!-- golang.org/issue/31481 -->
<code>-modcacherw</code> is a new flag that instructs the <code>go</code>
command to leave newly-created directories in the module cache at their
default permissions rather than making them read-only.
The use of this flag makes it more likely that tests or other tools will
accidentally add files not included in the module's verified checksum.
However, it allows the use of <code>rm</code> <code>-rf</code>
(instead of <code>go</code> <code>clean</code> <code>-modcache</code>)
to remove the module cache.
</p>
<p><!-- golang.org/issue/34506 -->
<code>-modfile=file</code> is a new flag that instructs the <code>go</code>
command to read (and possibly write) an alternate <code>go.mod</code> file
instead of the one in the module root directory. A file
named <code>go.mod</code> must still be present in order to determine the
module root directory, but it is not accessed. When <code>-modfile</code> is
specified, an alternate <code>go.sum</code> file is also used: its path is
derived from the <code>-modfile</code> flag by trimming the <code>.mod</code>
extension and appending <code>.sum</code>.
</p>
<h4 id="go-env-vars">Environment variables</h4>
<p><!-- golang.org/issue/32966 -->
<code>GOINSECURE</code> is a new environment variable that instructs
the <code>go</code> command to not require an HTTPS connection, and to skip
certificate validation, when fetching certain modules directly from their
origins. Like the existing <code>GOPRIVATE</code> variable, the value
of <code>GOINSECURE</code> is a comma-separated list of glob patterns.
</p>
<h4 id="commands-outside-modules">Commands outside modules</h4>
<p><!-- golang.org/issue/32027 -->
When module-aware mode is enabled explicitly (by setting
<code>GO111MODULE=on</code>), most module commands have more
limited functionality if no <code>go.mod</code> file is present. For
example, <code>go</code> <code>build</code>,
<code>go</code> <code>run</code>, and other build commands can only build
packages in the standard library and packages specified as <code>.go</code>
files on the command line.
</p>
<p>
Previously, the <code>go</code> command would resolve each package path
to the latest version of a module but would not record the module path
or version. This resulted in <a href="https://golang.org/issue/32027">slow,
non-reproducible builds</a>.
</p>
<p>
<code>go</code> <code>get</code> continues to work as before, as do
<code>go</code> <code>mod</code> <code>download</code> and
<code>go</code> <code>list</code> <code>-m</code> with explicit versions.
</p>
<h4 id="incompatible-versions"><code>+incompatible</code> versions</h4>
<!-- golang.org/issue/34165 -->
<p>
If the latest version of a module contains a <code>go.mod</code> file,
<code>go</code> <code>get</code> will no longer upgrade to an
<a href="/cmd/go/#hdr-Module_compatibility_and_semantic_versioning">incompatible</a>
major version of that module unless such a version is requested explicitly
or is already required.
<code>go</code> <code>list</code> also omits incompatible major versions
for such a module when fetching directly from version control, but may
include them if reported by a proxy.
</p>
<h4 id="go.mod"><code>go.mod</code> file maintenance</h4>
<!-- golang.org/issue/34822 -->
<p>
<code>go</code> commands other than
<code>go</code> <code>mod</code> <code>tidy</code> no longer
remove a <code>require</code> directive that specifies a version of an indirect dependency
that is already implied by other (transitive) dependencies of the main
module.
</p>
<p>
<code>go</code> commands other than
<code>go</code> <code>mod</code> <code>tidy</code> no longer
edit the <code>go.mod</code> file if the changes are only cosmetic.
</p>
<p>
When <code>-mod=readonly</code> is set, <code>go</code> commands will no
longer fail due to a missing <code>go</code> directive or an erroneous
<code>//&nbsp;indirect</code> comment.
</p>
<h4 id="module-downloading">Module downloading</h4>
<p><!-- golang.org/issue/26092 -->
The <code>go</code> command now supports Subversion repositories in module mode.
</p>
<p><!-- golang.org/issue/30748 -->
The <code>go</code> command now includes snippets of plain-text error messages
from module proxies and other HTTP servers.
An error message will only be shown if it is valid UTF-8 and consists of only
graphic characters and spaces.
</p>
<h4 id="go-test">Testing</h4>
<p><!-- golang.org/issue/24929 -->
<code>go test -v</code> now streams <code>t.Log</code> output as it happens,
rather than at the end of all tests.
</p>
<h2 id="runtime">Runtime</h2>
<p><!-- CL 190098 -->
This release improves the performance of most uses
of <code>defer</code> to incur almost zero overhead compared to
calling the deferred function directly.
As a result, <code>defer</code> can now be used in
performance-critical code without overhead concerns.
</p>
<p><!-- CL 201760, CL 201762 and many others -->
Goroutines are now asynchronously preemptible.
As a result, loops without function calls no longer potentially
deadlock the scheduler or significantly delay garbage collection.
This is supported on all platforms except <code>windows/arm</code>,
<code>darwin/arm</code>, <code>js/wasm</code>, and
<code>plan9/*</code>.
</p>
<p>
A consequence of the implementation of preemption is that on Unix
systems, including Linux and macOS systems, programs built with Go
1.14 will receive more signals than programs built with earlier
releases.
This means that programs that use packages
like <a href="/pkg/syscall/"><code>syscall</code></a>
or <a href="https://godoc.org/golang.org/x/sys/unix"><code>golang.org/x/sys/unix</code></a>
will see more slow system calls fail with <code>EINTR</code> errors.
Those programs will have to handle those errors in some way, most
likely looping to try the system call again. For more
information about this
see <a href="http://man7.org/linux/man-pages/man7/signal.7.html"><code>man
7 signal</code></a> for Linux systems or similar documentation for
other systems.
</p>
<p><!-- CL 201765, CL 195701 and many others -->
The page allocator is more efficient and incurs significantly less
lock contention at high values of <code>GOMAXPROCS</code>.
This is most noticeable as lower latency and higher throughput for
large allocations being done in parallel and at a high rate.
</p>
<p><!-- CL 171844 and many others -->
Internal timers, used by
<a href="/pkg/time/#After"><code>time.After</code></a>,
<a href="/pkg/time/#Tick"><code>time.Tick</code></a>,
<a href="/pkg/net/#Conn"><code>net.Conn.SetDeadline</code></a>,
and friends, are more efficient, with less lock contention and fewer
context switches.
This is a performance improvement that should not cause any user
visible changes.
</p>
<h2 id="compiler">Compiler</h2>
<p><!-- CL 162237 -->
This release adds <code>-d=checkptr</code> as a compile-time option
for adding instrumentation to check that Go code is following
<code>unsafe.Pointer</code> safety rules dynamically.
This option is enabled by default (except on Windows) with
the <code>-race</code> or <code>-msan</code> flags, and can be
disabled with <code>-gcflags=all=-d=checkptr=0</code>.
Specifically, <code>-d=checkptr</code> checks the following:
</p>
<ol>
<li>
When converting <code>unsafe.Pointer</code> to <code>*T</code>,
the resulting pointer must be aligned appropriately
for <code>T</code>.
</li>
<li>
If the result of pointer arithmetic points into a Go heap object,
one of the <code>unsafe.Pointer</code>-typed operands must point
into the same object.
</li>
</ol>
<p>
Using <code>-d=checkptr</code> is not currently recommended on
Windows because it causes false alerts in the standard library.
</p>
<p><!-- CL 204338 -->
The compiler can now emit machine-readable logs of key optimizations
using the <code>-json</code> flag, including inlining, escape
analysis, bounds-check elimination, and nil-check elimination.
</p>
<p><!-- CL 196959 -->
Detailed escape analysis diagnostics (<code>-m=2</code>) now work again.
This had been dropped from the new escape analysis implementation in
the previous release.
</p>
<p><!-- CL 196217 -->
All Go symbols in macOS binaries now begin with an underscore,
following platform conventions.
</p>
<p><!-- CL 202117 -->
This release includes experimental support for compiler-inserted
coverage instrumentation for fuzzing.
See <a href="https://golang.org/issue/14565">issue 14565</a> for more
details.
This API may change in future releases.
</p>
<p><!-- CL 174704 --><!-- CL 196784 -->
Bounds check elimination now uses information from slice creation and can
eliminate checks for indexes with types smaller than <code>int</code>.
</p>
<h2 id="library">Core library</h2>
<h3 id="hash/maphash">New byte sequence hashing package</h3>
<p> <!-- golang.org/issue/28322, CL 186877 -->
Go 1.14 includes a new package,
<a href="/pkg/hash/maphash/"><code>hash/maphash</code></a>,
which provides hash functions on byte sequences.
These hash functions are intended to be used to implement hash tables or
other data structures that need to map arbitrary strings or byte
sequences to a uniform distribution on unsigned 64-bit integers.
</p>
<p>
The hash functions are collision-resistant but not cryptographically secure.
</p>
<p>
The hash value of a given byte sequence is consistent within a
single process, but will be different in different processes.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
As always, there are various minor changes and updates to the library,
made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
in mind.
</p>
<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
<dd>
<p><!-- CL 191976 -->
Support for SSL version 3.0 (SSLv3) has been removed. Note that SSLv3 is the
<a href="https://tools.ietf.org/html/rfc7568">cryptographically broken</a>
protocol predating TLS.
</p>
<p><!-- CL 191999 -->
TLS 1.3 can't be disabled via the <code>GODEBUG</code> environment
variable anymore. Use the
<a href="/pkg/crypto/tls/#Config.MaxVersion"><code>Config.MaxVersion</code></a>
field to configure TLS versions.
</p>
<p><!-- CL 205059 -->
When multiple certificate chains are provided through the
<a href="/pkg/crypto/tls/#Config.Certificates"><code>Config.Certificates</code></a>
field, the first one compatible with the peer is now automatically
selected. This allows for example providing an ECDSA and an RSA
certificate, and letting the package automatically select the best one.
Note that the performance of this selection is going to be poor unless the
<a href="/pkg/crypto/tls/#Certificate.Leaf"><code>Certificate.Leaf</code></a>
field is set. The
<a href="/pkg/crypto/tls/#Config.NameToCertificate"><code>Config.NameToCertificate</code></a>
field, which only supports associating a single certificate with
a give name, is now deprecated and should be left as <code>nil</code>.
Similarly the
<a href="/pkg/crypto/tls/#Config.BuildNameToCertificate"><code>Config.BuildNameToCertificate</code></a>
method, which builds the <code>NameToCertificate</code> field
from the leaf certificates, is now deprecated and should not be
called.
</p>
<p><!-- CL 175517 -->
The new <a href="/pkg/crypto/tls/#CipherSuites"><code>CipherSuites</code></a>
and <a href="/pkg/crypto/tls/#InsecureCipherSuites"><code>InsecureCipherSuites</code></a>
functions return a list of currently implemented cipher suites.
The new <a href="/pkg/crypto/tls/#CipherSuiteName"><code>CipherSuiteName</code></a>
function returns a name for a cipher suite ID.
</p>
<p><!-- CL 205058, 205057 -->
The new <a href="/pkg/crypto/tls/#ClientHelloInfo.SupportsCertificate">
<code>(*ClientHelloInfo).SupportsCertificate</code></a> and
<a href="/pkg/crypto/tls/#CertificateRequestInfo.SupportsCertificate">
<code>(*CertificateRequestInfo).SupportsCertificate</code></a>
methods expose whether a peer supports a certain certificate.
</p>
<p><!-- CL 174329 -->
The <code>tls</code> package no longer supports the legacy Next Protocol
Negotiation (NPN) extension and now only supports ALPN. In previous
releases it supported both. There are no API changes and applications
should function identically as before. Most other clients and servers have
already removed NPN support in favor of the standardized ALPN.
</p>
<p><!-- CL 205063, 205062 -->
RSA-PSS signatures are now used when supported in TLS 1.2 handshakes. This
won't affect most applications, but custom
<a href="/pkg/crypto/tls/#Certificate.PrivateKey"><code>Certificate.PrivateKey</code></a>
implementations that don't support RSA-PSS signatures will need to use the new
<a href="/pkg/crypto/tls/#Certificate.SupportedSignatureAlgorithms">
<code>Certificate.SupportedSignatureAlgorithms</code></a>
field to disable them.
</p>
<p><!-- CL 205059, 205059 -->
<a href="/pkg/crypto/tls/#Config.Certificates"><code>Config.Certificates</code></a> and
<a href="/pkg/crypto/tls/#Config.GetCertificate"><code>Config.GetCertificate</code></a>
can now both be nil if
<a href="/pkg/crypto/tls/#Config.GetConfigForClient"><code>Config.GetConfigForClient</code></a>
is set. If the callbacks return neither certificates nor an error, the
<code>unrecognized_name</code> is now sent.
</p>
<p><!-- CL 205058 -->
The new <a href="/pkg/crypto/tls/#CertificateRequestInfo.Version"><code>CertificateRequestInfo.Version</code></a>
field provides the TLS version to client certificates callbacks.
</p>
<p><!-- CL 205068 -->
The new <code>TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256</code> and
<code>TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256</code> constants use
the final names for the cipher suites previously referred to as
<code>TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305</code> and
<code>TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305</code>.
</p>
</dd>
</dl><!-- crypto/tls -->
<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
<dd>
<p><!-- CL 204046 -->
<a href="/pkg/crypto/x509/#Certificate.CreateCRL"><code>Certificate.CreateCRL</code></a>
now supports Ed25519 issuers.
</p>
</dd>
</dl>
<dl id="debug/dwarf"><dt><a href="/pkg/debug/dwarf/">debug/dwarf</a></dt>
<dd>
<p><!-- CL 175138 -->
The <code>debug/dwarf</code> package now supports reading DWARF
version 5.
</p>
<p>
The new
method <a href="/pkg/debug/dwarf/#Data.AddSection"><code>(*Data).AddSection</code></a>
supports adding arbitrary new DWARF sections from the input file
to the DWARF <code>Data</code>.
</p>
<p><!-- CL 192698 -->
The new
method <a href="/pkg/debug/dwarf/#Reader.ByteOrder"><code>(*Reader).ByteOrder</code></a>
returns the byte order of the current compilation unit.
This may be used to interpret attributes that are encoded in the
native ordering, such as location descriptions.
</p>
<p><!-- CL 192699 -->
The new
method <a href="/pkg/debug/dwarf/#LineReader.Files"><code>(*LineReader).Files</code></a>
returns the file name table from a line reader.
This may be used to interpret the value of DWARF attributes such
as <code>AttrDeclFile</code>.
</p>
</dd>
</dl><!-- debug/dwarf -->
<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>
<dd>
<p><!-- CL 126624 -->
<a href="/pkg/encoding/asn1/#Unmarshal"><code>Unmarshal</code></a>
now supports ASN.1 string type BMPString, represented by the new
<a href="/pkg/encoding/asn1/#TagBMPString"><code>TagBMPString</code></a>
constant.
</p>
</dd>
</dl><!-- encoding/asn1 -->
<dl id="encoding/json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
<dd>
<p><!-- CL 200677 -->
The <a href="/pkg/encoding/json/#Decoder"><code>Decoder</code></a>
type supports a new
method <a href="/pkg/encoding/json/#Decoder.InputOffset"><code>InputOffset</code></a>
that returns the input stream byte offset of the current
decoder position.
</p>
<p><!-- CL 200217 -->
<a href="/pkg/encoding/json/#Compact"><code>Compact</code></a> no longer
escapes the <code>U+2028</code> and <code>U+2029</code> characters, which
was never a documented feature. For proper escaping, see <a
href="/pkg/encoding/json/#HTMLEscape"><code>HTMLEscape</code></a>.
</p>
<p><!-- CL 195045 -->
<a href="/pkg/encoding/json/#Number"><code>Number</code></a> no longer
accepts invalid numbers, to follow the documented behavior more closely.
If a program needs to accept invalid numbers like the empty string,
consider wrapping the type with <a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>.
</p>
<p><!-- CL 200237 -->
<a href="/pkg/encoding/json/#Unmarshal"><code>Unmarshal</code></a>
can now support map keys with string underlying type which implement
<a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>.
</p>
</dd>
</dl><!-- encoding/json -->
<dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
<dd>
<p><!-- CL 203820, 211657 -->
The <a href="/pkg/go/build/#Context"><code>Context</code></a>
type has a new field <code>Dir</code> which may be used to set
the working directory for the build.
The default is the current directory of the running process.
In module mode, this is used to locate the main module.
</p>
</dd>
</dl><!-- go/build -->
<dl id="go/doc"><dt><a href="/pkg/go/doc/">go/doc</a></dt>
<dd>
<p><!-- CL 204830 -->
The new
function <a href="/pkg/go/doc/#NewFromFiles"><code>NewFromFiles</code></a>
computes package documentation from a list
of <code>*ast.File</code>'s and associates examples with the
appropriate package elements.
The new information is available in a new <code>Examples</code>
field
in the <a href="/pkg/go/doc/#Package"><code>Package</code></a>, <a href="/pkg/go/doc/#Type"><code>Type</code></a>,
and <a href="/pkg/go/doc/#Func"><code>Func</code></a> types, and a
new <a href="/pkg/go/doc/#Example.Suffix"><code>Suffix</code></a>
field in
the <a href="/pkg/go/doc/#Example"><code>Example</code></a>
type.
</p>
</dd>
</dl><!-- go/doc -->
<dl id="io/ioutil"><dt><a href="/pkg/io/ioutil/">io/ioutil</a></dt>
<dd>
<p><!-- CL 198488 -->
<a href="/pkg/io/ioutil/#TempDir"><code>TempDir</code></a> can now create directories
whose names have predictable prefixes and suffixes.
As with <a href="/pkg/io/ioutil/#TempFile"><code>TempFile</code></a>, if the pattern
contains a '*', the random string replaces the last '*'.
</p>
</dd>
</dl>
<dl id="log"><dt><a href="/pkg/log/">log</a></dt>
<dd>
<p><!-- CL 186182 -->
The
new <a href="https://tip.golang.org/pkg/log/#pkg-constants"><code>Lmsgprefix</code></a>
flag may be used to tell the logging functions to emit the
optional output prefix immediately before the log message rather
than at the start of the line.
</p>
</dd>
</dl><!-- log -->
<dl id="math"><dt><a href="/pkg/math/">math</a></dt>
<dd>
<p><!-- CL 127458 -->
The new <a href="/pkg/math/#FMA"><code>FMA</code></a> function
computes <code>x*y+z</code> in floating point with no
intermediate rounding of the <code>x*y</code>
computation. Several architectures implement this computation
using dedicated hardware instructions for additional performance.
</p>
</dd>
</dl><!-- math -->
<dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
<dd>
<p><!-- CL 164972 -->
The <a href="/pkg/math/big/#Int.GCD"><code>GCD</code></a> method
now allows the inputs <code>a</code> and <code>b</code> to be
zero or negative.
</p>
</dd>
</dl><!-- math/big -->
<dl id="math/bits"><dt><a href="/pkg/math/bits/">math/bits</a></dt>
<dd>
<p><!-- CL 197838 -->
The new functions
<a href="/pkg/math/bits/#Rem"><code>Rem</code></a>,
<a href="/pkg/math/bits/#Rem32"><code>Rem32</code></a>, and
<a href="/pkg/math/bits/#Rem64"><code>Rem64</code></a>
support computing a remainder even when the quotient overflows.
</p>
</dd>
</dl><!-- math/bits -->
<dl id="mime"><dt><a href="/pkg/mime/">mime</a></dt>
<dd>
<p><!-- CL 186927 -->
The default type of <code>.js</code> and <code>.mjs</code> files
is now <code>text/javascript</code> rather
than <code>application/javascript</code>.
This is in accordance
with <a href="https://datatracker.ietf.org/doc/draft-ietf-dispatch-javascript-mjs/">an
IETF draft</a> that treats <code>application/javascript</code> as obsolete.
</p>
</dd>
</dl><!-- mime -->
<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
<dd>
<p>
The
new <a href="/pkg/mime/multipart/#Reader"><code>Reader</code></a>
method <a href="/pkg/mime/multipart/#Reader.NextRawPart"><code>NextRawPart</code></a>
supports fetching the next MIME part without transparently
decoding <code>quoted-printable</code> data.
</p>
</dd>
</dl><!-- mime/multipart -->
<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
<dd>
<p><!-- CL 200760 -->
The new <a href="/pkg/net/http/#Header"><code>Header</code></a>
method <a href="/pkg/net/http/#Header.Values"><code>Values</code></a>
can be used to fetch all values associated with a
canonicalized key.
</p>
<p><!-- CL 61291 -->
The
new <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
field <a href="/pkg/net/http/#Transport.DialTLSContext"><code>DialTLSContext</code></a>
can be used to specify an optional dial function for creating
TLS connections for non-proxied HTTPS requests.
