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832 строки
35 KiB
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<title>Clang Compiler User's Manual</title>
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<div id="content">
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<h1>Clang Compiler User's Manual</h1>
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<ul>
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<li><a href="#intro">Introduction</a>
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<ul>
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<li><a href="#terminology">Terminology</a></li>
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<li><a href="#basicusage">Basic Usage</a></li>
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</ul>
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</li>
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<li><a href="#commandline">Command Line Options</a>
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<ul>
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<li><a href="#cl_diagnostics">Options to Control Error and Warning
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Messages</a></li>
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</ul>
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</li>
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<li><a href="#general_features">Language and Target-Independent Features</a>
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<ul>
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<li><a href="#diagnostics">Controlling Errors and Warnings</a></li>
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<ul>
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<li><a href="#diagnostics_display">Controlling How Clang Displays Diagnostics</a></li>
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<li><a href="#diagnostics_mappings">Diagnostic Mappings</a></li>
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<li><a href="#diagnostics_commandline">Controlling Diagnostics via Command Line Flags</a></li>
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<li><a href="#diagnostics_pragmas">Controlling Diagnostics via Pragmas</a></li>
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</ul>
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<li><a href="#precompiledheaders">Precompiled Headers</a></li>
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</ul>
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</li>
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<li><a href="#c">C Language Features</a>
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<ul>
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<li><a href="#c_ext">Extensions supported by clang</a></li>
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<li><a href="#c_modes">Differences between various standard modes</a></li>
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<li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li>
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<li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li>
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<li><a href="#c_ms">Microsoft extensions</a></li>
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</ul>
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</li>
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<li><a href="#objc">Objective-C Language Features</a>
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<ul>
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<li><a href="#objc_incompatibilities">Intentional Incompatibilities with
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GCC</a></li>
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</ul>
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</li>
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<li><a href="#cxx">C++ Language Features</a>
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</li>
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<li><a href="#objcxx">Objective C++ Language Features</a>
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</li>
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<li><a href="#target_features">Target-Specific Features and Limitations</a>
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<ul>
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<li><a href="#target_arch">CPU Architectures Features and Limitations</a>
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<ul>
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<li><a href="#target_arch_x86">X86</a></li>
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<li><a href="#target_arch_arm">ARM</a></li>
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<li><a href="#target_arch_other">Other platforms</a></li>
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</ul>
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</li>
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<li><a href="#target_os">Operating System Features and Limitations</a>
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<ul>
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<li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li>
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<li>Linux, etc.</li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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<!-- ======================================================================= -->
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<h2 id="intro">Introduction</h2>
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<!-- ======================================================================= -->
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<p>The Clang Compiler is an open-source compiler for the C family of programming
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languages, aiming to be the best in class implementation of these languages.
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Clang builds on the LLVM optimizer and code generator, allowing it to provide
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high-quality optimization and code generation support for many targets. For
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more general information, please see the <a href="http://clang.llvm.org">Clang
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Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p>
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<p>This document describes important notes about using Clang as a compiler for
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an end-user, documenting the supported features, command line options, etc. If
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you are interested in using Clang to build a tool that processes code, please
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see <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are
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interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang
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Static Analyzer</a>, please see its web page.</p>
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<p>Clang is designed to support the C family of programming languages, which
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includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a
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href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many
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dialects of those. For language-specific information, please see the
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corresponding language specific section:</p>
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<ul>
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<li><a href="#c">C Language</a>: K&R C, ANSI C89, ISO C90, ISO C94
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(C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li>
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<li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus
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variants depending on base language.</li>
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<li><a href="#cxx">C++ Language Features</a></li>
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<li><a href="#objcxx">Objective C++ Language</a></li>
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</ul>
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<p>In addition to these base languages and their dialects, Clang supports a
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broad variety of language extensions, which are documented in the corresponding
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language section. These extensions are provided to be compatible with the GCC,
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Microsoft, and other popular compilers as well as to improve functionality
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through Clang-specific features. The Clang driver and language features are
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intentionally designed to be as compatible with the GNU GCC compiler as
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reasonably possible, easing migration from GCC to Clang. In most cases, code
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"just works".</p>
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<p>In addition to language specific features, Clang has a variety of features
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that depend on what CPU architecture or operating system is being compiled for.
