Go support for Protocol Buffers - Google's data interchange format
Copyright 2010 Google Inc.
http://code.google.com/p/goprotobuf/

This software implements Go bindings for protocol buffers.  For
information about protocol buffers themselves, see
	http://code.google.com/apis/protocolbuffers/
To use this software, you must first install the standard C++
implementation of protocol buffers from
	http://code.google.com/p/protobuf/
And of course you must also install the Go compiler and tools from
	http://code.google.com/p/go/
See
	http://golang.org/doc/install.html
for details or, if you are using gccgo, follow the instructions at
	http://golang.org/doc/gccgo_install.html

This software has two parts: a 'protocol compiler plugin' that
generates Go source files that, once compiled, can access and manage
protocol buffers; and a library that implements run-time support for
encoding (marshaling), decoding (unmarshaling), and accessing protocol
buffers.

There is no support for RPC in Go using protocol buffers.  It may come
once a standard RPC protocol develops for protobufs.

Extensions are not supported by the plugin in this release, although
they are in the library.  The work will be completed soon.

There are no insertion points in the plugin.

To install this code:

The simplest way is to run goinstall.

	# Grab the code from the repository and install the proto package.
	goinstall goprotobuf.googlecode.com/hg/proto

	# Compile and install the compiler plugin
	cd $GOROOT/src/pkg/goprotobuf.googlecode.com/hg/compiler
	make install

The compiler plugin, protoc-gen-go, will be installed in $GOBIN,
defaulting to $HOME/bin.  It must be in your $PATH for the protocol
compiler, protoc, to find it.

Once the software is installed, there are two steps to using it.
First you must compile the protocol buffer definitions and then import
them, with the support library, into your program.

To compile the protocol buffer definition, write a Makefile in the
style shown in the commentin the file Make.protobuf.  If your Makefile
includes Make.protobuf, the rest should follow automatically.  The
generated code can be compiled separately or as part of a normal Go
package.

The generated files will be suffixed .pb.go.  See the Test code below
for an example using such a file.



The package comment for the proto library contains text describing
the interface provided in Go for protocol buffers. Here is an edited
version.

==========

The proto package converts data structures to and from the
wire format of protocol buffers.  It works in concert with the
Go source code generated for .proto files by the protocol compiler.

A summary of the properties of the protocol buffer interface
for a protocol buffer variable v:

  - Names are turned from camel_case to CamelCase for export.
  - There are no methods on v to set and get fields; just treat
  	them as structure fields.
  - The zero value for a struct is its correct initialization state.
	All desired fields must be set before marshaling.
  - A Reset() method will restore a protobuf struct to its zero state.
  - Each type T has a method NewT() to create a new instance. It
	is equivalent to new(T).
  - Non-repeated fields are pointers to the values; nil means unset.
	That is, optional or required field int32 f becomes F *int32.
  - Repeated fields are slices.
  - Helper functions are available to simplify the getting and setting of fields:
  	foo.String = proto.String("hello") // set field
  	s := proto.GetString(foo.String)  // get field
  - Constants are defined to hold the default values of all fields that
	have them.  They have the form Default_StructName_FieldName.
  - Enums are given type names and maps between names to values,
  	plus a helper function to create values.  Enum values are prefixed
  	with the enum's type name.
  - Nested groups and enums have type names prefixed with the name of
  	the surrounding message type.
  - Marshal and Unmarshal are functions to encode and decode the wire format.

Consider file test.proto, containing

	package example;
	
	enum FOO { X = 17; };
	
	message Test {
	  required string label = 1;
	  optional int32 type = 2 [default=77];
	  repeated int64 reps = 3;
	  optional group OptionalGroup = 4 {
	    required string RequiredField = 5;
	  };
	}

To build a package from test.proto and some other Go files, write a
Makefile like this:

	include $(GOROOT)/src/Make.$(GOARCH)

	TARG=path/to/example
	GOFILES=\
		test.pb.go\
		other.go

	include $(GOROOT)/src/Make.pkg
	include $(GOROOT)/src/pkg/goprotobuf.googlecode.com/hg/Make.protobuf


To create and play with a Test object from the example package,

	package main

	import (
		"log"

		"goprotobuf.googlecode.com/hg/proto"
		"path/to/example"
	)

	func main() {
		test := &example.Test {
			Label: proto.String("hello"),
			Type: proto.Int32(17),
			Optionalgroup: &example.Test_OptionalGroup {
				RequiredField: proto.String("good bye"),
			},
		}
		data, err := proto.Marshal(test)
		if err != nil {
			log.Exit("marshaling error:", err)
		}
		newTest := example.NewTest()
		err = proto.Unmarshal(data, newTest)
		if err != nil {
			log.Exit("unmarshaling error:", err)
		}
		// Now test and newTest contain the same data.
		if proto.GetString(test.Label) != proto.GetString(newTest.Label) {
			log.Exit("data mismatch %q %q", proto.GetString(test.Label), proto.GetString(newTest.Label))
		}
		// etc.
	}