This new field can be used instead
of <a href="/pkg/net/http/#Transport.DialTLS"><code>DialTLS</code></a>,
which is now considered deprecated; <code>DialTLS</code> will
continue to work, but new code should
use <code>DialTLSContext</code>, which allows the transport to
cancel dials as soon as they are no longer needed.
</p>
<p><!-- CL 192518, CL 194218 -->
On Windows, <a href="/pkg/net/http/#ServeFile"><code>ServeFile</code></a> now correctly
serves files larger than 2GB.
</p>
</dd>
</dl><!-- net/http -->
<dl id="net/http/httptest"><dt><a href="/pkg/net/http/httptest/">net/http/httptest</a></dt>
<dd>
<p><!-- CL 201557 -->
The
new <a href="/pkg/net/http/httptest/#Server"><code>Server</code></a>
field <a href="/pkg/net/http/httptest/#Server.EnableHTTP2"><code>EnableHTTP2</code></a>
supports enabling HTTP/2 on the test server.
</p>
</dd>
</dl><!-- net/http/httptest -->
<dl id="net/textproto"><dt><a href="/pkg/net/textproto/">net/textproto</a></dt>
<dd>
<p><!-- CL 200760 -->
The
new <a href="/pkg/net/textproto/#MIMEHeader"><code>MIMEHeader</code></a>
method <a href="/pkg/net/textproto/#MIMEHeader.Values"><code>Values</code></a>
can be used to fetch all values associated with a canonicalized
key.
</p>
</dd>
</dl><!-- net/textproto -->
<dl id="net/url"><dt><a href="/pkg/net/url/">net/url</a></dt>
<dd>
<p><!-- CL 185117 -->
When parsing of a URL fails
(for example by <a href="/pkg/net/url/#Parse"><code>Parse</code></a>
or <a href="/pkg/net/url/#ParseRequestURI"><code>ParseRequestURI</code></a>),
the resulting <a href="/pkg/net/url/#Error.Error"><code>Error</code></a> message
will now quote the unparsable URL.
This provides clearer structure and consistency with other parsing errors.
</p>
</dd>
</dl><!-- net/url -->
<dl id="os/signal"><dt><a href="/pkg/os/signal/">os/signal</a></dt>
<dd>
<p><!-- CL 187739 -->
On Windows,
the <code>CTRL_CLOSE_EVENT</code>, <code>CTRL_LOGOFF_EVENT</code>,
and <code>CTRL_SHUTDOWN_EVENT</code> events now generate
a <code>syscall.SIGTERM</code> signal, similar to how Control-C
and Control-Break generate a <code>syscall.SIGINT</code> signal.
</p>
</dd>
</dl><!-- os/signal -->
<dl id="plugin"><dt><a href="/pkg/plugin/">plugin</a></dt>
<dd>
<p><!-- CL 191617 -->
The <code>plugin</code> package now supports <code>freebsd/amd64</code>.
</p>
</dd>
</dl><!-- plugin -->
<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
<dd>
<p><!-- CL 85661 -->
<a href="/pkg/reflect#StructOf"><code>StructOf</code></a> now
supports creating struct types with unexported fields, by
setting the <code>PkgPath</code> field in
a <code>StructField</code> element.
</p>
</dd>
</dl><!-- reflect -->
<dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
<dd>
<p><!-- CL 200081 -->
<code>runtime.Goexit</code> can no longer be aborted by a
recursive <code>panic</code>/<code>recover</code>.
</p>
<p><!-- CL 188297, CL 191785 -->
On macOS, <code>SIGPIPE</code> is no longer forwarded to signal
handlers installed before the Go runtime is initialized.
This is necessary because macOS delivers <code>SIGPIPE</code>
<a href="https://golang.org/issue/33384">to the main thread</a>
rather than the thread writing to the closed pipe.
</p>
</dd>
</dl><!-- runtime -->
<dl id="runtime/pprof"><dt><a href="/pkg/runtime/pprof/">runtime/pprof</a></dt>
<dd>
<p><!-- CL 204636, 205097 -->
The generated profile no longer includes the pseudo-PCs used for inline
marks. Symbol information of inlined functions is encoded in
<a href="https://github.com/google/pprof/blob/5e96527/proto/profile.proto#L177-L184">the format</a>
the pprof tool expects. This is a fix for the regression introduced
during recent releases.
</p>
</dd>
</dl><!-- runtime/pprof -->
<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
<dd>
<p>
The <a href="/pkg/strconv/#NumError"><code>NumError</code></a>
type now has
an <a href="/pkg/strconv/#NumError.Unwrap"><code>Unwrap</code></a>
method that may be used to retrieve the reason that a conversion
failed.
This supports using <code>NumError</code> values
with <a href="/pkg/errors/#Is"><code>errors.Is</code></a> to see
if the underlying error
is <a href="/pkg/strconv/#pkg-variables"><code>strconv.ErrRange</code></a>
or <a href="/pkg/strconv/#pkg-variables"><code>strconv.ErrSyntax</code></a>.
</p>
</dd>
</dl><!-- strconv -->
<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
<dd>
<p><!-- CL 200577 -->
Unlocking a highly contended <code>Mutex</code> now directly
yields the CPU to the next goroutine waiting for
that <code>Mutex</code>. This significantly improves the
performance of highly contended mutexes on high CPU count
machines.
</p>
</dd>
</dl><!-- sync -->
<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
<dd>
<p><!-- CL 201359 -->
The testing package now supports cleanup functions, called after
a test or benchmark has finished, by calling
<a href="/pkg/testing#T.Cleanup"><code>T.Cleanup</code></a> or
<a href="/pkg/testing#B.Cleanup"><code>B.Cleanup</code></a> respectively.
</p>
</dd>
</dl><!-- testing -->
<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
<dd>
<p><!-- CL 206124 -->
The text/template package now correctly reports errors when a
parenthesized argument is used as a function.
This most commonly shows up in erroneous cases like
<code>{{if (eq .F "a") or (eq .F "b")}}</code>.
This should be written as <code>{{if or (eq .F "a") (eq .F "b")}}</code>.
The erroneous case never worked as expected, and will now be
reported with an error <code>can't give argument to non-function</code>.
</p>
</dd>
</dl><!-- text/template -->
<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
<dd>
<p>
The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
support throughout the system has been upgraded from Unicode 11.0 to
<a href="https://www.unicode.org/versions/Unicode12.0.0/">Unicode 12.0</a>,
which adds 554 new characters, including four new scripts, and 61 new emoji.
</p>
</dd>
</dl><!-- unicode -->

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<!--{
"Title": "Go 1.2 Release Notes",
"Path": "/doc/go1.2",
"Template": true
}-->
<h2 id="introduction">Introduction to Go 1.2</h2>
<p>
Since the release of <a href="/doc/go1.1.html">Go version 1.1</a> in April, 2013,
the release schedule has been shortened to make the release process more efficient.
This release, Go version 1.2 or Go 1.2 for short, arrives roughly six months after 1.1,
while 1.1 took over a year to appear after 1.0.
Because of the shorter time scale, 1.2 is a smaller delta than the step from 1.0 to 1.1,
but it still has some significant developments, including
a better scheduler and one new language feature.
Of course, Go 1.2 keeps the <a href="/doc/go1compat.html">promise
of compatibility</a>.
The overwhelming majority of programs built with Go 1.1 (or 1.0 for that matter)
will run without any changes whatsoever when moved to 1.2,
although the introduction of one restriction
to a corner of the language may expose already-incorrect code
(see the discussion of the <a href="#use_of_nil">use of nil</a>).
</p>
<h2 id="language">Changes to the language</h2>
<p>
In the interest of firming up the specification, one corner case has been clarified,
with consequences for programs.
There is also one new language feature.
</p>
<h3 id="use_of_nil">Use of nil</h3>
<p>
The language now specifies that, for safety reasons,
certain uses of nil pointers are guaranteed to trigger a run-time panic.
For instance, in Go 1.0, given code like
</p>
<pre>
type T struct {
X [1<<24]byte
Field int32
}
func main() {
var x *T
...
}
</pre>
<p>
the <code>nil</code> pointer <code>x</code> could be used to access memory incorrectly:
the expression <code>x.Field</code> could access memory at address <code>1<<24</code>.
To prevent such unsafe behavior, in Go 1.2 the compilers now guarantee that any indirection through
a nil pointer, such as illustrated here but also in nil pointers to arrays, nil interface values,
nil slices, and so on, will either panic or return a correct, safe non-nil value.
In short, any expression that explicitly or implicitly requires evaluation of a nil address is an error.
The implementation may inject extra tests into the compiled program to enforce this behavior.
</p>
<p>
Further details are in the
<a href="//golang.org/s/go12nil">design document</a>.
</p>
<p>
<em>Updating</em>:
Most code that depended on the old behavior is erroneous and will fail when run.
Such programs will need to be updated by hand.
</p>
<h3 id="three_index">Three-index slices</h3>
<p>
Go 1.2 adds the ability to specify the capacity as well as the length when using a slicing operation
on an existing array or slice.
A slicing operation creates a new slice by describing a contiguous section of an already-created array or slice:
</p>
<pre>
var array [10]int
slice := array[2:4]
</pre>
<p>
The capacity of the slice is the maximum number of elements that the slice may hold, even after reslicing;
it reflects the size of the underlying array.
In this example, the capacity of the <code>slice</code> variable is 8.
</p>
<p>
Go 1.2 adds new syntax to allow a slicing operation to specify the capacity as well as the length.
A second
colon introduces the capacity value, which must be less than or equal to the capacity of the
source slice or array, adjusted for the origin. For instance,
</p>
<pre>
slice = array[2:4:7]
</pre>
<p>
sets the slice to have the same length as in the earlier example but its capacity is now only 5 elements (7-2).
It is impossible to use this new slice value to access the last three elements of the original array.
</p>
<p>
In this three-index notation, a missing first index (<code>[:i:j]</code>) defaults to zero but the other
two indices must always be specified explicitly.
It is possible that future releases of Go may introduce default values for these indices.
</p>
<p>
Further details are in the
<a href="//golang.org/s/go12slice">design document</a>.
</p>
<p>
<em>Updating</em>:
This is a backwards-compatible change that affects no existing programs.
</p>
<h2 id="impl">Changes to the implementations and tools</h2>
<h3 id="preemption">Pre-emption in the scheduler</h3>
<p>
In prior releases, a goroutine that was looping forever could starve out other
goroutines on the same thread, a serious problem when GOMAXPROCS
provided only one user thread.
In Go 1.2, this is partially addressed: The scheduler is invoked occasionally
upon entry to a function.
This means that any loop that includes a (non-inlined) function call can
be pre-empted, allowing other goroutines to run on the same thread.
</p>
<h3 id="thread_limit">Limit on the number of threads</h3>
<p>
Go 1.2 introduces a configurable limit (default 10,000) to the total number of threads
a single program may have in its address space, to avoid resource starvation
issues in some environments.
Note that goroutines are multiplexed onto threads so this limit does not directly
limit the number of goroutines, only the number that may be simultaneously blocked
in a system call.
In practice, the limit is hard to reach.
</p>
<p>
The new <a href="/pkg/runtime/debug/#SetMaxThreads"><code>SetMaxThreads</code></a> function in the
<a href="/pkg/runtime/debug/"><code>runtime/debug</code></a> package controls the thread count limit.
</p>
<p>
<em>Updating</em>:
Few functions will be affected by the limit, but if a program dies because it hits the
limit, it could be modified to call <code>SetMaxThreads</code> to set a higher count.
Even better would be to refactor the program to need fewer threads, reducing consumption
of kernel resources.
</p>
<h3 id="stack_size">Stack size</h3>
<p>
In Go 1.2, the minimum size of the stack when a goroutine is created has been lifted from 4KB to 8KB.
Many programs were suffering performance problems with the old size, which had a tendency
to introduce expensive stack-segment switching in performance-critical sections.
The new number was determined by empirical testing.
</p>
<p>
At the other end, the new function <a href="/pkg/runtime/debug/#SetMaxStack"><code>SetMaxStack</code></a>
in the <a href="/pkg/runtime/debug"><code>runtime/debug</code></a> package controls
the <em>maximum</em> size of a single goroutine's stack.
The default is 1GB on 64-bit systems and 250MB on 32-bit systems.
Before Go 1.2, it was too easy for a runaway recursion to consume all the memory on a machine.
</p>
<p>
<em>Updating</em>:
The increased minimum stack size may cause programs with many goroutines to use
more memory. There is no workaround, but plans for future releases
include new stack management technology that should address the problem better.
</p>
<h3 id="cgo_and_cpp">Cgo and C++</h3>
<p>
The <a href="/cmd/cgo/"><code>cgo</code></a> command will now invoke the C++
compiler to build any pieces of the linked-to library that are written in C++;
<a href="/cmd/cgo/">the documentation</a> has more detail.
</p>
<h3 id="go_tools_godoc">Godoc and vet moved to the go.tools subrepository</h3>
<p>
Both binaries are still included with the distribution, but the source code for the
godoc and vet commands has moved to the
<a href="//code.google.com/p/go.tools">go.tools</a> subrepository.
</p>
<p>
Also, the core of the godoc program has been split into a
<a href="https://code.google.com/p/go/source/browse/?repo=tools#hg%2Fgodoc">library</a>,
while the command itself is in a separate
<a href="https://code.google.com/p/go/source/browse/?repo=tools#hg%2Fcmd%2Fgodoc">directory</a>.
The move allows the code to be updated easily and the separation into a library and command
makes it easier to construct custom binaries for local sites and different deployment methods.
</p>
<p>
<em>Updating</em>:
Since godoc and vet are not part of the library,
no client Go code depends on the their source and no updating is required.
</p>
<p>
The binary distributions available from <a href="//golang.org">golang.org</a>
include these binaries, so users of these distributions are unaffected.
</p>
<p>
When building from source, users must use "go get" to install godoc and vet.
(The binaries will continue to be installed in their usual locations, not
<code>$GOPATH/bin</code>.)
</p>
<pre>
$ go get code.google.com/p/go.tools/cmd/godoc
$ go get code.google.com/p/go.tools/cmd/vet
</pre>
<h3 id="gccgo">Status of gccgo</h3>
<p>
We expect the future GCC 4.9 release to include gccgo with full
support for Go 1.2.
In the current (4.8.2) release of GCC, gccgo implements Go 1.1.2.
</p>
<h3 id="gc_changes">Changes to the gc compiler and linker</h3>
<p>
Go 1.2 has several semantic changes to the workings of the gc compiler suite.
Most users will be unaffected by them.
</p>
<p>
The <a href="/cmd/cgo/"><code>cgo</code></a> command now
works when C++ is included in the library being linked against.
See the <a href="/cmd/cgo/"><code>cgo</code></a> documentation
for details.
</p>
<p>
The gc compiler displayed a vestigial detail of its origins when
a program had no <code>package</code> clause: it assumed
the file was in package <code>main</code>.
The past has been erased, and a missing <code>package</code> clause
is now an error.
</p>
<p>
On the ARM, the toolchain supports "external linking", which
is a step towards being able to build shared libraries with the gc
toolchain and to provide dynamic linking support for environments
in which that is necessary.
</p>
<p>
In the runtime for the ARM, with <code>5a</code>, it used to be possible to refer
to the runtime-internal <code>m</code> (machine) and <code>g</code>
(goroutine) variables using <code>R9</code> and <code>R10</code> directly.
It is now necessary to refer to them by their proper names.
</p>
<p>
Also on the ARM, the <code>5l</code> linker (sic) now defines the
<code>MOVBS</code> and <code>MOVHS</code> instructions
as synonyms of <code>MOVB</code> and <code>MOVH</code>,
to make clearer the separation between signed and unsigned
sub-word moves; the unsigned versions already existed with a
<code>U</code> suffix.
</p>
<h3 id="cover">Test coverage</h3>
<p>
One major new feature of <a href="/pkg/go/"><code>go test</code></a> is
that it can now compute and, with help from a new, separately installed
"go tool cover" program, display test coverage results.
</p>
<p>
The cover tool is part of the
<a href="https://code.google.com/p/go/source/checkout?repo=tools"><code>go.tools</code></a>
subrepository.
It can be installed by running
</p>
<pre>
$ go get code.google.com/p/go.tools/cmd/cover
</pre>
<p>
The cover tool does two things.
First, when "go test" is given the <code>-cover</code> flag, it is run automatically
to rewrite the source for the package and insert instrumentation statements.
The test is then compiled and run as usual, and basic coverage statistics are reported:
</p>
<pre>
$ go test -cover fmt
ok fmt 0.060s coverage: 91.4% of statements
$
</pre>
<p>
Second, for more detailed reports, different flags to "go test" can create a coverage profile file,
which the cover program, invoked with "go tool cover", can then analyze.
</p>
<p>
Details on how to generate and analyze coverage statistics can be found by running the commands
</p>
<pre>
$ go help testflag
$ go tool cover -help
</pre>
<h3 id="go_doc">The go doc command is deleted</h3>
<p>
The "go doc" command is deleted.
Note that the <a href="/cmd/godoc/"><code>godoc</code></a> tool itself is not deleted,
just the wrapping of it by the <a href="/cmd/go/"><code>go</code></a> command.
All it did was show the documents for a package by package path,
which godoc itself already does with more flexibility.
It has therefore been deleted to reduce the number of documentation tools and,
as part of the restructuring of godoc, encourage better options in future.
</p>
<p>
<em>Updating</em>: For those who still need the precise functionality of running
</p>
<pre>
$ go doc
</pre>
<p>
in a directory, the behavior is identical to running
</p>
<pre>
$ godoc .
</pre>
<h3 id="gocmd">Changes to the go command</h3>
<p>
The <a href="/cmd/go/"><code>go get</code></a> command
now has a <code>-t</code> flag that causes it to download the dependencies
of the tests run by the package, not just those of the package itself.
By default, as before, dependencies of the tests are not downloaded.
</p>
<h2 id="performance">Performance</h2>
<p>
There are a number of significant performance improvements in the standard library; here are a few of them.
</p>
<ul>
<li>
The <a href="/pkg/compress/bzip2/"><code>compress/bzip2</code></a>
decompresses about 30% faster.
</li>
<li>
The <a href="/pkg/crypto/des/"><code>crypto/des</code></a> package
is about five times faster.
</li>
<li>
The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
encodes about 30% faster.
</li>
<li>
Networking performance on Windows and BSD systems is about 30% faster through the use
of an integrated network poller in the runtime, similar to what was done for Linux and OS X
in Go 1.1.
</li>
</ul>
<h2 id="library">Changes to the standard library</h2>
<h3 id="archive_tar_zip">The archive/tar and archive/zip packages</h3>
<p>
The
<a href="/pkg/archive/tar/"><code>archive/tar</code></a>
and
<a href="/pkg/archive/zip/"><code>archive/zip</code></a>
packages have had a change to their semantics that may break existing programs.
The issue is that they both provided an implementation of the
<a href="/pkg/os/#FileInfo"><code>os.FileInfo</code></a>
interface that was not compliant with the specification for that interface.
In particular, their <code>Name</code> method returned the full
path name of the entry, but the interface specification requires that
the method return only the base name (final path element).
</p>
<p>
<em>Updating</em>: Since this behavior was newly implemented and
a bit obscure, it is possible that no code depends on the broken behavior.
If there are programs that do depend on it, they will need to be identified
and fixed manually.
</p>
<h3 id="encoding">The new encoding package</h3>
<p>
There is a new package, <a href="/pkg/encoding/"><code>encoding</code></a>,
that defines a set of standard encoding interfaces that may be used to
build custom marshalers and unmarshalers for packages such as
<a href="/pkg/encoding/xml/"><code>encoding/xml</code></a>,
<a href="/pkg/encoding/json/"><code>encoding/json</code></a>,
and
<a href="/pkg/encoding/binary/"><code>encoding/binary</code></a>.
These new interfaces have been used to tidy up some implementations in
the standard library.
</p>
<p>
The new interfaces are called
<a href="/pkg/encoding/#BinaryMarshaler"><code>BinaryMarshaler</code></a>,
<a href="/pkg/encoding/#BinaryUnmarshaler"><code>BinaryUnmarshaler</code></a>,
<a href="/pkg/encoding/#TextMarshaler"><code>TextMarshaler</code></a>,
and
<a href="/pkg/encoding/#TextUnmarshaler"><code>TextUnmarshaler</code></a>.
Full details are in the <a href="/pkg/encoding/">documentation</a> for the package
and a separate <a href="//golang.org/s/go12encoding">design document</a>.
</p>
<h3 id="fmt_indexed_arguments">The fmt package</h3>
<p>
The <a href="/pkg/fmt/"><code>fmt</code></a> package's formatted print
routines such as <a href="/pkg/fmt/#Printf"><code>Printf</code></a>
now allow the data items to be printed to be accessed in arbitrary order
by using an indexing operation in the formatting specifications.
Wherever an argument is to be fetched from the argument list for formatting,
either as the value to be formatted or as a width or specification integer,
a new optional indexing notation <code>[</code><em>n</em><code>]</code>
fetches argument <em>n</em> instead.
The value of <em>n</em> is 1-indexed.
After such an indexing operating, the next argument to be fetched by normal
processing will be <em>n</em>+1.
</p>
<p>
For example, the normal <code>Printf</code> call
</p>
<pre>
fmt.Sprintf("%c %c %c\n", 'a', 'b', 'c')
</pre>
<p>
would create the string <code>"a b c"</code>, but with indexing operations like this,
</p>
<pre>
fmt.Sprintf("%[3]c %[1]c %c\n", 'a', 'b', 'c')
</pre>
<p>
the result is "<code>"c a b"</code>. The <code>[3]</code> index accesses the third formatting
argument, which is <code>'c'</code>, <code>[1]</code> accesses the first, <code>'a'</code>,
and then the next fetch accesses the argument following that one, <code>'b'</code>.
</p>
<p>
The motivation for this feature is programmable format statements to access
the arguments in different order for localization, but it has other uses:
</p>
<pre>
log.Printf("trace: value %v of type %[1]T\n", expensiveFunction(a.b[c]))
</pre>
<p>
<em>Updating</em>: The change to the syntax of format specifications
is strictly backwards compatible, so it affects no working programs.
</p>
<h3 id="text_template">The text/template and html/template packages</h3>
<p>
The
<a href="/pkg/text/template/"><code>text/template</code></a> package
has a couple of changes in Go 1.2, both of which are also mirrored in the
<a href="/pkg/html/template/"><code>html/template</code></a> package.
</p>
<p>
First, there are new default functions for comparing basic types.
The functions are listed in this table, which shows their names and
the associated familiar comparison operator.
</p>
<table cellpadding="0" summary="Template comparison functions">
<tr>
<th width="50"></th><th width="100">Name</th> <th width="50">Operator</th>
</tr>
<tr>
<td></td><td><code>eq</code></td> <td><code>==</code></td>
</tr>
<tr>
<td></td><td><code>ne</code></td> <td><code>!=</code></td>
</tr>
<tr>
<td></td><td><code>lt</code></td> <td><code>&lt;</code></td>
</tr>
<tr>
<td></td><td><code>le</code></td> <td><code>&lt;=</code></td>
</tr>
<tr>
<td></td><td><code>gt</code></td> <td><code>&gt;</code></td>
</tr>
<tr>
<td></td><td><code>ge</code></td> <td><code>&gt;=</code></td>
</tr>
</table>
<p>
These functions behave slightly differently from the corresponding Go operators.
First, they operate only on basic types (<code>bool</code>, <code>int</code>,
<code>float64</code>, <code>string</code>, etc.).
(Go allows comparison of arrays and structs as well, under some circumstances.)
Second, values can be compared as long as they are the same sort of value:
any signed integer value can be compared to any other signed integer value for example. (Go
does not permit comparing an <code>int8</code> and an <code>int16</code>).
Finally, the <code>eq</code> function (only) allows comparison of the first
argument with one or more following arguments. The template in this example,
</p>
<pre>
{{"{{"}}if eq .A 1 2 3 {{"}}"}} equal {{"{{"}}else{{"}}"}} not equal {{"{{"}}end{{"}}"}}
</pre>
<p>
reports "equal" if <code>.A</code> is equal to <em>any</em> of 1, 2, or 3.
</p>
<p>
The second change is that a small addition to the grammar makes "if else if" chains easier to write.
Instead of writing,
</p>
<pre>
{{"{{"}}if eq .A 1{{"}}"}} X {{"{{"}}else{{"}}"}} {{"{{"}}if eq .A 2{{"}}"}} Y {{"{{"}}end{{"}}"}} {{"{{"}}end{{"}}"}}
</pre>
<p>
one can fold the second "if" into the "else" and have only one "end", like this:
</p>
<pre>
{{"{{"}}if eq .A 1{{"}}"}} X {{"{{"}}else if eq .A 2{{"}}"}} Y {{"{{"}}end{{"}}"}}
</pre>
<p>
The two forms are identical in effect; the difference is just in the syntax.
</p>
<p>
<em>Updating</em>: Neither the "else if" change nor the comparison functions
affect existing programs. Those that
already define functions called <code>eq</code> and so on through a function
map are unaffected because the associated function map will override the new
default function definitions.
</p>
<h3 id="new_packages">New packages</h3>
<p>
There are two new packages.
</p>
<ul>
<li>
The <a href="/pkg/encoding/"><code>encoding</code></a> package is
<a href="#encoding">described above</a>.
</li>
<li>
The <a href="/pkg/image/color/palette/"><code>image/color/palette</code></a> package
provides standard color palettes.
</li>
</ul>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
The following list summarizes a number of minor changes to the library, mostly additions.
See the relevant package documentation for more information about each change.
</p>
<ul>
<li>
The <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package
adds the
<a href="/pkg/archive/zip/#File.DataOffset"><code>DataOffset</code></a> accessor
to return the offset of a file's (possibly compressed) data within the archive.
</li>
<li>
The <a href="/pkg/bufio/"><code>bufio</code></a> package
adds <a href="/pkg/bufio/#Reader.Reset"><code>Reset</code></a>
methods to <a href="/pkg/bufio/#Reader"><code>Reader</code></a> and
<a href="/pkg/bufio/#Writer"><code>Writer</code></a>.
These methods allow the <a href="/pkg/io/#Reader"><code>Readers</code></a>
and <a href="/pkg/io/#Writer"><code>Writers</code></a>
to be re-used on new input and output readers and writers, saving
allocation overhead.
</li>
<li>
The <a href="/pkg/compress/bzip2/"><code>compress/bzip2</code></a>
can now decompress concatenated archives.
</li>
<li>
The <a href="/pkg/compress/flate/"><code>compress/flate</code></a>
package adds a <a href="/pkg/compress/flate/#Writer.Reset"><code>Reset</code></a>
method on the <a href="/pkg/compress/flate/#Writer"><code>Writer</code></a>,
to make it possible to reduce allocation when, for instance, constructing an
archive to hold multiple compressed files.
</li>
<li>
The <a href="/pkg/compress/gzip/"><code>compress/gzip</code></a> package's
<a href="/pkg/compress/gzip/#Writer"><code>Writer</code></a> type adds a
<a href="/pkg/compress/gzip/#Writer.Reset"><code>Reset</code></a>
so it may be reused.
</li>
<li>
The <a href="/pkg/compress/zlib/"><code>compress/zlib</code></a> package's
<a href="/pkg/compress/zlib/#Writer"><code>Writer</code></a> type adds a
<a href="/pkg/compress/zlib/#Writer.Reset"><code>Reset</code></a>
so it may be reused.
</li>
<li>
The <a href="/pkg/container/heap/"><code>container/heap</code></a> package
adds a <a href="/pkg/container/heap/#Fix"><code>Fix</code></a>
method to provide a more efficient way to update an item's position in the heap.
</li>
<li>
The <a href="/pkg/container/list/"><code>container/list</code></a> package
adds the <a href="/pkg/container/list/#List.MoveBefore"><code>MoveBefore</code></a>
and
<a href="/pkg/container/list/#List.MoveAfter"><code>MoveAfter</code></a>
methods, which implement the obvious rearrangement.
</li>
<li>
The <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package
adds the a new GCM mode (Galois Counter Mode), which is almost always
used with AES encryption.
</li>
<li>
The
<a href="/pkg/crypto/md5/"><code>crypto/md5</code></a> package
adds a new <a href="/pkg/crypto/md5/#Sum"><code>Sum</code></a> function
to simplify hashing without sacrificing performance.
</li>
<li>
Similarly, the
<a href="/pkg/crypto/md5/"><code>crypto/sha1</code></a> package
adds a new <a href="/pkg/crypto/sha1/#Sum"><code>Sum</code></a> function.
</li>
<li>
Also, the
<a href="/pkg/crypto/sha256/"><code>crypto/sha256</code></a> package
adds <a href="/pkg/crypto/sha256/#Sum256"><code>Sum256</code></a>
and <a href="/pkg/crypto/sha256/#Sum224"><code>Sum224</code></a> functions.
</li>
<li>
Finally, the <a href="/pkg/crypto/sha512/"><code>crypto/sha512</code></a> package
adds <a href="/pkg/crypto/sha512/#Sum512"><code>Sum512</code></a> and
<a href="/pkg/crypto/sha512/#Sum384"><code>Sum384</code></a> functions.
</li>
<li>
The <a href="/pkg/crypto/x509/"><code>crypto/x509</code></a> package
adds support for reading and writing arbitrary extensions.
</li>
<li>
The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package adds
support for TLS 1.1, 1.2 and AES-GCM.
</li>
<li>
The <a href="/pkg/database/sql/"><code>database/sql</code></a> package adds a
<a href="/pkg/database/sql/#DB.SetMaxOpenConns"><code>SetMaxOpenConns</code></a>
method on <a href="/pkg/database/sql/#DB"><code>DB</code></a> to limit the
number of open connections to the database.
</li>
<li>
The <a href="/pkg/encoding/csv/"><code>encoding/csv</code></a> package
now always allows trailing commas on fields.
</li>
<li>
The <a href="/pkg/encoding/gob/"><code>encoding/gob</code></a> package
now treats channel and function fields of structures as if they were unexported,
even if they are not. That is, it ignores them completely. Previously they would
trigger an error, which could cause unexpected compatibility problems if an
embedded structure added such a field.
The package also now supports the generic <code>BinaryMarshaler</code> and
<code>BinaryUnmarshaler</code> interfaces of the
<a href="/pkg/encoding/"><code>encoding</code></a> package
described above.
</li>
<li>
The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
now will always escape ampersands as "\u0026" when printing strings.
It will now accept but correct invalid UTF-8 in
<a href="/pkg/encoding/json/#Marshal"><code>Marshal</code></a>
(such input was previously rejected).
Finally, it now supports the generic encoding interfaces of the
<a href="/pkg/encoding/"><code>encoding</code></a> package
described above.
</li>
<li>
The <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package
now allows attributes stored in pointers to be marshaled.
It also supports the generic encoding interfaces of the
<a href="/pkg/encoding/"><code>encoding</code></a> package
described above through the new
<a href="/pkg/encoding/xml/#Marshaler"><code>Marshaler</code></a>,
<a href="/pkg/encoding/xml/#Unmarshaler"><code>Unmarshaler</code></a>,
and related
<a href="/pkg/encoding/xml/#MarshalerAttr"><code>MarshalerAttr</code></a> and
<a href="/pkg/encoding/xml/#UnmarshalerAttr"><code>UnmarshalerAttr</code></a>
interfaces.
The package also adds a
<a href="/pkg/encoding/xml/#Encoder.Flush"><code>Flush</code></a> method
to the
<a href="/pkg/encoding/xml/#Encoder"><code>Encoder</code></a>
type for use by custom encoders. See the documentation for
<a href="/pkg/encoding/xml/#Encoder.EncodeToken"><code>EncodeToken</code></a>
to see how to use it.
</li>
<li>
The <a href="/pkg/flag/"><code>flag</code></a> package now
has a <a href="/pkg/flag/#Getter"><code>Getter</code></a> interface
to allow the value of a flag to be retrieved. Due to the
Go 1 compatibility guidelines, this method cannot be added to the existing
<a href="/pkg/flag/#Value"><code>Value</code></a>
interface, but all the existing standard flag types implement it.
The package also now exports the <a href="/pkg/flag/#CommandLine"><code>CommandLine</code></a>
flag set, which holds the flags from the command line.
</li>
<li>
The <a href="/pkg/go/ast/"><code>go/ast</code></a> package's
<a href="/pkg/go/ast/#SliceExpr"><code>SliceExpr</code></a> struct
has a new boolean field, <code>Slice3</code>, which is set to true
when representing a slice expression with three indices (two colons).
The default is false, representing the usual two-index form.
</li>
<li>
The <a href="/pkg/go/build/"><code>go/build</code></a> package adds
the <code>AllTags</code> field
to the <a href="/pkg/go/build/#Package"><code>Package</code></a> type,
to make it easier to process build tags.
</li>
<li>
The <a href="/pkg/image/draw/"><code>image/draw</code></a> package now
exports an interface, <a href="/pkg/image/draw/#Drawer"><code>Drawer</code></a>,
that wraps the standard <a href="/pkg/image/draw/#Draw"><code>Draw</code></a> method.
The Porter-Duff operators now implement this interface, in effect binding an operation to
the draw operator rather than providing it explicitly.
Given a paletted image as its destination, the new
<a href="/pkg/image/draw/#FloydSteinberg"><code>FloydSteinberg</code></a>
implementation of the
<a href="/pkg/image/draw/#Drawer"><code>Drawer</code></a>
interface will use the Floyd-Steinberg error diffusion algorithm to draw the image.
To create palettes suitable for such processing, the new
<a href="/pkg/image/draw/#Quantizer"><code>Quantizer</code></a> interface
represents implementations of quantization algorithms that choose a palette
given a full-color image.
There are no implementations of this interface in the library.
</li>
<li>
The <a href="/pkg/image/gif/"><code>image/gif</code></a> package
can now create GIF files using the new
<a href="/pkg/image/gif/#Encode"><code>Encode</code></a>
and <a href="/pkg/image/gif/#EncodeAll"><code>EncodeAll</code></a>
functions.
Their options argument allows specification of an image
<a href="/pkg/image/draw/#Quantizer"><code>Quantizer</code></a> to use;
if it is <code>nil</code>, the generated GIF will use the
<a href="/pkg/image/color/palette/#Plan9"><code>Plan9</code></a>
color map (palette) defined in the new
<a href="/pkg/image/color/palette/"><code>image/color/palette</code></a> package.
The options also specify a
<a href="/pkg/image/draw/#Drawer"><code>Drawer</code></a>
to use to create the output image;
if it is <code>nil</code>, Floyd-Steinberg error diffusion is used.
</li>
<li>
The <a href="/pkg/io/#Copy"><code>Copy</code></a> method of the
<a href="/pkg/io/"><code>io</code></a> package now prioritizes its
arguments differently.
If one argument implements <a href="/pkg/io/#WriterTo"><code>WriterTo</code></a>
and the other implements <a href="/pkg/io/#ReaderFrom"><code>ReaderFrom</code></a>,
<a href="/pkg/io/#Copy"><code>Copy</code></a> will now invoke
<a href="/pkg/io/#WriterTo"><code>WriterTo</code></a> to do the work,
so that less intermediate buffering is required in general.
</li>
<li>
The <a href="/pkg/net/"><code>net</code></a> package requires cgo by default
because the host operating system must in general mediate network call setup.
On some systems, though, it is possible to use the network without cgo, and useful
to do so, for instance to avoid dynamic linking.
The new build tag <code>netgo</code> (off by default) allows the construction of a
<code>net</code> package in pure Go on those systems where it is possible.
</li>
<li>
The <a href="/pkg/net/"><code>net</code></a> package adds a new field
<code>DualStack</code> to the <a href="/pkg/net/#Dialer"><code>Dialer</code></a>
struct for TCP connection setup using a dual IP stack as described in
<a href="https://tools.ietf.org/html/rfc6555">RFC 6555</a>.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package will no longer
transmit cookies that are incorrect according to
<a href="https://tools.ietf.org/html/rfc6265">RFC 6265</a>.
It just logs an error and sends nothing.
Also,
the <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#ReadResponse"><code>ReadResponse</code></a>
function now permits the <code>*Request</code> parameter to be <code>nil</code>,
whereupon it assumes a GET request.
Finally, an HTTP server will now serve HEAD
requests transparently, without the need for special casing in handler code.
While serving a HEAD request, writes to a
<a href="/pkg/net/http/#Handler"><code>Handler</code></a>'s
<a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a>
are absorbed by the
<a href="/pkg/net/http/#Server"><code>Server</code></a>
and the client receives an empty body as required by the HTTP specification.
</li>
<li>
The <a href="/pkg/os/exec/"><code>os/exec</code></a> package's
<a href="/pkg/os/exec/#Cmd.StdinPipe"><code>Cmd.StdinPipe</code></a> method
returns an <code>io.WriteCloser</code>, but has changed its concrete
implementation from <code>*os.File</code> to an unexported type that embeds
<code>*os.File</code>, and it is now safe to close the returned value.
Before Go 1.2, there was an unavoidable race that this change fixes.
Code that needs access to the methods of <code>*os.File</code> can use an
interface type assertion, such as <code>wc.(interface{ Sync() error })</code>.
</li>
<li>
The <a href="/pkg/runtime/"><code>runtime</code></a> package relaxes
the constraints on finalizer functions in
<a href="/pkg/runtime/#SetFinalizer"><code>SetFinalizer</code></a>: the
actual argument can now be any type that is assignable to the formal type of
the function, as is the case for any normal function call in Go.
</li>
<li>
The <a href="/pkg/sort/"><code>sort</code></a> package has a new
<a href="/pkg/sort/#Stable"><code>Stable</code></a> function that implements
stable sorting. It is less efficient than the normal sort algorithm, however.
</li>
<li>
The <a href="/pkg/strings/"><code>strings</code></a> package adds
an <a href="/pkg/strings/#IndexByte"><code>IndexByte</code></a>
function for consistency with the <a href="/pkg/bytes/"><code>bytes</code></a> package.
</li>
<li>
The <a href="/pkg/sync/atomic/"><code>sync/atomic</code></a> package
adds a new set of swap functions that atomically exchange the argument with the
value stored in the pointer, returning the old value.
The functions are
<a href="/pkg/sync/atomic/#SwapInt32"><code>SwapInt32</code></a>,
<a href="/pkg/sync/atomic/#SwapInt64"><code>SwapInt64</code></a>,
<a href="/pkg/sync/atomic/#SwapUint32"><code>SwapUint32</code></a>,
<a href="/pkg/sync/atomic/#SwapUint64"><code>SwapUint64</code></a>,
<a href="/pkg/sync/atomic/#SwapUintptr"><code>SwapUintptr</code></a>,
and
<a href="/pkg/sync/atomic/#SwapPointer"><code>SwapPointer</code></a>,
which swaps an <code>unsafe.Pointer</code>.
</li>
<li>
The <a href="/pkg/syscall/"><code>syscall</code></a> package now implements
<a href="/pkg/syscall/#Sendfile"><code>Sendfile</code></a> for Darwin.
</li>
<li>
The <a href="/pkg/testing/"><code>testing</code></a> package
now exports the <a href="/pkg/testing/#TB"><code>TB</code></a> interface.
It records the methods in common with the
<a href="/pkg/testing/#T"><code>T</code></a>
and
<a href="/pkg/testing/#B"><code>B</code></a> types,
to make it easier to share code between tests and benchmarks.
Also, the
<a href="/pkg/testing/#AllocsPerRun"><code>AllocsPerRun</code></a>
function now quantizes the return value to an integer (although it
still has type <code>float64</code>), to round off any error caused by
initialization and make the result more repeatable.
</li>
<li>
The <a href="/pkg/text/template/"><code>text/template</code></a> package
now automatically dereferences pointer values when evaluating the arguments
to "escape" functions such as "html", to bring the behavior of such functions
in agreement with that of other printing functions such as "printf".
</li>
<li>
In the <a href="/pkg/time/"><code>time</code></a> package, the
<a href="/pkg/time/#Parse"><code>Parse</code></a> function
and
<a href="/pkg/time/#Time.Format"><code>Format</code></a>
method
now handle time zone offsets with seconds, such as in the historical
date "1871-01-01T05:33:02+00:34:08".
Also, pattern matching in the formats for those routines is stricter: a non-lowercase letter
must now follow the standard words such as "Jan" and "Mon".
</li>
<li>
The <a href="/pkg/unicode/"><code>unicode</code></a> package
adds <a href="/pkg/unicode/#In"><code>In</code></a>,
a nicer-to-use but equivalent version of the original
<a href="/pkg/unicode/#IsOneOf"><code>IsOneOf</code></a>,
to see whether a character is a member of a Unicode category.
</li>
</ul>

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<!--{
"Title": "Go 1.3 Release Notes",
"Path": "/doc/go1.3",
"Template": true
}-->
<h2 id="introduction">Introduction to Go 1.3</h2>
<p>
The latest Go release, version 1.3, arrives six months after 1.2,
and contains no language changes.
It focuses primarily on implementation work, providing
precise garbage collection,
a major refactoring of the compiler toolchain that results in
faster builds, especially for large projects,
significant performance improvements across the board,
and support for DragonFly BSD, Solaris, Plan 9 and Google's Native Client architecture (NaCl).
It also has an important refinement to the memory model regarding synchronization.
As always, Go 1.3 keeps the <a href="/doc/go1compat.html">promise
of compatibility</a>,
and almost everything
will continue to compile and run without change when moved to 1.3.
</p>
<h2 id="os">Changes to the supported operating systems and architectures</h2>
<h3 id="win2000">Removal of support for Windows 2000</h3>
<p>
Microsoft stopped supporting Windows 2000 in 2010.
Since it has <a href="https://codereview.appspot.com/74790043">implementation difficulties</a>
regarding exception handling (signals in Unix terminology),
as of Go 1.3 it is not supported by Go either.
</p>
<h3 id="dragonfly">Support for DragonFly BSD</h3>
<p>
Go 1.3 now includes experimental support for DragonFly BSD on the <code>amd64</code> (64-bit x86) and <code>386</code> (32-bit x86) architectures.
It uses DragonFly BSD 3.6 or above.
</p>
<h3 id="freebsd">Support for FreeBSD</h3>
<p>
It was not announced at the time, but since the release of Go 1.2, support for Go on FreeBSD
requires FreeBSD 8 or above.
</p>
<p>
As of Go 1.3, support for Go on FreeBSD requires that the kernel be compiled with the
<code>COMPAT_FREEBSD32</code> flag configured.
</p>
<p>
In concert with the switch to EABI syscalls for ARM platforms, Go 1.3 will run only on FreeBSD 10.
The x86 platforms, 386 and amd64, are unaffected.
</p>
<h3 id="nacl">Support for Native Client</h3>
<p>
Support for the Native Client virtual machine architecture has returned to Go with the 1.3 release.
It runs on the 32-bit Intel architectures (<code>GOARCH=386</code>) and also on 64-bit Intel, but using
32-bit pointers (<code>GOARCH=amd64p32</code>).
There is not yet support for Native Client on ARM.
Note that this is Native Client (NaCl), not Portable Native Client (PNaCl).
Details about Native Client are <a href="https://developers.google.com/native-client/dev/">here</a>;
how to set up the Go version is described <a href="//golang.org/wiki/NativeClient">here</a>.
</p>
<h3 id="netbsd">Support for NetBSD</h3>
<p>
As of Go 1.3, support for Go on NetBSD requires NetBSD 6.0 or above.
</p>
<h3 id="openbsd">Support for OpenBSD</h3>
<p>
As of Go 1.3, support for Go on OpenBSD requires OpenBSD 5.5 or above.
</p>
<h3 id="plan9">Support for Plan 9</h3>
<p>
Go 1.3 now includes experimental support for Plan 9 on the <code>386</code> (32-bit x86) architecture.
It requires the <code>Tsemacquire</code> syscall, which has been in Plan 9 since June, 2012.
</p>
<h3 id="solaris">Support for Solaris</h3>
<p>
Go 1.3 now includes experimental support for Solaris on the <code>amd64</code> (64-bit x86) architecture.
It requires illumos, Solaris 11 or above.
</p>
<h2 id="memory">Changes to the memory model</h2>
<p>
The Go 1.3 memory model <a href="https://codereview.appspot.com/75130045">adds a new rule</a>
concerning sending and receiving on buffered channels,
to make explicit that a buffered channel can be used as a simple
semaphore, using a send into the
channel to acquire and a receive from the channel to release.
This is not a language change, just a clarification about an expected property of communication.
</p>
<h2 id="impl">Changes to the implementations and tools</h2>
<h3 id="stacks">Stack</h3>
<p>
Go 1.3 has changed the implementation of goroutine stacks away from the old,
"segmented" model to a contiguous model.
When a goroutine needs more stack
than is available, its stack is transferred to a larger single block of memory.
The overhead of this transfer operation amortizes well and eliminates the old "hot spot"
problem when a calculation repeatedly steps across a segment boundary.
Details including performance numbers are in this
<a href="//golang.org/s/contigstacks">design document</a>.
</p>
<h3 id="garbage_collector">Changes to the garbage collector</h3>
<p>
For a while now, the garbage collector has been <em>precise</em> when examining
values in the heap; the Go 1.3 release adds equivalent precision to values on the stack.
This means that a non-pointer Go value such as an integer will never be mistaken for a
pointer and prevent unused memory from being reclaimed.
</p>
<p>
Starting with Go 1.3, the runtime assumes that values with pointer type
contain pointers and other values do not.
This assumption is fundamental to the precise behavior of both stack expansion
and garbage collection.
Programs that use <a href="/pkg/unsafe/">package unsafe</a>
to store integers in pointer-typed values are illegal and will crash if the runtime detects the behavior.
Programs that use <a href="/pkg/unsafe/">package unsafe</a> to store pointers
in integer-typed values are also illegal but more difficult to diagnose during execution.
Because the pointers are hidden from the runtime, a stack expansion or garbage collection
may reclaim the memory they point at, creating
<a href="//en.wikipedia.org/wiki/Dangling_pointer">dangling pointers</a>.
</p>
<p>
<em>Updating</em>: Code that uses <code>unsafe.Pointer</code> to convert
an integer-typed value held in memory into a pointer is illegal and must be rewritten.
Such code can be identified by <code>go vet</code>.
</p>
<h3 id="map">Map iteration</h3>
<p>
Iterations over small maps no longer happen in a consistent order.
Go 1 defines that &ldquo;<a href="//golang.org/ref/spec#For_statements">The iteration order over maps
is not specified and is not guaranteed to be the same from one iteration to the next.</a>&rdquo;
To keep code from depending on map iteration order,
Go 1.0 started each map iteration at a random index in the map.
A new map implementation introduced in Go 1.1 neglected to randomize
iteration for maps with eight or fewer entries, although the iteration order
can still vary from system to system.
This has allowed people to write Go 1.1 and Go 1.2 programs that
depend on small map iteration order and therefore only work reliably on certain systems.
Go 1.3 reintroduces random iteration for small maps in order to flush out these bugs.
</p>
<p>
<em>Updating</em>: If code assumes a fixed iteration order for small maps,
it will break and must be rewritten not to make that assumption.
Because only small maps are affected, the problem arises most often in tests.
</p>
<h3 id="liblink">The linker</h3>
<p>
As part of the general <a href="//golang.org/s/go13linker">overhaul</a> to
the Go linker, the compilers and linkers have been refactored.
The linker is still a C program, but now the instruction selection phase that
was part of the linker has been moved to the compiler through the creation of a new
library called <code>liblink</code>.
By doing instruction selection only once, when the package is first compiled,
this can speed up compilation of large projects significantly.
</p>
<p>
<em>Updating</em>: Although this is a major internal change, it should have no
effect on programs.
</p>
<h3 id="gccgo">Status of gccgo</h3>
<p>
GCC release 4.9 will contain the Go 1.2 (not 1.3) version of gccgo.
The release schedules for the GCC and Go projects do not coincide,
which means that 1.3 will be available in the development branch but
that the next GCC release, 4.10, will likely have the Go 1.4 version of gccgo.
</p>
<h3 id="gocmd">Changes to the go command</h3>
<p>
The <a href="/cmd/go/"><code>cmd/go</code></a> command has several new
features.
The <a href="/cmd/go/"><code>go run</code></a> and
<a href="/cmd/go/"><code>go test</code></a> subcommands
support a new <code>-exec</code> option to specify an alternate
way to run the resulting binary.
Its immediate purpose is to support NaCl.
</p>
<p>
The test coverage support of the <a href="/cmd/go/"><code>go test</code></a>
subcommand now automatically sets the coverage mode to <code>-atomic</code>
when the race detector is enabled, to eliminate false reports about unsafe
access to coverage counters.
</p>
<p>
The <a href="/cmd/go/"><code>go test</code></a> subcommand
now always builds the package, even if it has no test files.
Previously, it would do nothing if no test files were present.
</p>
<p>
The <a href="/cmd/go/"><code>go build</code></a> subcommand
supports a new <code>-i</code> option to install dependencies
of the specified target, but not the target itself.
</p>
<p>
Cross compiling with <a href="/cmd/cgo/"><code>cgo</code></a> enabled
is now supported.
The CC_FOR_TARGET and CXX_FOR_TARGET environment
variables are used when running all.bash to specify the cross compilers
for C and C++ code, respectively.
</p>
<p>
Finally, the go command now supports packages that import Objective-C
files (suffixed <code>.m</code>) through cgo.
</p>
<h3 id="cgo">Changes to cgo</h3>
<p>
The <a href="/cmd/cgo/"><code>cmd/cgo</code></a> command,
which processes <code>import "C"</code> declarations in Go packages,
has corrected a serious bug that may cause some packages to stop compiling.
Previously, all pointers to incomplete struct types translated to the Go type <code>*[0]byte</code>,
with the effect that the Go compiler could not diagnose passing one kind of struct pointer
to a function expecting another.
Go 1.3 corrects this mistake by translating each different
incomplete struct to a different named type.
</p>
<p>
Given the C declaration <code>typedef struct S T</code> for an incomplete <code>struct S</code>,
some Go code used this bug to refer to the types <code>C.struct_S</code> and <code>C.T</code> interchangeably.
Cgo now explicitly allows this use, even for completed struct types.
However, some Go code also used this bug to pass (for example) a <code>*C.FILE</code>
from one package to another.
This is not legal and no longer works: in general Go packages
should avoid exposing C types and names in their APIs.
</p>
<p>
<em>Updating</em>: Code confusing pointers to incomplete types or
passing them across package boundaries will no longer compile
and must be rewritten.
If the conversion is correct and must be preserved,
use an explicit conversion via <a href="/pkg/unsafe/#Pointer"><code>unsafe.Pointer</code></a>.
</p>
<h3 id="swig">SWIG 3.0 required for programs that use SWIG</h3>
<p>
For Go programs that use SWIG, SWIG version 3.0 is now required.
The <a href="/cmd/go"><code>cmd/go</code></a> command will now link the
SWIG generated object files directly into the binary, rather than
building and linking with a shared library.
</p>
<h3 id="gc_flag">Command-line flag parsing</h3>
<p>
In the gc toolchain, the assemblers now use the
same command-line flag parsing rules as the Go flag package, a departure
from the traditional Unix flag parsing.
This may affect scripts that invoke the tool directly.
For example,
<code>go tool 6a -SDfoo</code> must now be written
<code>go tool 6a -S -D foo</code>.
(The same change was made to the compilers and linkers in <a href="/doc/go1.1#gc_flag">Go 1.1</a>.)
</p>
<h3 id="godoc">Changes to godoc</h3>
<p>
When invoked with the <code>-analysis</code> flag,
<a href="//godoc.org/golang.org/x/tools/cmd/godoc">godoc</a>
now performs sophisticated <a href="/lib/godoc/analysis/help.html">static
analysis</a> of the code it indexes.
The results of analysis are presented in both the source view and the
package documentation view, and include the call graph of each package
and the relationships between
definitions and references,
types and their methods,
interfaces and their implementations,
send and receive operations on channels,
functions and their callers, and
call sites and their callees.
</p>
<h3 id="misc">Miscellany</h3>
<p>
The program <code>misc/benchcmp</code> that compares
performance across benchmarking runs has been rewritten.
Once a shell and awk script in the main repository, it is now a Go program in the <code>go.tools</code> repo.
Documentation is <a href="//godoc.org/golang.org/x/tools/cmd/benchcmp">here</a>.
</p>
<p>
For the few of us that build Go distributions, the tool <code>misc/dist</code> has been
moved and renamed; it now lives in <code>misc/makerelease</code>, still in the main repository.
</p>
<h2 id="performance">Performance</h2>
<p>
The performance of Go binaries for this release has improved in many cases due to changes
in the runtime and garbage collection, plus some changes to libraries.
Significant instances include:
</p>
<ul>
<li>
The runtime handles defers more efficiently, reducing the memory footprint by about two kilobytes
per goroutine that calls defer.
</li>
<li>
The garbage collector has been sped up, using a concurrent sweep algorithm,
better parallelization, and larger pages.
The cumulative effect can be a 50-70% reduction in collector pause time.
</li>
<li>
The race detector (see <a href="/doc/articles/race_detector.html">this guide</a>)
is now about 40% faster.
</li>
<li>
The regular expression package <a href="/pkg/regexp/"><code>regexp</code></a>
is now significantly faster for certain simple expressions due to the implementation of
a second, one-pass execution engine.
The choice of which engine to use is automatic;
the details are hidden from the user.
</li>
</ul>
<p>
Also, the runtime now includes in stack dumps how long a goroutine has been blocked,
which can be useful information when debugging deadlocks or performance issues.
</p>
<h2 id="library">Changes to the standard library</h2>
<h3 id="new_packages">New packages</h3>
<p>
A new package <a href="/pkg/debug/plan9obj/"><code>debug/plan9obj</code></a> was added to the standard library.
It implements access to Plan 9 <a href="https://9p.io/magic/man2html/6/a.out">a.out</a> object files.
</p>
<h3 id="major_library_changes">Major changes to the library</h3>
<p>
A previous bug in <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a>
made it possible to skip verification in TLS inadvertently.
In Go 1.3, the bug is fixed: one must specify either ServerName or
InsecureSkipVerify, and if ServerName is specified it is enforced.
This may break existing code that incorrectly depended on insecure
behavior.
</p>
<p>
There is an important new type added to the standard library: <a href="/pkg/sync/#Pool"><code>sync.Pool</code></a>.
It provides an efficient mechanism for implementing certain types of caches whose memory
can be reclaimed automatically by the system.
</p>
<p>
The <a href="/pkg/testing/"><code>testing</code></a> package's benchmarking helper,
<a href="/pkg/testing/#B"><code>B</code></a>, now has a
<a href="/pkg/testing/#B.RunParallel"><code>RunParallel</code></a> method
to make it easier to run benchmarks that exercise multiple CPUs.
</p>
<p>
<em>Updating</em>: The crypto/tls fix may break existing code, but such
code was erroneous and should be updated.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
The following list summarizes a number of minor changes to the library, mostly additions.
See the relevant package documentation for more information about each change.
</p>
<ul>
<li> In the <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package,
a new <a href="/pkg/crypto/tls/#DialWithDialer"><code>DialWithDialer</code></a>
function lets one establish a TLS connection using an existing dialer, making it easier
to control dial options such as timeouts.
The package also now reports the TLS version used by the connection in the
<a href="/pkg/crypto/tls/#ConnectionState"><code>ConnectionState</code></a>
struct.
</li>
<li> The <a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>
function of the <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
now supports parsing (and elsewhere, serialization) of PKCS #10 certificate
signature requests.
</li>
<li>
The formatted print functions of the <code>fmt</code> package now define <code>%F</code>
as a synonym for <code>%f</code> when printing floating-point values.
</li>
<li>
The <a href="/pkg/math/big/"><code>math/big</code></a> package's
<a href="/pkg/math/big/#Int"><code>Int</code></a> and
<a href="/pkg/math/big/#Rat"><code>Rat</code></a> types
now implement
<a href="/pkg/encoding/#TextMarshaler"><code>encoding.TextMarshaler</code></a> and
<a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>.
</li>
<li>
The complex power function, <a href="/pkg/math/cmplx/#Pow"><code>Pow</code></a>,
now specifies the behavior when the first argument is zero.
It was undefined before.
The details are in the <a href="/pkg/math/cmplx/#Pow">documentation for the function</a>.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package now exposes the
properties of a TLS connection used to make a client request in the new
<a href="/pkg/net/http/#Response"><code>Response.TLS</code></a> field.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package now
allows setting an optional server error logger
with <a href="/pkg/net/http/#Server"><code>Server.ErrorLog</code></a>.
The default is still that all errors go to stderr.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package now
supports disabling HTTP keep-alive connections on the server
with <a href="/pkg/net/http/#Server.SetKeepAlivesEnabled"><code>Server.SetKeepAlivesEnabled</code></a>.
The default continues to be that the server does keep-alive (reuses
connections for multiple requests) by default.
Only resource-constrained servers or those in the process of graceful
shutdown will want to disable them.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package adds an optional
<a href="/pkg/net/http/#Transport"><code>Transport.TLSHandshakeTimeout</code></a>
setting to cap the amount of time HTTP client requests will wait for
TLS handshakes to complete.
It's now also set by default
on <a href="/pkg/net/http#DefaultTransport"><code>DefaultTransport</code></a>.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#DefaultTransport"><code>DefaultTransport</code></a>,
used by the HTTP client code, now
enables <a href="http://en.wikipedia.org/wiki/Keepalive#TCP_keepalive">TCP
keep-alives</a> by default.
Other <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
values with a nil <code>Dial</code> field continue to function the same
as before: no TCP keep-alives are used.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package
now enables <a href="http://en.wikipedia.org/wiki/Keepalive#TCP_keepalive">TCP
keep-alives</a> for incoming server requests when
<a href="/pkg/net/http/#ListenAndServe"><code>ListenAndServe</code></a>
or
<a href="/pkg/net/http/#ListenAndServeTLS"><code>ListenAndServeTLS</code></a>
are used.
When a server is started otherwise, TCP keep-alives are not enabled.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package now
provides an
optional <a href="/pkg/net/http/#Server"><code>Server.ConnState</code></a>
callback to hook various phases of a server connection's lifecycle
(see <a href="/pkg/net/http/#ConnState"><code>ConnState</code></a>).
This can be used to implement rate limiting or graceful shutdown.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package's HTTP
client now has an
optional <a href="/pkg/net/http/#Client"><code>Client.Timeout</code></a>
field to specify an end-to-end timeout on requests made using the
client.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#Request.ParseMultipartForm"><code>Request.ParseMultipartForm</code></a>
method will now return an error if the body's <code>Content-Type</code>
is not <code>multipart/form-data</code>.
Prior to Go 1.3 it would silently fail and return <code>nil</code>.
Code that relies on the previous behavior should be updated.
</li>
<li> In the <a href="/pkg/net/"><code>net</code></a> package,
the <a href="/pkg/net/#Dialer"><code>Dialer</code></a> struct now
has a <code>KeepAlive</code> option to specify a keep-alive period for the connection.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#Transport"><code>Transport</code></a>
now closes <a href="/pkg/net/http/#Request"><code>Request.Body</code></a>
consistently, even on error.
</li>
<li>
The <a href="/pkg/os/exec/"><code>os/exec</code></a> package now implements
what the documentation has always said with regard to relative paths for the binary.
In particular, it only calls <a href="/pkg/os/exec/#LookPath"><code>LookPath</code></a>
when the binary's file name contains no path separators.
</li>
<li>
The <a href="/pkg/reflect/#Value.SetMapIndex"><code>SetMapIndex</code></a>
function in the <a href="/pkg/reflect/"><code>reflect</code></a> package
no longer panics when deleting from a <code>nil</code> map.
</li>
<li>
If the main goroutine calls
<a href="/pkg/runtime/#Goexit"><code>runtime.Goexit</code></a>
and all other goroutines finish execution, the program now always crashes,
reporting a detected deadlock.
Earlier versions of Go handled this situation inconsistently: most instances
were reported as deadlocks, but some trivial cases exited cleanly instead.
</li>
<li>
The runtime/debug package now has a new function
<a href="/pkg/runtime/debug/#WriteHeapDump"><code>debug.WriteHeapDump</code></a>
that writes out a description of the heap.
</li>
<li>
The <a href="/pkg/strconv/#CanBackquote"><code>CanBackquote</code></a>
function in the <a href="/pkg/strconv/"><code>strconv</code></a> package
now considers the <code>DEL</code> character, <code>U+007F</code>, to be
non-printing.
</li>
<li>
The <a href="/pkg/syscall/"><code>syscall</code></a> package now provides
<a href="/pkg/syscall/#SendmsgN"><code>SendmsgN</code></a>
as an alternate version of
<a href="/pkg/syscall/#Sendmsg"><code>Sendmsg</code></a>
that returns the number of bytes written.
</li>
<li>
On Windows, the <a href="/pkg/syscall/"><code>syscall</code></a> package now
supports the cdecl calling convention through the addition of a new function
<a href="/pkg/syscall/#NewCallbackCDecl"><code>NewCallbackCDecl</code></a>
alongside the existing function
<a href="/pkg/syscall/#NewCallback"><code>NewCallback</code></a>.
</li>
<li>
The <a href="/pkg/testing/"><code>testing</code></a> package now
diagnoses tests that call <code>panic(nil)</code>, which are almost always erroneous.
Also, tests now write profiles (if invoked with profiling flags) even on failure.
</li>
<li>
The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
support throughout the system has been upgraded from
Unicode 6.2.0 to <a href="http://www.unicode.org/versions/Unicode6.3.0/">Unicode 6.3.0</a>.
</li>
</ul>

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<!--{
"Title": "Go 1.4 Release Notes",
"Path": "/doc/go1.4",
"Template": true
}-->
<h2 id="introduction">Introduction to Go 1.4</h2>
<p>
The latest Go release, version 1.4, arrives as scheduled six months after 1.3.
</p>
<p>
It contains only one tiny language change,
in the form of a backwards-compatible simple variant of <code>for</code>-<code>range</code> loop,
and a possibly breaking change to the compiler involving methods on pointers-to-pointers.
</p>
<p>
The release focuses primarily on implementation work, improving the garbage collector
and preparing the ground for a fully concurrent collector to be rolled out in the
next few releases.
Stacks are now contiguous, reallocated when necessary rather than linking on new
"segments";
this release therefore eliminates the notorious "hot stack split" problem.
There are some new tools available including support in the <code>go</code> command
for build-time source code generation.
The release also adds support for ARM processors on Android and Native Client (NaCl)
and for AMD64 on Plan 9.
</p>
<p>
As always, Go 1.4 keeps the <a href="/doc/go1compat.html">promise
of compatibility</a>,
and almost everything
will continue to compile and run without change when moved to 1.4.
</p>
<h2 id="language">Changes to the language</h2>
<h3 id="forrange">For-range loops</h3>
<p>
Up until Go 1.3, <code>for</code>-<code>range</code> loop had two forms
</p>
<pre>
for i, v := range x {
...
}
</pre>
<p>
and
</p>
<pre>
for i := range x {
...
}
</pre>
<p>
If one was not interested in the loop values, only the iteration itself, it was still
necessary to mention a variable (probably the <a href="/ref/spec#Blank_identifier">blank identifier</a>, as in
<code>for</code> <code>_</code> <code>=</code> <code>range</code> <code>x</code>), because
the form
</p>
<pre>
for range x {
...
}
</pre>
<p>
was not syntactically permitted.
</p>
<p>
This situation seemed awkward, so as of Go 1.4 the variable-free form is now legal.
The pattern arises rarely but the code can be cleaner when it does.
</p>
<p>
<em>Updating</em>: The change is strictly backwards compatible to existing Go
programs, but tools that analyze Go parse trees may need to be modified to accept
this new form as the
<code>Key</code> field of <a href="/pkg/go/ast/#RangeStmt"><code>RangeStmt</code></a>
may now be <code>nil</code>.
</p>
<h3 id="methodonpointertopointer">Method calls on **T</h3>
<p>
Given these declarations,
</p>
<pre>
type T int
func (T) M() {}
var x **T
</pre>
<p>
both <code>gc</code> and <code>gccgo</code> accepted the method call
</p>
<pre>
x.M()
</pre>
<p>
which is a double dereference of the pointer-to-pointer <code>x</code>.
The Go specification allows a single dereference to be inserted automatically,
but not two, so this call is erroneous according to the language definition.
It has therefore been disallowed in Go 1.4, which is a breaking change,
although very few programs will be affected.
</p>
<p>
<em>Updating</em>: Code that depends on the old, erroneous behavior will no longer
compile but is easy to fix by adding an explicit dereference.
</p>
<h2 id="os">Changes to the supported operating systems and architectures</h2>
<h3 id="android">Android</h3>
<p>
Go 1.4 can build binaries for ARM processors running the Android operating system.
It can also build a <code>.so</code> library that can be loaded by an Android application
using the supporting packages in the <a href="https://golang.org/x/mobile">mobile</a> subrepository.
A brief description of the plans for this experimental port are available
<a href="https://golang.org/s/go14android">here</a>.
</p>
<h3 id="naclarm">NaCl on ARM</h3>
<p>
The previous release introduced Native Client (NaCl) support for the 32-bit x86
(<code>GOARCH=386</code>)
and 64-bit x86 using 32-bit pointers (GOARCH=amd64p32).
The 1.4 release adds NaCl support for ARM (GOARCH=arm).
</p>
<h3 id="plan9amd64">Plan9 on AMD64</h3>
<p>
This release adds support for the Plan 9 operating system on AMD64 processors,
provided the kernel supports the <code>nsec</code> system call and uses 4K pages.
</p>
<h2 id="compatibility">Changes to the compatibility guidelines</h2>
<p>
The <a href="/pkg/unsafe/"><code>unsafe</code></a> package allows one
to defeat Go's type system by exploiting internal details of the implementation
or machine representation of data.
It was never explicitly specified what use of <code>unsafe</code> meant
with respect to compatibility as specified in the
<a href="go1compat.html">Go compatibility guidelines</a>.
The answer, of course, is that we can make no promise of compatibility
for code that does unsafe things.
</p>
<p>
We have clarified this situation in the documentation included in the release.
The <a href="go1compat.html">Go compatibility guidelines</a> and the
docs for the <a href="/pkg/unsafe/"><code>unsafe</code></a> package
are now explicit that unsafe code is not guaranteed to remain compatible.
</p>
<p>
<em>Updating</em>: Nothing technical has changed; this is just a clarification
of the documentation.
</p>
<h2 id="impl">Changes to the implementations and tools</h2>
<h3 id="runtime">Changes to the runtime</h3>
<p>
Prior to Go 1.4, the runtime (garbage collector, concurrency support, interface management,
maps, slices, strings, ...) was mostly written in C, with some assembler support.
In 1.4, much of the code has been translated to Go so that the garbage collector can scan
the stacks of programs in the runtime and get accurate information about what variables
are active.
This change was large but should have no semantic effect on programs.
</p>
<p>
This rewrite allows the garbage collector in 1.4 to be fully precise,
meaning that it is aware of the location of all active pointers in the program.
This means the heap will be smaller as there will be no false positives keeping non-pointers alive.
Other related changes also reduce the heap size, which is smaller by 10%-30% overall
relative to the previous release.
</p>
<p>
A consequence is that stacks are no longer segmented, eliminating the "hot split" problem.
When a stack limit is reached, a new, larger stack is allocated, all active frames for
the goroutine are copied there, and any pointers into the stack are updated.
Performance can be noticeably better in some cases and is always more predictable.
Details are available in <a href="https://golang.org/s/contigstacks">the design document</a>.
</p>
<p>
The use of contiguous stacks means that stacks can start smaller without triggering performance issues,
so the default starting size for a goroutine's stack in 1.4 has been reduced from 8192 bytes to 2048 bytes.
</p>
<p>
As preparation for the concurrent garbage collector scheduled for the 1.5 release,
writes to pointer values in the heap are now done by a function call,
called a write barrier, rather than directly from the function updating the value.
In this next release, this will permit the garbage collector to mediate writes to the heap while it is running.
This change has no semantic effect on programs in 1.4, but was
included in the release to test the compiler and the resulting performance.
</p>
<p>
The implementation of interface values has been modified.
In earlier releases, the interface contained a word that was either a pointer or a one-word
scalar value, depending on the type of the concrete object stored.
This implementation was problematical for the garbage collector,
so as of 1.4 interface values always hold a pointer.
In running programs, most interface values were pointers anyway,
so the effect is minimal, but programs that store integers (for example) in
interfaces will see more allocations.
</p>
<p>
As of Go 1.3, the runtime crashes if it finds a memory word that should contain
a valid pointer but instead contains an obviously invalid pointer (for example, the value 3).
Programs that store integers in pointer values may run afoul of this check and crash.
In Go 1.4, setting the <a href="/pkg/runtime/"><code>GODEBUG</code></a> variable
<code>invalidptr=0</code> disables
the crash as a workaround, but we cannot guarantee that future releases will be
able to avoid the crash; the correct fix is to rewrite code not to alias integers and pointers.
</p>
<h3 id="asm">Assembly</h3>
<p>
The language accepted by the assemblers <code>cmd/5a</code>, <code>cmd/6a</code>
and <code>cmd/8a</code> has had several changes,
mostly to make it easier to deliver type information to the runtime.
</p>
<p>
First, the <code>textflag.h</code> file that defines flags for <code>TEXT</code> directives
has been copied from the linker source directory to a standard location so it can be
included with the simple directive
</p>
<pre>
#include "textflag.h"
</pre>
<p>
The more important changes are in how assembler source can define the necessary
type information.
For most programs it will suffice to move data
definitions (<code>DATA</code> and <code>GLOBL</code> directives)
out of assembly into Go files
and to write a Go declaration for each assembly function.
The <a href="/doc/asm#runtime">assembly document</a> describes what to do.
</p>
<p>
<em>Updating</em>:
Assembly files that include <code>textflag.h</code> from its old
location will still work, but should be updated.
For the type information, most assembly routines will need no change,
but all should be examined.
Assembly source files that define data,
functions with non-empty stack frames, or functions that return pointers
need particular attention.
A description of the necessary (but simple) changes
is in the <a href="/doc/asm#runtime">assembly document</a>.
</p>
<p>
More information about these changes is in the <a href="/doc/asm">assembly document</a>.
</p>
<h3 id="gccgo">Status of gccgo</h3>
<p>
The release schedules for the GCC and Go projects do not coincide.
GCC release 4.9 contains the Go 1.2 version of gccgo.
The next release, GCC 5, will likely have the Go 1.4 version of gccgo.
</p>
<h3 id="internalpackages">Internal packages</h3>
<p>
Go's package system makes it easy to structure programs into components with clean boundaries,
but there are only two forms of access: local (unexported) and global (exported).
Sometimes one wishes to have components that are not exported,
for instance to avoid acquiring clients of interfaces to code that is part of a public repository
but not intended for use outside the program to which it belongs.
</p>
<p>
The Go language does not have the power to enforce this distinction, but as of Go 1.4 the
<a href="/cmd/go/"><code>go</code></a> command introduces
a mechanism to define "internal" packages that may not be imported by packages outside
the source subtree in which they reside.
</p>
<p>
To create such a package, place it in a directory named <code>internal</code> or in a subdirectory of a directory
named internal.
When the <code>go</code> command sees an import of a package with <code>internal</code> in its path,
it verifies that the package doing the import
is within the tree rooted at the parent of the <code>internal</code> directory.
For example, a package <code>.../a/b/c/internal/d/e/f</code>
can be imported only by code in the directory tree rooted at <code>.../a/b/c</code>.
It cannot be imported by code in <code>.../a/b/g</code> or in any other repository.
</p>
<p>
For Go 1.4, the internal package mechanism is enforced for the main Go repository;
from 1.5 and onward it will be enforced for any repository.
</p>
<p>
Full details of the mechanism are in
<a href="https://golang.org/s/go14internal">the design document</a>.
</p>
<h3 id="canonicalimports">Canonical import paths</h3>
<p>
Code often lives in repositories hosted by public services such as <code>github.com</code>,
meaning that the import paths for packages begin with the name of the hosting service,
<code>github.com/rsc/pdf</code> for example.
One can use
<a href="/cmd/go/#hdr-Remote_import_paths">an existing mechanism</a>
to provide a "custom" or "vanity" import path such as
<code>rsc.io/pdf</code>, but
that creates two valid import paths for the package.
That is a problem: one may inadvertently import the package through the two
distinct paths in a single program, which is wasteful;
miss an update to a package because the path being used is not recognized to be
out of date;
or break clients using the old path by moving the package to a different hosting service.
</p>
<p>
Go 1.4 introduces an annotation for package clauses in Go source that identify a canonical
import path for the package.
If an import is attempted using a path that is not canonical,
the <a href="/cmd/go/"><code>go</code></a> command
will refuse to compile the importing package.
</p>
<p>
The syntax is simple: put an identifying comment on the package line.
For our example, the package clause would read:
</p>
<pre>
package pdf // import "rsc.io/pdf"
</pre>
<p>
With this in place,
the <code>go</code> command will
refuse to compile a package that imports <code>github.com/rsc/pdf</code>,
ensuring that the code can be moved without breaking users.
</p>
<p>
The check is at build time, not download time, so if <code>go</code> <code>get</code>
fails because of this check, the mis-imported package has been copied to the local machine
and should be removed manually.
</p>
<p>
To complement this new feature, a check has been added at update time to verify
that the local package's remote repository matches that of its custom import.
The <code>go</code> <code>get</code> <code>-u</code> command will fail to
update a package if its remote repository has changed since it was first
downloaded.
The new <code>-f</code> flag overrides this check.
</p>
<p>
Further information is in
<a href="https://golang.org/s/go14customimport">the design document</a>.
</p>
<h3 id="subrepo">Import paths for the subrepositories</h3>
<p>
The Go project subrepositories (<code>code.google.com/p/go.tools</code> and so on)
are now available under custom import paths replacing <code>code.google.com/p/go.</code> with <code>golang.org/x/</code>,
as in <code>golang.org/x/tools</code>.
We will add canonical import comments to the code around June 1, 2015,
at which point Go 1.4 and later will stop accepting the old <code>code.google.com</code> paths.
</p>
<p>
<em>Updating</em>: All code that imports from subrepositories should change
to use the new <code>golang.org</code> paths.
Go 1.0 and later can resolve and import the new paths, so updating will not break
compatibility with older releases.
Code that has not updated will stop compiling with Go 1.4 around June 1, 2015.
</p>
<h3 id="gogenerate">The go generate subcommand</h3>
<p>
The <a href="/cmd/go/"><code>go</code></a> command has a new subcommand,
<a href="/cmd/go/#hdr-Generate_Go_files_by_processing_source"><code>go generate</code></a>,
to automate the running of tools to generate source code before compilation.
For example, it can be used to run the <a href="/cmd/yacc"><code>yacc</code></a>
compiler-compiler on a <code>.y</code> file to produce the Go source file implementing the grammar,
or to automate the generation of <code>String</code> methods for typed constants using the new
<a href="https://godoc.org/golang.org/x/tools/cmd/stringer">stringer</a>
tool in the <code>golang.org/x/tools</code> subrepository.
</p>
<p>
For more information, see the
<a href="https://golang.org/s/go1.4-generate">design document</a>.
</p>
<h3 id="filenames">Change to file name handling</h3>
<p>
Build constraints, also known as build tags, control compilation by including or excluding files
(see the documentation <a href="/pkg/go/build/"><code>/go/build</code></a>).
Compilation can also be controlled by the name of the file itself by "tagging" the file with
a suffix (before the <code>.go</code> or <code>.s</code> extension) with an underscore
and the name of the architecture or operating system.
For instance, the file <code>gopher_arm.go</code> will only be compiled if the target
processor is an ARM.
</p>
<p>
Before Go 1.4, a file called just <code>arm.go</code> was similarly tagged, but this behavior
can break sources when new architectures are added, causing files to suddenly become tagged.
In 1.4, therefore, a file will be tagged in this manner only if the tag (architecture or operating
system name) is preceded by an underscore.
</p>
<p>
<em>Updating</em>: Packages that depend on the old behavior will no longer compile correctly.
Files with names like <code>windows.go</code> or <code>amd64.go</code> should either
have explicit build tags added to the source or be renamed to something like
<code>os_windows.go</code> or <code>support_amd64.go</code>.
</p>
<h3 id="gocmd">Other changes to the go command</h3>
<p>
There were a number of minor changes to the
<a href="/cmd/go/"><code>cmd/go</code></a>
command worth noting.
</p>
<ul>
<li>
Unless <a href="/cmd/cgo/"><code>cgo</code></a> is being used to build the package,
the <code>go</code> command now refuses to compile C source files,
since the relevant C compilers
(<a href="/cmd/6c/"><code>6c</code></a> etc.)
are intended to be removed from the installation in some future release.
(They are used today only to build part of the runtime.)
It is difficult to use them correctly in any case, so any extant uses are likely incorrect,
so we have disabled them.
</li>
<li>
The <a href="/cmd/go/#hdr-Test_packages"><code>go</code> <code>test</code></a>
subcommand has a new flag, <code>-o</code>, to set the name of the resulting binary,
corresponding to the same flag in other subcommands.
The non-functional <code>-file</code> flag has been removed.
</li>
<li>
The <a href="/cmd/go/#hdr-Test_packages"><code>go</code> <code>test</code></a>
subcommand will compile and link all <code>*_test.go</code> files in the package,
even when there are no <code>Test</code> functions in them.
It previously ignored such files.
</li>
<li>
The behavior of the
<a href="/cmd/go/#hdr-Test_packages"><code>go</code> <code>build</code></a>
subcommand's
<code>-a</code> flag has been changed for non-development installations.
For installations running a released distribution, the <code>-a</code> flag will no longer
rebuild the standard library and commands, to avoid overwriting the installation's files.
</li>
</ul>
<h3 id="pkg">Changes to package source layout</h3>
<p>
In the main Go source repository, the source code for the packages was kept in
the directory <code>src/pkg</code>, which made sense but differed from
other repositories, including the Go subrepositories.
In Go 1.4, the<code> pkg</code> level of the source tree is now gone, so for example
the <a href="/pkg/fmt/"><code>fmt</code></a> package's source, once kept in
directory <code>src/pkg/fmt</code>, now lives one level higher in <code>src/fmt</code>.
</p>
<p>
<em>Updating</em>: Tools like <code>godoc</code> that discover source code
need to know about the new location. All tools and services maintained by the Go team
have been updated.
</p>
<h3 id="swig">SWIG</h3>
<p>
Due to runtime changes in this release, Go 1.4 requires SWIG 3.0.3.
</p>
<h3 id="misc">Miscellany</h3>
<p>
The standard repository's top-level <code>misc</code> directory used to contain
Go support for editors and IDEs: plugins, initialization scripts and so on.
Maintaining these was becoming time-consuming
and needed external help because many of the editors listed were not used by
members of the core team.
It also required us to make decisions about which plugin was best for a given
editor, even for editors we do not use.
</p>
<p>
The Go community at large is much better suited to managing this information.
In Go 1.4, therefore, this support has been removed from the repository.
Instead, there is a curated, informative list of what's available on
a <a href="//golang.org/wiki/IDEsAndTextEditorPlugins">wiki page</a>.
</p>
<h2 id="performance">Performance</h2>
<p>
Most programs will run about the same speed or slightly faster in 1.4 than in 1.3;
some will be slightly slower.
There are many changes, making it hard to be precise about what to expect.
</p>
<p>
As mentioned above, much of the runtime was translated to Go from C,
which led to some reduction in heap sizes.
It also improved performance slightly because the Go compiler is better
at optimization, due to things like inlining, than the C compiler used to build
the runtime.
</p>
<p>
The garbage collector was sped up, leading to measurable improvements for
garbage-heavy programs.
On the other hand, the new write barriers slow things down again, typically
by about the same amount but, depending on their behavior, some programs
may be somewhat slower or faster.
</p>
<p>
Library changes that affect performance are documented below.
</p>
<h2 id="library">Changes to the standard library</h2>
<h3 id="new_packages">New packages</h3>
<p>
There are no new packages in this release.
</p>
<h3 id="major_library_changes">Major changes to the library</h3>
<h4 id="scanner">bufio.Scanner</h4>
<p>
The <a href="/pkg/bufio/#Scanner"><code>Scanner</code></a> type in the
<a href="/pkg/bufio/"><code>bufio</code></a> package
has had a bug fixed that may require changes to custom
<a href="/pkg/bufio/#SplitFunc"><code>split functions</code></a>.
The bug made it impossible to generate an empty token at EOF; the fix
changes the end conditions seen by the split function.
Previously, scanning stopped at EOF if there was no more data.
As of 1.4, the split function will be called once at EOF after input is exhausted,
so the split function can generate a final empty token
as the documentation already promised.
</p>
<p>
<em>Updating</em>: Custom split functions may need to be modified to
handle empty tokens at EOF as desired.
</p>
<h4 id="syscall">syscall</h4>
<p>
The <a href="/pkg/syscall/"><code>syscall</code></a> package is now frozen except
for changes needed to maintain the core repository.
In particular, it will no longer be extended to support new or different system calls
that are not used by the core.
The reasons are described at length in <a href="https://golang.org/s/go1.4-syscall">a
separate document</a>.
</p>
<p>
A new subrepository, <a href="https://golang.org/x/sys">golang.org/x/sys</a>,
has been created to serve as the location for new developments to support system
calls on all kernels.
It has a nicer structure, with three packages that each hold the implementation of
system calls for one of
<a href="https://godoc.org/golang.org/x/sys/unix">Unix</a>,
<a href="https://godoc.org/golang.org/x/sys/windows">Windows</a> and
<a href="https://godoc.org/golang.org/x/sys/plan9">Plan 9</a>.
These packages will be curated more generously, accepting all reasonable changes
that reflect kernel interfaces in those operating systems.
See the documentation and the article mentioned above for more information.
</p>
<p>
<em>Updating</em>: Existing programs are not affected as the <code>syscall</code>
package is largely unchanged from the 1.3 release.
Future development that requires system calls not in the <code>syscall</code> package
should build on <code>golang.org/x/sys</code> instead.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<p>
The following list summarizes a number of minor changes to the library, mostly additions.
See the relevant package documentation for more information about each change.
</p>
<ul>
<li>
The <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package's
<a href="/pkg/archive/zip/#Writer"><code>Writer</code></a> now supports a
<a href="/pkg/archive/zip/#Writer.Flush"><code>Flush</code></a> method.
</li>
<li>
The <a href="/pkg/compress/flate/"><code>compress/flate</code></a>,
<a href="/pkg/compress/gzip/"><code>compress/gzip</code></a>,
and <a href="/pkg/compress/zlib/"><code>compress/zlib</code></a>
packages now support a <code>Reset</code> method
for the decompressors, allowing them to reuse buffers and improve performance.
The <a href="/pkg/compress/gzip/"><code>compress/gzip</code></a> package also has a
<a href="/pkg/compress/gzip/#Reader.Multistream"><code>Multistream</code></a> method to control support
for multistream files.
</li>
<li>
The <a href="/pkg/crypto/"><code>crypto</code></a> package now has a
<a href="/pkg/crypto/#Signer"><code>Signer</code></a> interface, implemented by the
<code>PrivateKey</code> types in
<a href="/pkg/crypto/ecdsa"><code>crypto/ecdsa</code></a> and
<a href="/pkg/crypto/rsa"><code>crypto/rsa</code></a>.
</li>
<li>
The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
now supports ALPN as defined in <a href="https://tools.ietf.org/html/rfc7301">RFC 7301</a>.
</li>
<li>
The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
now supports programmatic selection of server certificates
through the new <a href="/pkg/crypto/tls/#Config.CertificateForName"><code>CertificateForName</code></a> function
of the <a href="/pkg/crypto/tls/#Config"><code>Config</code></a> struct.
</li>
<li>
Also in the crypto/tls package, the server now supports
<a href="https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00">TLS_FALLBACK_SCSV</a>
to help clients detect fallback attacks.
(The Go client does not support fallback at all, so it is not vulnerable to
those attacks.)
</li>
<li>
The <a href="/pkg/database/sql/"><code>database/sql</code></a> package can now list all registered
<a href="/pkg/database/sql/#Drivers"><code>Drivers</code></a>.
</li>
<li>
The <a href="/pkg/debug/dwarf/"><code>debug/dwarf</code></a> package now supports
<a href="/pkg/debug/dwarf/#UnspecifiedType"><code>UnspecifiedType</code></a>s.
</li>
<li>
In the <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package,
optional elements with a default value will now only be omitted if they have that value.
</li>
<li>
The <a href="/pkg/encoding/csv/"><code>encoding/csv</code></a> package no longer
quotes empty strings but does quote the end-of-data marker <code>\.</code> (backslash dot).
This is permitted by the definition of CSV and allows it to work better with Postgres.
</li>
<li>
The <a href="/pkg/encoding/gob/"><code>encoding/gob</code></a> package has been rewritten to eliminate
the use of unsafe operations, allowing it to be used in environments that do not permit use of the
<a href="/pkg/unsafe/"><code>unsafe</code></a> package.
For typical uses it will be 10-30% slower, but the delta is dependent on the type of the data and
in some cases, especially involving arrays, it can be faster.
There is no functional change.
</li>
<li>
The <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package's
<a href="/pkg/encoding/xml/#Decoder"><code>Decoder</code></a> can now report its input offset.
</li>
<li>
In the <a href="/pkg/fmt/"><code>fmt</code></a> package,
formatting of pointers to maps has changed to be consistent with that of pointers
to structs, arrays, and so on.
For instance, <code>&amp;map[string]int{"one":</code> <code>1}</code> now prints by default as
<code>&amp;map[one:</code> <code>1]</code> rather than as a hexadecimal pointer value.
</li>
<li>
The <a href="/pkg/image/"><code>image</code></a> package's
<a href="/pkg/image/#Image"><code>Image</code></a>
implementations like
<a href="/pkg/image/#RGBA"><code>RGBA</code></a> and
<a href="/pkg/image/#Gray"><code>Gray</code></a> have specialized
<a href="/pkg/image/#RGBA.RGBAAt"><code>RGBAAt</code></a> and
<a href="/pkg/image/#Gray.GrayAt"><code>GrayAt</code></a> methods alongside the general
<a href="/pkg/image/#Image.At"><code>At</code></a> method.
</li>
<li>
The <a href="/pkg/image/png/"><code>image/png</code></a> package now has an
<a href="/pkg/image/png/#Encoder"><code>Encoder</code></a>
type to control the compression level used for encoding.
</li>
<li>
The <a href="/pkg/math/"><code>math</code></a> package now has a
<a href="/pkg/math/#Nextafter32"><code>Nextafter32</code><a/> function.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#Request"><code>Request</code></a> type
has a new <a href="/pkg/net/http/#Request.BasicAuth"><code>BasicAuth</code></a> method
that returns the username and password from authenticated requests using the
HTTP Basic Authentication
Scheme.
</li>
<li>The <a href="/pkg/net/http/"><code>net/http</code></a> package's
<a href="/pkg/net/http/#Request"><code>Transport</code></a> type
has a new <a href="/pkg/net/http/#Transport.DialTLS"><code>DialTLS</code></a> hook
that allows customizing the behavior of outbound TLS connections.
</li>
<li>
The <a href="/pkg/net/http/httputil/"><code>net/http/httputil</code></a> package's
<a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a> type
has a new field,
<a href="/pkg/net/http/#ReverseProxy.ErrorLog"><code>ErrorLog</code></a>, that
provides user control of logging.
</li>
<li>
The <a href="/pkg/os/"><code>os</code></a> package
now implements symbolic links on the Windows operating system
through the <a href="/pkg/os/#Symlink"><code>Symlink</code></a> function.
Other operating systems already have this functionality.
There is also a new <a href="/pkg/os/#Unsetenv"><code>Unsetenv</code></a> function.
</li>
<li>
The <a href="/pkg/reflect/"><code>reflect</code></a> package's
<a href="/pkg/reflect/#Type"><code>Type</code></a> interface
has a new method, <a href="/pkg/reflect/#type.Comparable"><code>Comparable</code></a>,
that reports whether the type implements general comparisons.
</li>
<li>
Also in the <a href="/pkg/reflect/"><code>reflect</code></a> package, the
<a href="/pkg/reflect/#Value"><code>Value</code></a> interface is now three instead of four words
because of changes to the implementation of interfaces in the runtime.
This saves memory but has no semantic effect.
</li>
<li>
The <a href="/pkg/runtime/"><code>runtime</code></a> package
now implements monotonic clocks on Windows,
as it already did for the other systems.
</li>
<li>
The <a href="/pkg/runtime/"><code>runtime</code></a> package's
<a href="/pkg/runtime/#MemStats.Mallocs"><code>Mallocs</code></a> counter
now counts very small allocations that were missed in Go 1.3.
This may break tests using <a href="/pkg/runtime/#ReadMemStats"><code>ReadMemStats</code></a>
or <a href="/pkg/testing/#AllocsPerRun"><code>AllocsPerRun</code></a>
due to the more accurate answer.
</li>
<li>
In the <a href="/pkg/runtime/"><code>runtime</code></a> package,
an array <a href="/pkg/runtime/#MemStats.PauseEnd"><code>PauseEnd</code></a>
has been added to the
<a href="/pkg/runtime/#MemStats"><code>MemStats</code></a>
and <a href="/pkg/runtime/#GCStats"><code>GCStats</code></a> structs.
This array is a circular buffer of times when garbage collection pauses ended.
The corresponding pause durations are already recorded in
<a href="/pkg/runtime/#MemStats.PauseNs"><code>PauseNs</code></a>
</li>
<li>
The <a href="/pkg/runtime/race/"><code>runtime/race</code></a> package
now supports FreeBSD, which means the
<a href="/pkg/cmd/go/"><code>go</code></a> command's <code>-race</code>
flag now works on FreeBSD.
</li>
<li>
The <a href="/pkg/sync/atomic/"><code>sync/atomic</code></a> package
has a new type, <a href="/pkg/sync/atomic/#Value"><code>Value</code></a>.
<code>Value</code> provides an efficient mechanism for atomic loads and
stores of values of arbitrary type.
</li>
<li>
In the <a href="/pkg/syscall/"><code>syscall</code></a> package's
implementation on Linux, the
<a href="/pkg/syscall/#Setuid"><code>Setuid</code></a>
and <a href="/pkg/syscall/#Setgid"><code>Setgid</code></a> have been disabled
because those system calls operate on the calling thread, not the whole process, which is
different from other platforms and not the expected result.
</li>
<li>
The <a href="/pkg/testing/"><code>testing</code></a> package
has a new facility to provide more control over running a set of tests.
If the test code contains a function
<pre>
func TestMain(m *<a href="/pkg/testing/#M"><code>testing.M</code></a>)
</pre>
that function will be called instead of running the tests directly.
The <code>M</code> struct contains methods to access and run the tests.
</li>
<li>
Also in the <a href="/pkg/testing/"><code>testing</code></a> package,
a new <a href="/pkg/testing/#Coverage"><code>Coverage</code></a>
function reports the current test coverage fraction,
enabling individual tests to report how much they are contributing to the
overall coverage.
</li>
<li>
The <a href="/pkg/text/scanner/"><code>text/scanner</code></a> package's
<a href="/pkg/text/scanner/#Scanner"><code>Scanner</code></a> type
has a new function,
<a href="/pkg/text/scanner/#Scanner.IsIdentRune"><code>IsIdentRune</code></a>,
allowing one to control the definition of an identifier when scanning.
</li>
<li>
The <a href="/pkg/text/template/"><code>text/template</code></a> package's boolean
functions <code>eq</code>, <code>lt</code>, and so on have been generalized to allow comparison
of signed and unsigned integers, simplifying their use in practice.
(Previously one could only compare values of the same signedness.)
All negative values compare less than all unsigned values.
</li>
<li>
The <code>time</code> package now uses the standard symbol for the micro prefix,
the micro symbol (U+00B5 'µ'), to print microsecond durations.
<a href="/pkg/time/#ParseDuration"><code>ParseDuration</code></a> still accepts <code>us</code>
but the package no longer prints microseconds as <code>us</code>.
<br>
<em>Updating</em>: Code that depends on the output format of durations
but does not use ParseDuration will need to be updated.
</li>
</ul>

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<!--{
"Title": "Go 1.6 Release Notes",
"Path": "/doc/go1.6",
"Template": true
}-->
<!--
Edit .,s;^PKG:([a-z][A-Za-z0-9_/]+);<a href="/pkg/\1/"><code>\1</code></a>;g
Edit .,s;^([a-z][A-Za-z0-9_/]+)\.([A-Z][A-Za-z0-9_]+\.)?([A-Z][A-Za-z0-9_]+)([ .',]|$);<a href="/pkg/\1/#\2\3"><code>\3</code></a>\4;g
-->
<style>
main ul li { margin: 0.5em 0; }
</style>
<h2 id="introduction">Introduction to Go 1.6</h2>
<p>
The latest Go release, version 1.6, arrives six months after 1.5.
Most of its changes are in the implementation of the language, runtime, and libraries.
There are no changes to the language specification.
As always, the release maintains the Go 1 <a href="/doc/go1compat.html">promise of compatibility</a>.
We expect almost all Go programs to continue to compile and run as before.
</p>
<p>
The release adds new ports to <a href="#ports">Linux on 64-bit MIPS and Android on 32-bit x86</a>;
defined and enforced <a href="#cgo">rules for sharing Go pointers with C</a>;
transparent, automatic <a href="#http2">support for HTTP/2</a>;
and a new mechanism for <a href="#template">template reuse</a>.
</p>
<h2 id="language">Changes to the language</h2>
<p>
There are no language changes in this release.
</p>
<h2 id="ports">Ports</h2>
<p>
Go 1.6 adds experimental ports to
Linux on 64-bit MIPS (<code>linux/mips64</code> and <code>linux/mips64le</code>).
These ports support <code>cgo</code> but only with internal linking.
</p>
<p>
Go 1.6 also adds an experimental port to Android on 32-bit x86 (<code>android/386</code>).
</p>
<p>
On FreeBSD, Go 1.6 defaults to using <code>clang</code>, not <code>gcc</code>, as the external C compiler.
</p>
<p>
On Linux on little-endian 64-bit PowerPC (<code>linux/ppc64le</code>),
Go 1.6 now supports <code>cgo</code> with external linking and
is roughly feature complete.
</p>
<p>
On NaCl, Go 1.5 required SDK version pepper-41.
Go 1.6 adds support for later SDK versions.
</p>
<p>
On 32-bit x86 systems using the <code>-dynlink</code> or <code>-shared</code> compilation modes,
the register CX is now overwritten by certain memory references and should
be avoided in hand-written assembly.
See the <a href="/doc/asm#x86">assembly documentation</a> for details.
</p>
<h2 id="tools">Tools</h2>
<h3 id="cgo">Cgo</h3>
<p>
There is one major change to <a href="/cmd/cgo/"><code>cgo</code></a>, along with one minor change.
</p>
<p>
The major change is the definition of rules for sharing Go pointers with C code,
to ensure that such C code can coexist with Go's garbage collector.
Briefly, Go and C may share memory allocated by Go
when a pointer to that memory is passed to C as part of a <code>cgo</code> call,
provided that the memory itself contains no pointers to Go-allocated memory,
and provided that C does not retain the pointer after the call returns.
These rules are checked by the runtime during program execution:
if the runtime detects a violation, it prints a diagnosis and crashes the program.
The checks can be disabled by setting the environment variable
<code>GODEBUG=cgocheck=0</code>, but note that the vast majority of
code identified by the checks is subtly incompatible with garbage collection
in one way or another.
Disabling the checks will typically only lead to more mysterious failure modes.
Fixing the code in question should be strongly preferred
over turning off the checks.
See the <a href="/cmd/cgo/#hdr-Passing_pointers"><code>cgo</code> documentation</a> for more details.
</p>
<p>
The minor change is
the addition of explicit <code>C.complexfloat</code> and <code>C.complexdouble</code> types,
separate from Go's <code>complex64</code> and <code>complex128</code>.
Matching the other numeric types, C's complex types and Go's complex type are
no longer interchangeable.
</p>
<h3 id="compiler">Compiler Toolchain</h3>
<p>
The compiler toolchain is mostly unchanged.
Internally, the most significant change is that the parser is now hand-written
instead of generated from <a href="/cmd/yacc/">yacc</a>.
</p>
<p>
The compiler, linker, and <code>go</code> command have a new flag <code>-msan</code>,
analogous to <code>-race</code> and only available on linux/amd64,
that enables interoperation with the <a href="https://clang.llvm.org/docs/MemorySanitizer.html">Clang MemorySanitizer</a>.
Such interoperation is useful mainly for testing a program containing suspect C or C++ code.
</p>
<p>
The linker has a new option <code>-libgcc</code> to set the expected location
of the C compiler support library when linking <a href="/cmd/cgo/"><code>cgo</code></a> code.
The option is only consulted when using <code>-linkmode=internal</code>,
and it may be set to <code>none</code> to disable the use of a support library.
</p>
<p>
The implementation of <a href="/doc/go1.5#link">build modes started in Go 1.5</a> has been expanded to more systems.
This release adds support for the <code>c-shared</code> mode on <code>android/386</code>, <code>android/amd64</code>,
<code>android/arm64</code>, <code>linux/386</code>, and <code>linux/arm64</code>;
for the <code>shared</code> mode on <code>linux/386</code>, <code>linux/arm</code>, <code>linux/amd64</code>, and <code>linux/ppc64le</code>;
and for the new <code>pie</code> mode (generating position-independent executables) on
<code>android/386</code>, <code>android/amd64</code>, <code>android/arm</code>, <code>android/arm64</code>, <code>linux/386</code>,
<code>linux/amd64</code>, <code>linux/arm</code>, <code>linux/arm64</code>, and <code>linux/ppc64le</code>.
See the <a href="https://golang.org/s/execmodes">design document</a> for details.
</p>
<p>
As a reminder, the linker's <code>-X</code> flag changed in Go 1.5.
In Go 1.4 and earlier, it took two arguments, as in
</p>
<pre>
-X importpath.name value
</pre>
<p>
Go 1.5 added an alternative syntax using a single argument
that is itself a <code>name=value</code> pair:
</p>
<pre>
-X importpath.name=value
</pre>
<p>
In Go 1.5 the old syntax was still accepted, after printing a warning
suggesting use of the new syntax instead.
Go 1.6 continues to accept the old syntax and print the warning.
Go 1.7 will remove support for the old syntax.
</p>
<h3 id="gccgo">Gccgo</h3>
<p>
The release schedules for the GCC and Go projects do not coincide.
GCC release 5 contains the Go 1.4 version of gccgo.
The next release, GCC 6, will have the Go 1.6.1 version of gccgo.
</p>
<h3 id="go_command">Go command</h3>
<p>
The <a href="/cmd/go"><code>go</code></a> command's basic operation
is unchanged, but there are a number of changes worth noting.
</p>
<p>
Go 1.5 introduced experimental support for vendoring,
enabled by setting the <code>GO15VENDOREXPERIMENT</code> environment variable to <code>1</code>.
Go 1.6 keeps the vendoring support, no longer considered experimental,
and enables it by default.
It can be disabled explicitly by setting
the <code>GO15VENDOREXPERIMENT</code> environment variable to <code>0</code>.
Go 1.7 will remove support for the environment variable.
</p>
<p>
The most likely problem caused by enabling vendoring by default happens
in source trees containing an existing directory named <code>vendor</code> that
does not expect to be interpreted according to new vendoring semantics.
In this case, the simplest fix is to rename the directory to anything other
than <code>vendor</code> and update any affected import paths.
</p>
<p>
For details about vendoring,
see the documentation for the <a href="/cmd/go/#hdr-Vendor_Directories"><code>go</code> command</a>
and the <a href="https://golang.org/s/go15vendor">design document</a>.
</p>
<p>
There is a new build flag, <code>-msan</code>,
that compiles Go with support for the LLVM memory sanitizer.
This is intended mainly for use when linking against C or C++ code
that is being checked with the memory sanitizer.
</p>
<h3 id="doc_command">Go doc command</h3>
<p>
Go 1.5 introduced the
<a href="/cmd/go/#hdr-Show_documentation_for_package_or_symbol"><code>go doc</code></a> command,
which allows references to packages using only the package name, as in
<code>go</code> <code>doc</code> <code>http</code>.
In the event of ambiguity, the Go 1.5 behavior was to use the package
with the lexicographically earliest import path.
In Go 1.6, ambiguity is resolved by preferring import paths with
fewer elements, breaking ties using lexicographic comparison.
An important effect of this change is that original copies of packages
are now preferred over vendored copies.
Successful searches also tend to run faster.
</p>
<h3 id="vet_command">Go vet command</h3>
<p>
The <a href="/cmd/vet"><code>go vet</code></a> command now diagnoses
passing function or method values as arguments to <code>Printf</code>,
such as when passing <code>f</code> where <code>f()</code> was intended.
</p>
<h2 id="performance">Performance</h2>
<p>
As always, the changes are so general and varied that precise statements
about performance are difficult to make.
Some programs may run faster, some slower.
On average the programs in the Go 1 benchmark suite run a few percent faster in Go 1.6
than they did in Go 1.5.
The garbage collector's pauses are even lower than in Go 1.5,
especially for programs using
a large amount of memory.
</p>
<p>
There have been significant optimizations bringing more than 10% improvements
to implementations of the
<a href="/pkg/compress/bzip2/"><code>compress/bzip2</code></a>,
<a href="/pkg/compress/gzip/"><code>compress/gzip</code></a>,
<a href="/pkg/crypto/aes/"><code>crypto/aes</code></a>,
<a href="/pkg/crypto/elliptic/"><code>crypto/elliptic</code></a>,
<a href="/pkg/crypto/ecdsa/"><code>crypto/ecdsa</code></a>, and
<a href="/pkg/sort/"><code>sort</code></a> packages.
</p>
<h2 id="library">Core library</h2>
<h3 id="http2">HTTP/2</h3>
<p>
Go 1.6 adds transparent support in the
<a href="/pkg/net/http/"><code>net/http</code></a> package
for the new <a href="https://http2.github.io/">HTTP/2 protocol</a>.
Go clients and servers will automatically use HTTP/2 as appropriate when using HTTPS.
There is no exported API specific to details of the HTTP/2 protocol handling,
just as there is no exported API specific to HTTP/1.1.
</p>
<p>
Programs that must disable HTTP/2 can do so by setting
<a href="/pkg/net/http/#Transport"><code>Transport.TLSNextProto</code></a> (for clients)
or
<a href="/pkg/net/http/#Server"><code>Server.TLSNextProto</code></a> (for servers)
to a non-nil, empty map.
</p>
<p>
Programs that must adjust HTTP/2 protocol-specific details can import and use
<a href="https://golang.org/x/net/http2"><code>golang.org/x/net/http2</code></a>,
in particular its
<a href="https://godoc.org/golang.org/x/net/http2/#ConfigureServer">ConfigureServer</a>
and
<a href="https://godoc.org/golang.org/x/net/http2/#ConfigureTransport">ConfigureTransport</a>
functions.
</p>
<h3 id="runtime">Runtime</h3>
<p>
The runtime has added lightweight, best-effort detection of concurrent misuse of maps.
As always, if one goroutine is writing to a map, no other goroutine should be
reading or writing the map concurrently.
If the runtime detects this condition, it prints a diagnosis and crashes the program.
The best way to find out more about the problem is to run the program
under the
<a href="https://blog.golang.org/race-detector">race detector</a>,
which will more reliably identify the race
and give more detail.
</p>
<p>
For program-ending panics, the runtime now by default
prints only the stack of the running goroutine,
not all existing goroutines.
Usually only the current goroutine is relevant to a panic,
so omitting the others significantly reduces irrelevant output
in a crash message.
To see the stacks from all goroutines in crash messages, set the environment variable
<code>GOTRACEBACK</code> to <code>all</code>
or call
<a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback</code></a>
before the crash, and rerun the program.
See the <a href="/pkg/runtime/#hdr-Environment_Variables">runtime documentation</a> for details.
</p>
<p>
<em>Updating</em>:
Uncaught panics intended to dump the state of the entire program,
such as when a timeout is detected or when explicitly handling a received signal,
should now call <code>debug.SetTraceback("all")</code> before panicking.
Searching for uses of
<a href="/pkg/os/signal/#Notify"><code>signal.Notify</code></a> may help identify such code.
</p>
<p>
On Windows, Go programs in Go 1.5 and earlier forced
the global Windows timer resolution to 1ms at startup
by calling <code>timeBeginPeriod(1)</code>.
Go no longer needs this for good scheduler performance,
and changing the global timer resolution caused problems on some systems,
so the call has been removed.
</p>
<p>
When using <code>-buildmode=c-archive</code> or
<code>-buildmode=c-shared</code> to build an archive or a shared
library, the handling of signals has changed.
In Go 1.5 the archive or shared library would install a signal handler
for most signals.
In Go 1.6 it will only install a signal handler for the
synchronous signals needed to handle run-time panics in Go code:
SIGBUS, SIGFPE, SIGSEGV.
See the <a href="/pkg/os/signal">os/signal</a> package for more
details.
</p>
<h3 id="reflect">Reflect</h3>
<p>
The
<a href="/pkg/reflect/"><code>reflect</code></a> package has
<a href="https://golang.org/issue/12367">resolved a long-standing incompatibility</a>
between the gc and gccgo toolchains
regarding embedded unexported struct types containing exported fields.
Code that walks data structures using reflection, especially to implement
serialization in the spirit
of the
<a href="/pkg/encoding/json/"><code>encoding/json</code></a> and
<a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> packages,
may need to be updated.
</p>
<p>
The problem arises when using reflection to walk through
an embedded unexported struct-typed field
into an exported field of that struct.
In this case, <code>reflect</code> had incorrectly reported
the embedded field as exported, by returning an empty <code>Field.PkgPath</code>.
Now it correctly reports the field as unexported
but ignores that fact when evaluating access to exported fields
contained within the struct.
</p>
<p>
<em>Updating</em>:
Typically, code that previously walked over structs and used
</p>
<pre>
f.PkgPath != ""
</pre>
<p>
to exclude inaccessible fields
should now use
</p>
<pre>
f.PkgPath != "" &amp;&amp; !f.Anonymous
</pre>
<p>
For example, see the changes to the implementations of
<a href="https://go-review.googlesource.com/#/c/14011/2/src/encoding/json/encode.go"><code>encoding/json</code></a> and
<a href="https://go-review.googlesource.com/#/c/14012/2/src/encoding/xml/typeinfo.go"><code>encoding/xml</code></a>.
</p>
<h3 id="sort">Sorting</h3>
<p>
In the
<a href="/pkg/sort/"><code>sort</code></a>
package,
the implementation of
<a href="/pkg/sort/#Sort"><code>Sort</code></a>
has been rewritten to make about 10% fewer calls to the
<a href="/pkg/sort/#Interface"><code>Interface</code></a>'s
<code>Less</code> and <code>Swap</code>
methods, with a corresponding overall time savings.
The new algorithm does choose a different ordering than before
for values that compare equal (those pairs for which <code>Less(i,</code> <code>j)</code> and <code>Less(j,</code> <code>i)</code> are false).
</p>
<p>
<em>Updating</em>:
The definition of <code>Sort</code> makes no guarantee about the final order of equal values,
but the new behavior may still break programs that expect a specific order.
Such programs should either refine their <code>Less</code> implementations
to report the desired order
or should switch to
<a href="/pkg/sort/#Stable"><code>Stable</code></a>,
which preserves the original input order
of equal values.
</p>
<h3 id="template">Templates</h3>
<p>
In the
<a href="/pkg/text/template/">text/template</a> package,
there are two significant new features to make writing templates easier.
</p>
<p>
First, it is now possible to <a href="/pkg/text/template/#hdr-Text_and_spaces">trim spaces around template actions</a>,
which can make template definitions more readable.
A minus sign at the beginning of an action says to trim space before the action,
and a minus sign at the end of an action says to trim space after the action.
For example, the template
</p>
<pre>
{{"{{"}}23 -}}
&lt;
{{"{{"}}- 45}}
</pre>
<p>
formats as <code>23&lt;45</code>.
</p>
<p>
Second, the new <a href="/pkg/text/template/#hdr-Actions"><code>{{"{{"}}block}}</code> action</a>,
combined with allowing redefinition of named templates,
provides a simple way to define pieces of a template that
can be replaced in different instantiations.
There is <a href="/pkg/text/template/#example_Template_block">an example</a>
in the <code>text/template</code> package that demonstrates this new feature.
</p>
<h3 id="minor_library_changes">Minor changes to the library</h3>
<ul>
<li>
The <a href="/pkg/archive/tar/"><code>archive/tar</code></a> package's
implementation corrects many bugs in rare corner cases of the file format.
One visible change is that the
<a href="/pkg/archive/tar/#Reader"><code>Reader</code></a> type's
<a href="/pkg/archive/tar/#Reader.Read"><code>Read</code></a> method
now presents the content of special file types as being empty,
returning <code>io.EOF</code> immediately.
</li>
<li>
In the <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package, the
<a href="/pkg/archive/zip/#Reader"><code>Reader</code></a> type now has a
<a href="/pkg/archive/zip/#Reader.RegisterDecompressor"><code>RegisterDecompressor</code></a> method,
and the
<a href="/pkg/archive/zip/#Writer"><code>Writer</code></a> type now has a
<a href="/pkg/archive/zip/#Writer.RegisterCompressor"><code>RegisterCompressor</code></a> method,
enabling control over compression options for individual zip files.
These take precedence over the pre-existing global
<a href="/pkg/archive/zip/#RegisterDecompressor"><code>RegisterDecompressor</code></a> and
<a href="/pkg/archive/zip/#RegisterCompressor"><code>RegisterCompressor</code></a> functions.
</li>
<li>
The <a href="/pkg/bufio/"><code>bufio</code></a> package's
<a href="/pkg/bufio/#Scanner"><code>Scanner</code></a> type now has a
<a href="/pkg/bufio/#Scanner.Buffer"><code>Buffer</code></a> method,
to specify an initial buffer and maximum buffer size to use during scanning.
This makes it possible, when needed, to scan tokens larger than
<code>MaxScanTokenSize</code>.
Also for the <code>Scanner</code>, the package now defines the
<a href="/pkg/bufio/#ErrFinalToken"><code>ErrFinalToken</code></a> error value, for use by
<a href="/pkg/bufio/#SplitFunc">split functions</a> to abort processing or to return a final empty token.
</li>
<li>
The <a href="/pkg/compress/flate/"><code>compress/flate</code></a> package
has deprecated its
<a href="/pkg/compress/flate/#ReadError"><code>ReadError</code></a> and
<a href="/pkg/compress/flate/#WriteError"><code>WriteError</code></a> error implementations.
In Go 1.5 they were only rarely returned when an error was encountered;
now they are never returned, although they remain defined for compatibility.
</li>
<li>
The <a href="/pkg/compress/flate/"><code>compress/flate</code></a>,
<a href="/pkg/compress/gzip/"><code>compress/gzip</code></a>, and
<a href="/pkg/compress/zlib/"><code>compress/zlib</code></a> packages
now report
<a href="/pkg/io/#ErrUnexpectedEOF"><code>io.ErrUnexpectedEOF</code></a> for truncated input streams, instead of
<a href="/pkg/io/#EOF"><code>io.EOF</code></a>.
</li>
<li>
The <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package now
overwrites the destination buffer in the event of a GCM decryption failure.
This is to allow the AESNI code to avoid using a temporary buffer.
</li>
<li>
The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
has a variety of minor changes.
It now allows
<a href="/pkg/crypto/tls/#Listen"><code>Listen</code></a>
to succeed when the
<a href="/pkg/crypto/tls/#Config"><code>Config</code></a>
has a nil <code>Certificates</code>, as long as the <code>GetCertificate</code> callback is set,
it adds support for RSA with AES-GCM cipher suites,
and
it adds a
<a href="/pkg/crypto/tls/#RecordHeaderError"><code>RecordHeaderError</code></a>
to allow clients (in particular, the <a href="/pkg/net/http/"><code>net/http</code></a> package)
to report a better error when attempting a TLS connection to a non-TLS server.
</li>
<li>
The <a href="/pkg/crypto/x509/"><code>crypto/x509</code></a> package
now permits certificates to contain negative serial numbers
(technically an error, but unfortunately common in practice),
and it defines a new
<a href="/pkg/crypto/x509/#InsecureAlgorithmError"><code>InsecureAlgorithmError</code></a>
to give a better error message when rejecting a certificate
signed with an insecure algorithm like MD5.
</li>
<li>
The <a href="/pkg/debug/dwarf"><code>debug/dwarf</code></a> and
<a href="/pkg/debug/elf/"><code>debug/elf</code></a> packages
together add support for compressed DWARF sections.
User code needs no updating: the sections are decompressed automatically when read.
</li>
<li>
The <a href="/pkg/debug/elf/"><code>debug/elf</code></a> package
adds support for general compressed ELF sections.
User code needs no updating: the sections are decompressed automatically when read.
However, compressed
<a href="/pkg/debug/elf/#Section"><code>Sections</code></a> do not support random access:
they have a nil <code>ReaderAt</code> field.
</li>
<li>
The <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package
now exports
<a href="/pkg/encoding/asn1/#pkg-constants">tag and class constants</a>
useful for advanced parsing of ASN.1 structures.
</li>
<li>
Also in the <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package,
<a href="/pkg/encoding/asn1/#Unmarshal"><code>Unmarshal</code></a> now rejects various non-standard integer and length encodings.
</li>
<li>
The <a href="/pkg/encoding/base64"><code>encoding/base64</code></a> package's
<a href="/pkg/encoding/base64/#Decoder"><code>Decoder</code></a> has been fixed
to process the final bytes of its input. Previously it processed as many four-byte tokens as
possible but ignored the remainder, up to three bytes.
The <code>Decoder</code> therefore now handles inputs in unpadded encodings (like
<a href="/pkg/encoding/base64/#RawURLEncoding">RawURLEncoding</a>) correctly,
but it also rejects inputs in padded encodings that are truncated or end with invalid bytes,
such as trailing spaces.
</li>
<li>
The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
now checks the syntax of a
<a href="/pkg/encoding/json/#Number"><code>Number</code></a>
before marshaling it, requiring that it conforms to the JSON specification for numeric values.
As in previous releases, the zero <code>Number</code> (an empty string) is marshaled as a literal 0 (zero).
</li>
<li>
The <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package's
<a href="/pkg/encoding/xml/#Marshal"><code>Marshal</code></a>
function now supports a <code>cdata</code> attribute, such as <code>chardata</code>
but encoding its argument in one or more <code>&lt;![CDATA[ ... ]]&gt;</code> tags.
</li>
<li>
Also in the <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package,
<a href="/pkg/encoding/xml/#Decoder"><code>Decoder</code></a>'s
<a href="/pkg/encoding/xml/#Decoder.Token"><code>Token</code></a> method
now reports an error when encountering EOF before seeing all open tags closed,
consistent with its general requirement that tags in the input be properly matched.
To avoid that requirement, use
<a href="/pkg/encoding/xml/#Decoder.RawToken"><code>RawToken</code></a>.
</li>
<li>
The <a href="/pkg/fmt/"><code>fmt</code></a> package now allows
any integer type as an argument to
<a href="/pkg/fmt/#Printf"><code>Printf</code></a>'s <code>*</code> width and precision specification.
In previous releases, the argument to <code>*</code> was required to have type <code>int</code>.
</li>
<li>
Also in the <a href="/pkg/fmt/"><code>fmt</code></a> package,
<a href="/pkg/fmt/#Scanf"><code>Scanf</code></a> can now scan hexadecimal strings using %X, as an alias for %x.
Both formats accept any mix of upper- and lower-case hexadecimal.
</li>
<li>
The <a href="/pkg/image/"><code>image</code></a>
and
<a href="/pkg/image/color/"><code>image/color</code></a> packages
add
<a href="/pkg/image/#NYCbCrA"><code>NYCbCrA</code></a>
and
<a href="/pkg/image/color/#NYCbCrA"><code>NYCbCrA</code></a>
types, to support Y'CbCr images with non-premultiplied alpha.
</li>
<li>
The <a href="/pkg/io/"><code>io</code></a> package's
<a href="/pkg/io/#MultiWriter"><code>MultiWriter</code></a>
implementation now implements a <code>WriteString</code> method,
for use by
<a href="/pkg/io/#WriteString"><code>WriteString</code></a>.
</li>
<li>
In the <a href="/pkg/math/big/"><code>math/big</code></a> package,
<a href="/pkg/math/big/#Int"><code>Int</code></a> adds
<a href="/pkg/math/big/#Int.Append"><code>Append</code></a>
and
<a href="/pkg/math/big/#Int.Text"><code>Text</code></a>
methods to give more control over printing.
</li>
<li>
Also in the <a href="/pkg/math/big/"><code>math/big</code></a> package,
<a href="/pkg/math/big/#Float"><code>Float</code></a> now implements
<a href="/pkg/encoding/#TextMarshaler"><code>encoding.TextMarshaler</code></a> and
<a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>,
allowing it to be serialized in a natural form by the
<a href="/pkg/encoding/json/"><code>encoding/json</code></a> and
<a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> packages.
</li>
<li>
Also in the <a href="/pkg/math/big/"><code>math/big</code></a> package,
<a href="/pkg/math/big/#Float"><code>Float</code></a>'s
<a href="/pkg/math/big/#Float.Append"><code>Append</code></a> method now supports the special precision argument -1.
As in
<a href="/pkg/strconv/#ParseFloat"><code>strconv.ParseFloat</code></a>,
precision -1 means to use the smallest number of digits necessary such that
<a href="/pkg/math/big/#Float.Parse"><code>Parse</code></a>
reading the result into a <code>Float</code> of the same precision
will yield the original value.
</li>
<li>
The <a href="/pkg/math/rand/"><code>math/rand</code></a> package
adds a
<a href="/pkg/math/rand/#Read"><code>Read</code></a>
function, and likewise
<a href="/pkg/math/rand/#Rand"><code>Rand</code></a> adds a
<a href="/pkg/math/rand/#Rand.Read"><code>Read</code></a> method.
These make it easier to generate pseudorandom test data.
Note that, like the rest of the package,
these should not be used in cryptographic settings;
for such purposes, use the <a href="/pkg/crypto/rand/"><code>crypto/rand</code></a> package instead.
</li>
<li>
The <a href="/pkg/net/"><code>net</code></a> package's
<a href="/pkg/net/#ParseMAC"><code>ParseMAC</code></a> function now accepts 20-byte IP-over-InfiniBand (IPoIB) link-layer addresses.
</li>
<li>
Also in the <a href="/pkg/net/"><code>net</code></a> package,
there have been a few changes to DNS lookups.
First, the
<a href="/pkg/net/#DNSError"><code>DNSError</code></a> error implementation now implements
<a href="/pkg/net/#Error"><code>Error</code></a>,
and in particular its new
<a href="/pkg/net/#DNSError.IsTemporary"><code>IsTemporary</code></a>
method returns true for DNS server errors.
Second, DNS lookup functions such as
<a href="/pkg/net/#LookupAddr"><code>LookupAddr</code></a>
now return rooted domain names (with a trailing dot)
on Plan 9 and Windows, to match the behavior of Go on Unix systems.
</li>
<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package has
a number of minor additions beyond the HTTP/2 support already discussed.
First, the
<a href="/pkg/net/http/#FileServer"><code>FileServer</code></a> now sorts its generated directory listings by file name.
Second, the
<a href="/pkg/net/http/#ServeFile"><code>ServeFile</code></a> function now refuses to serve a result
if the request's URL path contains &ldquo;..&rdquo; (dot-dot) as a path element.
Programs should typically use <code>FileServer</code> and
<a href="/pkg/net/http/#Dir"><code>Dir</code></a>
instead of calling <code>ServeFile</code> directly.
Programs that need to serve file content in response to requests for URLs containing dot-dot can
still call <a href="/pkg/net/http/#ServeContent"><code>ServeContent</code></a>.
Third, the
<a href="/pkg/net/http/#Client"><code>Client</code></a> now allows user code to set the
<code>Expect:</code> <code>100-continue</code> header (see
<a href="/pkg/net/http/#Transport"><code>Transport.ExpectContinueTimeout</code></a>).
Fourth, there are
<a href="/pkg/net/http/#pkg-constants">five new error codes</a>:
<code>StatusPreconditionRequired</code> (428),
<code>StatusTooManyRequests</code> (429),
<code>StatusRequestHeaderFieldsTooLarge</code> (431), and
<code>StatusNetworkAuthenticationRequired</code> (511) from RFC 6585,
as well as the recently-approved
<code>StatusUnavailableForLegalReasons</code> (451).
Fifth, the implementation and documentation of
<a href="/pkg/net/http/#CloseNotifier"><code>CloseNotifier</code></a>
has been substantially changed.
The <a href="/pkg/net/http/#Hijacker"><code>Hijacker</code></a>
interface now works correctly on connections that have previously
been used with <code>CloseNotifier</code>.
The documentation now describes when <code>CloseNotifier</code>
is expected to work.
</li>
<li>
Also in the <a href="/pkg/net/http/"><code>net/http</code></a> package,
there are a few changes related to the handling of a
<a href="/pkg/net/http/#Request"><code>Request</code></a> data structure with its <code>Method</code> field set to the empty string.
An empty <code>Method</code> field has always been documented as an alias for <code>"GET"</code>
and it remains so.
However, Go 1.6 fixes a few routines that did not treat an empty
<code>Method</code> the same as an explicit <code>"GET"</code>.
Most notably, in previous releases
<a href="/pkg/net/http/#Client"><code>Client</code></a> followed redirects only with
<code>Method</code> set explicitly to <code>"GET"</code>;
in Go 1.6 <code>Client</code> also follows redirects for the empty <code>Method</code>.
Finally,
<a href="/pkg/net/http/#NewRequest"><code>NewRequest</code></a> accepts a <code>method</code> argument that has not been
documented as allowed to be empty.
In past releases, passing an empty <code>method</code> argument resulted
in a <code>Request</code> with an empty <code>Method</code> field.
In Go 1.6, the resulting <code>Request</code> always has an initialized
<code>Method</code> field: if its argument is an empty string, <code>NewRequest</code>
sets the <code>Method</code> field in the returned <code>Request</code> to <code>"GET"</code>.
</li>
<li>
The <a href="/pkg/net/http/httptest/"><code>net/http/httptest</code></a> package's
<a href="/pkg/net/http/httptest/#ResponseRecorder"><code>ResponseRecorder</code></a> now initializes a default Content-Type header
using the same content-sniffing algorithm as in
<a href="/pkg/net/http/#Server"><code>http.Server</code></a>.
</li>
<li>
The <a href="/pkg/net/url/"><code>net/url</code></a> package's
<a href="/pkg/net/url/#Parse"><code>Parse</code></a> is now stricter and more spec-compliant regarding the parsing
of host names.
For example, spaces in the host name are no longer accepted.
</li>
<li>
Also in the <a href="/pkg/net/url/"><code>net/url</code></a> package,
the <a href="/pkg/net/url/#Error"><code>Error</code></a> type now implements
<a href="/pkg/net/#Error"><code>net.Error</code></a>.
</li>
<li>
The <a href="/pkg/os/"><code>os</code></a> package's
<a href="/pkg/os/#IsExist"><code>IsExist</code></a>,
<a href="/pkg/os/#IsNotExist"><code>IsNotExist</code></a>,
and
<a href="/pkg/os/#IsPermission"><code>IsPermission</code></a>
now return correct results when inquiring about an
<a href="/pkg/os/#SyscallError"><code>SyscallError</code></a>.
</li>
<li>
On Unix-like systems, when a write
to <a href="/pkg/os/#pkg-variables"><code>os.Stdout</code>
or <code>os.Stderr</code></a> (more precisely, an <code>os.File</code>
opened for file descriptor 1 or 2) fails due to a broken pipe error,
the program will raise a <code>SIGPIPE</code> signal.
By default this will cause the program to exit; this may be changed by
calling the
<a href="/pkg/os/signal"><code>os/signal</code></a>
<a href="/pkg/os/signal/#Notify"><code>Notify</code></a> function
for <code>syscall.SIGPIPE</code>.
A write to a broken pipe on a file descriptor other 1 or 2 will simply
return <code>syscall.EPIPE</code> (possibly wrapped in
<a href="/pkg/os#PathError"><code>os.PathError</code></a>
and/or <a href="/pkg/os#SyscallError"><code>os.SyscallError</code></a>)
to the caller.
The old behavior of raising an uncatchable <code>SIGPIPE</code> signal
after 10 consecutive writes to a broken pipe no longer occurs.
</li>
<li>
In the <a href="/pkg/os/exec/"><code>os/exec</code></a> package,
<a href="/pkg/os/exec/#Cmd"><code>Cmd</code></a>'s
<a href="/pkg/os/exec/#Cmd.Output"><code>Output</code></a> method continues to return an
<a href="/pkg/os/exec/#ExitError"><code>ExitError</code></a> when a command exits with an unsuccessful status.
If standard error would otherwise have been discarded,
the returned <code>ExitError</code> now holds a prefix and suffix
(currently 32 kB) of the failed command's standard error output,
for debugging or for inclusion in error messages.
The <code>ExitError</code>'s
<a href="/pkg/os/exec/#ExitError.String"><code>String</code></a>
method does not show the captured standard error;
programs must retrieve it from the data structure
separately.
</li>
<li>
On Windows, the <a href="/pkg/path/filepath/"><code>path/filepath</code></a> package's
<a href="/pkg/path/filepath/#Join"><code>Join</code></a> function now correctly handles the case when the base is a relative drive path.
For example, <code>Join(`c:`,</code> <code>`a`)</code> now
returns <code>`c:a`</code> instead of <code>`c:\a`</code> as in past releases.
This may affect code that expects the incorrect result.
</li>
<li>
In the <a href="/pkg/regexp/"><code>regexp</code></a> package,
the
<a href="/pkg/regexp/#Regexp"><code>Regexp</code></a> type has always been safe for use by
concurrent goroutines.
It uses a <a href="/pkg/sync/#Mutex"><code>sync.Mutex</code></a> to protect
a cache of scratch spaces used during regular expression searches.
Some high-concurrency servers using the same <code>Regexp</code> from many goroutines
have seen degraded performance due to contention on that mutex.
To help such servers, <code>Regexp</code> now has a
<a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a> method,
which makes a copy of a <code>Regexp</code> that shares most of the structure
of the original but has its own scratch space cache.
Two goroutines can use different copies of a <code>Regexp</code>
without mutex contention.
A copy does have additional space overhead, so <code>Copy</code>
should only be used when contention has been observed.
</li>
<li>
The <a href="/pkg/strconv/"><code>strconv</code></a> package adds
<a href="/pkg/strconv/#IsGraphic"><code>IsGraphic</code></a>,
similar to <a href="/pkg/strconv/#IsPrint"><code>IsPrint</code></a>.
It also adds
<a href="/pkg/strconv/#QuoteToGraphic"><code>QuoteToGraphic</code></a>,
<a href="/pkg/strconv/#QuoteRuneToGraphic"><code>QuoteRuneToGraphic</code></a>,
<a href="/pkg/strconv/#AppendQuoteToGraphic"><code>AppendQuoteToGraphic</code></a>,
and
<a href="/pkg/strconv/#AppendQuoteRuneToGraphic"><code>AppendQuoteRuneToGraphic</code></a>,
analogous to
<a href="/pkg/strconv/#QuoteToASCII"><code>QuoteToASCII</code></a>,
<a href="/pkg/strconv/#QuoteRuneToASCII"><code>QuoteRuneToASCII</code></a>,
and so on.
The <code>ASCII</code> family escapes all space characters except ASCII space (U+0020).
In contrast, the <code>Graphic</code> family does not escape any Unicode space characters (category Zs).
</li>
<li>
In the <a href="/pkg/testing/"><code>testing</code></a> package,
when a test calls
<a href="/pkg/testing/#T.Parallel">t.Parallel</a>,
that test is paused until all non-parallel tests complete, and then
that test continues execution with all other parallel tests.
Go 1.6 changes the time reported for such a test:
previously the time counted only the parallel execution,
but now it also counts the time from the start of testing
until the call to <code>t.Parallel</code>.
</li>
<li>
The <a href="/pkg/text/template/"><code>text/template</code></a> package
contains two minor changes, in addition to the <a href="#template">major changes</a>
described above.
First, it adds a new
<a href="/pkg/text/template/#ExecError"><code>ExecError</code></a> type
returned for any error during
<a href="/pkg/text/template/#Template.Execute"><code>Execute</code></a>
that does not originate in a <code>Write</code> to the underlying writer.
Callers can distinguish template usage errors from I/O errors by checking for
<code>ExecError</code>.
Second, the
<a href="/pkg/text/template/#Template.Funcs"><code>Funcs</code></a> method
now checks that the names used as keys in the
<a href="/pkg/text/template/#FuncMap"><code>FuncMap</code></a>
are identifiers that can appear in a template function invocation.
If not, <code>Funcs</code> panics.
</li>
<li>
The <a href="/pkg/time/"><code>time</code></a> package's
<a href="/pkg/time/#Parse"><code>Parse</code></a> function has always rejected any day of month larger than 31,
such as January 32.
In Go 1.6, <code>Parse</code> now also rejects February 29 in non-leap years,
February 30, February 31, April 31, June 31, September 31, and November 31.
</li>
</ul>

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<!--{
"Title": "Go 1 and the Future of Go Programs",
"Path": "/doc/go1compat"
}-->
<h2 id="introduction">Introduction</h2>
<p>
The release of Go version 1, Go 1 for short, is a major milestone
in the development of the language. Go 1 is a stable platform for
the growth of programs and projects written in Go.
</p>
<p>
Go 1 defines two things: first, the specification of the language;
and second, the specification of a set of core APIs, the "standard
packages" of the Go library. The Go 1 release includes their
implementation in the form of two compiler suites (gc and gccgo),
and the core libraries themselves.
</p>
<p>
It is intended that programs written to the Go 1 specification will
continue to compile and run correctly, unchanged, over the lifetime
of that specification. At some indefinite point, a Go 2 specification
may arise, but until that time, Go programs that work today should
continue to work even as future "point" releases of Go 1 arise (Go
1.1, Go 1.2, etc.).
</p>
<p>
Compatibility is at the source level. Binary compatibility for
compiled packages is not guaranteed between releases. After a point
release, Go source will need to be recompiled to link against the
new release.
</p>
<p>
The APIs may grow, acquiring new packages and features, but not in
a way that breaks existing Go 1 code.
</p>
<h2 id="expectations">Expectations</h2>
<p>
Although we expect that the vast majority of programs will maintain
this compatibility over time, it is impossible to guarantee that
no future change will break any program. This document is an attempt
to set expectations for the compatibility of Go 1 software in the
future. There are a number of ways in which a program that compiles
and runs today may fail to do so after a future point release. They
are all unlikely but worth recording.
</p>
<ul>
<li>
Security. A security issue in the specification or implementation
may come to light whose resolution requires breaking compatibility.
We reserve the right to address such security issues.
</li>
<li>
Unspecified behavior. The Go specification tries to be explicit
about most properties of the language, but there are some aspects
that are undefined. Programs that depend on such unspecified behavior
may break in future releases.
</li>
<li>
Specification errors. If it becomes necessary to address an
inconsistency or incompleteness in the specification, resolving the
issue could affect the meaning or legality of existing programs.
We reserve the right to address such issues, including updating the
implementations. Except for security issues, no incompatible changes
to the specification would be made.
</li>
<li>
Bugs. If a compiler or library has a bug that violates the
specification, a program that depends on the buggy behavior may
break if the bug is fixed. We reserve the right to fix such bugs.
</li>
<li>
Struct literals. For the addition of features in later point
releases, it may be necessary to add fields to exported structs in
the API. Code that uses unkeyed struct literals (such as pkg.T{3,
"x"}) to create values of these types would fail to compile after
such a change. However, code that uses keyed literals (pkg.T{A:
3, B: "x"}) will continue to compile after such a change. We will
update such data structures in a way that allows keyed struct
literals to remain compatible, although unkeyed literals may fail
to compile. (There are also more intricate cases involving nested
data structures or interfaces, but they have the same resolution.)
We therefore recommend that composite literals whose type is defined
in a separate package should use the keyed notation.
</li>
<li>
Methods. As with struct fields, it may be necessary to add methods
to types.
Under some circumstances, such as when the type is embedded in
a struct along with another type,
the addition of the new method may break
the struct by creating a conflict with an existing method of the other
embedded type.
We cannot protect against this rare case and do not guarantee compatibility
should it arise.
</li>
<li>
Dot imports. If a program imports a standard package
using <code>import . "path"</code>, additional names defined in the
imported package in future releases may conflict with other names
defined in the program. We do not recommend the use of <code>import .</code>
outside of tests, and using it may cause a program to fail
to compile in future releases.
</li>
<li>
Use of package <code>unsafe</code>. Packages that import
<a href="/pkg/unsafe/"><code>unsafe</code></a>
may depend on internal properties of the Go implementation.
We reserve the right to make changes to the implementation
that may break such programs.
</li>
</ul>
<p>
Of course, for all of these possibilities, should they arise, we
would endeavor whenever feasible to update the specification,
compilers, or libraries without affecting existing code.
</p>
<p>
These same considerations apply to successive point releases. For
instance, code that runs under Go 1.2 should be compatible with Go
1.2.1, Go 1.3, Go 1.4, etc., although not necessarily with Go 1.1
since it may use features added only in Go 1.2
</p>
<p>
Features added between releases, available in the source repository
but not part of the numbered binary releases, are under active
development. No promise of compatibility is made for software using
such features until they have been released.
</p>
<p>
Finally, although it is not a correctness issue, it is possible
that the performance of a program may be affected by
changes in the implementation of the compilers or libraries upon
which it depends.
No guarantee can be made about the performance of a
given program between releases.
</p>
<p>
Although these expectations apply to Go 1 itself, we hope similar
considerations would be made for the development of externally
developed software based on Go 1.
</p>
<h2 id="subrepos">Sub-repositories</h2>
<p>
Code in sub-repositories of the main go tree, such as
<a href="//golang.org/x/net">golang.org/x/net</a>,
may be developed under
looser compatibility requirements. However, the sub-repositories
will be tagged as appropriate to identify versions that are compatible
with the Go 1 point releases.
</p>
<h2 id="operating_systems">Operating systems</h2>
<p>
It is impossible to guarantee long-term compatibility with operating
system interfaces, which are changed by outside parties.
The <a href="/pkg/syscall/"><code>syscall</code></a> package
is therefore outside the purview of the guarantees made here.
As of Go version 1.4, the <code>syscall</code> package is frozen.
Any evolution of the system call interface must be supported elsewhere,
such as in the
<a href="//golang.org/x/sys">go.sys</a> subrepository.
For details and background, see
<a href="//golang.org/s/go1.4-syscall">this document</a>.
</p>
<h2 id="tools">Tools</h2>
<p>
Finally, the Go toolchain (compilers, linkers, build tools, and so
on) is under active development and may change behavior. This
means, for instance, that scripts that depend on the location and
properties of the tools may be broken by a point release.
</p>
<p>
These caveats aside, we believe that Go 1 will be a firm foundation
for the development of Go and its ecosystem.
</p>

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The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
The design is licensed under the Creative Commons 3.0 Attributions license.
Read this article for more details: https://blog.golang.org/gopher

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@ -0,0 +1,96 @@
<!--{
"Title": "Help",
"Path": "/help/",
"Template": true
}-->
<div id="manual-nav"></div>
<h2 id="help">Get help</h2>
<img class="gopher" src="/doc/gopher/help.png" alt=""/>
{{if not $.GoogleCN}}
<h3 id="mailinglist"><a href="https://groups.google.com/group/golang-nuts">Go Nuts Mailing List</a></h3>
<p>
Get help from Go users, and share your work on the official mailing list.
</p>
<p>
Search the <a href="https://groups.google.com/group/golang-nuts">golang-nuts</a>
archives and consult the <a href="/doc/go_faq.html">FAQ</a> and
<a href="//golang.org/wiki">wiki</a> before posting.
</p>
<h3 id="forum"><a href="https://forum.golangbridge.org/">Go Forum</a></h3>
<p>
The <a href="https://forum.golangbridge.org/">Go Forum</a> is a discussion
forum for Go programmers.
</p>
<h3 id="discord"><a href="https://discord.gg/64C346U">Gophers Discord</a></h3>
<p>
Get live support and talk with other gophers on the Go Discord.
</p>
<h3 id="slack"><a href="https://blog.gopheracademy.com/gophers-slack-community/">Gopher Slack</a></h3>
<p>Get live support from other users in the Go slack channel.</p>
<h3 id="irc"><a href="irc:irc.freenode.net/go-nuts">Go IRC Channel</a></h3>
<p>Get live support at <b>#go-nuts</b> on <b>irc.freenode.net</b>, the official
Go IRC channel.</p>
{{end}}
<h3 id="faq"><a href="/doc/faq">Frequently Asked Questions (FAQ)</a></h3>
<p>Answers to common questions about Go.</p>
{{if not $.GoogleCN}}
<h2 id="inform">Stay informed</h2>
<h3 id="announce"><a href="https://groups.google.com/group/golang-announce">Go Announcements Mailing List</a></h3>
<p>
Subscribe to
<a href="https://groups.google.com/group/golang-announce">golang-announce</a>
for important announcements, such as the availability of new Go releases.
</p>
<h3 id="blog"><a href="//blog.golang.org">Go Blog</a></h3>
<p>The Go project's official blog.</p>
<h3 id="twitter"><a href="https://twitter.com/golang">@golang at Twitter</a></h3>
<p>The Go project's official Twitter account.</p>
<h3 id="reddit"><a href="https://reddit.com/r/golang">golang sub-Reddit</a></h3>
<p>
The <a href="https://reddit.com/r/golang">golang sub-Reddit</a> is a place
for Go news and discussion.
</p>
<h3 id="gotime"><a href="https://changelog.com/gotime">Go Time Podcast</a></h3>
<p>
The <a href="https://changelog.com/gotime">Go Time podcast</a> is a panel of Go experts and special guests
discussing the Go programming language, the community, and everything in between.
</p>
{{end}}
<h2 id="community">Community resources</h2>
<h3 id="go_user_groups"><a href="/wiki/GoUserGroups">Go User Groups</a></h3>
<p>
Each month in places around the world, groups of Go programmers ("gophers")
meet to talk about Go. Find a chapter near you.
</p>
{{if not $.GoogleCN}}
<h3 id="playground"><a href="/play">Go Playground</a></h3>
<p>A place to write, run, and share Go code.</p>
<h3 id="wiki"><a href="/wiki">Go Wiki</a></h3>
<p>A wiki maintained by the Go community.</p>
{{end}}
<h3 id="conduct"><a href="/conduct">Code of Conduct</a></h3>
<p>
Guidelines for participating in Go community spaces
and a reporting process for handling issues.
</p>

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#nav-main li { display: inline; }

19
_content/doc/play/fib.go Normal file
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package main
import "fmt"
// fib returns a function that returns
// successive Fibonacci numbers.
func fib() func() int {
a, b := 0, 1
return func() int {
a, b = b, a+b
return a
}
}
func main() {
f := fib()
// Function calls are evaluated left-to-right.
fmt.Println(f(), f(), f(), f(), f())
}

9
_content/doc/play/hello.go Normal file
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// You can edit this code!
// Click here and start typing.
package main
import "fmt"
func main() {
fmt.Println("Hello, 世界")
}

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// An implementation of Conway's Game of Life.
package main
import (
"bytes"
"fmt"
"math/rand"
"time"
)
// Field represents a two-dimensional field of cells.
type Field struct {
s [][]bool
w, h int
}
// NewField returns an empty field of the specified width and height.
func NewField(w, h int) *Field {
s := make([][]bool, h)
for i := range s {
s[i] = make([]bool, w)
}
return &Field{s: s, w: w, h: h}
}
// Set sets the state of the specified cell to the given value.
func (f *Field) Set(x, y int, b bool) {
f.s[y][x] = b
}
// Alive reports whether the specified cell is alive.
// If the x or y coordinates are outside the field boundaries they are wrapped
// toroidally. For instance, an x value of -1 is treated as width-1.
func (f *Field) Alive(x, y int) bool {
x += f.w
x %= f.w
y += f.h
y %= f.h
return f.s[y][x]
}
// Next returns the state of the specified cell at the next time step.
func (f *Field) Next(x, y int) bool {
// Count the adjacent cells that are alive.
alive := 0
for i := -1; i <= 1; i++ {
for j := -1; j <= 1; j++ {
if (j != 0 || i != 0) && f.Alive(x+i, y+j) {
alive++
}
}
}
// Return next state according to the game rules:
// exactly 3 neighbors: on,
// exactly 2 neighbors: maintain current state,
// otherwise: off.
return alive == 3 || alive == 2 && f.Alive(x, y)
}
// Life stores the state of a round of Conway's Game of Life.
type Life struct {
a, b *Field
w, h int
}
// NewLife returns a new Life game state with a random initial state.
func NewLife(w, h int) *Life {
a := NewField(w, h)
for i := 0; i < (w * h / 4); i++ {
a.Set(rand.Intn(w), rand.Intn(h), true)
}
return &Life{
a: a, b: NewField(w, h),
w: w, h: h,
}
}
// Step advances the game by one instant, recomputing and updating all cells.
func (l *Life) Step() {
// Update the state of the next field (b) from the current field (a).
for y := 0; y < l.h; y++ {
for x := 0; x < l.w; x++ {
l.b.Set(x, y, l.a.Next(x, y))
}
}
// Swap fields a and b.
l.a, l.b = l.b, l.a
}
// String returns the game board as a string.
func (l *Life) String() string {
var buf bytes.Buffer
for y := 0; y < l.h; y++ {
for x := 0; x < l.w; x++ {
b := byte(' ')
if l.a.Alive(x, y) {
b = '*'
}
buf.WriteByte(b)
}
buf.WriteByte('\n')
}
return buf.String()
}
func main() {
l := NewLife(40, 15)
for i := 0; i < 300; i++ {
l.Step()
fmt.Print("\x0c", l) // Clear screen and print field.
time.Sleep(time.Second / 30)
}
}

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// Peano integers are represented by a linked
// list whose nodes contain no data
// (the nodes are the data).
// http://en.wikipedia.org/wiki/Peano_axioms
// This program demonstrates that Go's automatic
// stack management can handle heavily recursive
// computations.
package main
import "fmt"
// Number is a pointer to a Number
type Number *Number
// The arithmetic value of a Number is the
// count of the nodes comprising the list.
// (See the count function below.)
// -------------------------------------
// Peano primitives
func zero() *Number {
return nil
}
func isZero(x *Number) bool {
return x == nil
}
func add1(x *Number) *Number {
e := new(Number)
*e = x
return e
}
func sub1(x *Number) *Number {
return *x
}
func add(x, y *Number) *Number {
if isZero(y) {
return x
}
return add(add1(x), sub1(y))
}
func mul(x, y *Number) *Number {
if isZero(x) || isZero(y) {
return zero()
}
return add(mul(x, sub1(y)), x)
}
func fact(n *Number) *Number {
if isZero(n) {
return add1(zero())
}
return mul(fact(sub1(n)), n)
}
// -------------------------------------
// Helpers to generate/count Peano integers
func gen(n int) *Number {
if n > 0 {
return add1(gen(n - 1))
}
return zero()
}
func count(x *Number) int {
if isZero(x) {
return 0
}
return count(sub1(x)) + 1
}
// -------------------------------------
// Print i! for i in [0,9]
func main() {
for i := 0; i <= 9; i++ {
f := count(fact(gen(i)))
fmt.Println(i, "! =", f)
}
}

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