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Please see the <a href="target_features">Target-Specific Features and
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Limitations</a> section for more details.</p>
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<p>The rest of the introduction introduces some basic <a
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href="#terminology">compiler terminology</a> that is used throughout this manual
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and contains a basic <a href="#basicusage">introduction to using Clang</a>
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as a command line compiler.</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="terminology">Terminology</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic,
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optimizer</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="basicusage">Basic Usage</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>Intro to how to use a C compiler for newbies.</p>
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<p>
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compile + link
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compile then link
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debug info
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enabling optimizations
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picking a language to use, defaults to C99 by default. Autosenses based on
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extension.
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using a makefile
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</p>
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<!-- ======================================================================= -->
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<h2 id="commandline">Command Line Options</h2>
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<!-- ======================================================================= -->
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<p>
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This section is generally an index into other sections. It does not go into
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depth on the ones that are covered by other sections. However, the first part
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introduces the language selection and other high level options like -c, -g, etc.
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</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p><b>-Werror</b>: Turn warnings into errors.</p>
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<p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p>
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<p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is
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specified.</p>
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<p><b>-Wfoo</b>: Enable warning foo</p>
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<p><b>-Wno-foo</b>: Disable warning foo</p>
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<p><b>-w</b>: Disable all warnings.</p>
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<p><b>-pedantic</b>: Warn on language extensions.</p>
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<p><b>-pedantic-errors</b>: Error on language extensions.</p>
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<p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p>
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<!-- ================================================= -->
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<h4 id="cl_diag_formatting">Formatting of Diagnostics</h4>
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<!-- ================================================= -->
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<p>Clang aims to produce beautiful diagnostics by default, particularly for new
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users that first come to Clang. However, different people have different
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preferences, and sometimes Clang is driven by another program that wants to
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parse simple and consistent output, not a person. For these cases, Clang
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provides a wide range of options to control the exact output format of the
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diagnostics that it generates.</p>
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<dl>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in
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diagnostic.</dt>
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<dd>This option, which defaults to on, controls whether or not Clang prints the
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column number of a diagnostic. For example, when this is enabled, Clang will
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print something like:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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//
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</pre>
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<p>When this is disabled, Clang will print "test.c:28: warning..." with no
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column number.</p>
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</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print
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source file/line/column information in diagnostic.</dt>
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<dd>This option, which defaults to on, controls whether or not Clang prints the
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filename, line number and column number of a diagnostic. For example,
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when this is enabled, Clang will print something like:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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//
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</pre>
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<p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p>
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</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source
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line and ranges from source code in diagnostic.</dt>
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<dd>This option, which defaults to on, controls whether or not Clang prints the
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source line, source ranges, and caret when emitting a diagnostic. For example,
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when this is enabled, Clang will print something like:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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//
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</pre>
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<p>When this is disabled, Clang will just print:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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</pre>
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</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>:
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Enable <tt>[-Woption]</tt> information in diagnostic line.</dt>
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<dd>This option, which defaults to on,
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controls whether or not Clang prints the associated <A
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href="#cl_diag_warning_groups">warning group</a> option name when outputting
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a warning diagnostic. For example, in this output:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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//
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</pre>
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<p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing
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the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the
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diagnostic. This information tells you the flag needed to enable or disable the
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diagnostic, either from the command line or through <a
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href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>:
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Enable "FixIt" information in the diagnostics output.</dt>
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<dd>This option, which defaults to on, controls whether or not Clang prints the
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information on how to fix a specific diagnostic underneath it when it knows.
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For example, in this output:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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//
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</pre>
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<p>Passing <b>-fno-diagnostics-fixit-info</b> will prevent Clang from printing
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the "//" line at the end of the message. This information is useful for users
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who may not understand what is wrong, but can be confusing for machine
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parsing.</p>
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</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fdiagnostics-print-source-range-info">
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<b>-f[no-]diagnostics-print-source-range-info</b>:
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Print machine parsable information about source ranges.</dt>
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<dd>This option, which defaults to off, controls whether or not Clang prints
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information about source ranges in a machine parsable format after the
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file/line/column number information. The information is a simple sequence of
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brace enclosed ranges, where each range lists the start and end line/column
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locations. For example, in this output:</p>
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<pre>
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exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
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P = (P-42) + Gamma*4;
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~~~~~~ ^ ~~~~~~~
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</pre>
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<p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p>
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</dd>
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</dl>
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<!-- ===================================================== -->
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<h4 id="cl_diag_warning_groups">Individual Warning Groups</h4>
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<!-- ===================================================== -->
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<p>TODO: Generate this from tblgen. Define one anchor per warning group.</p>
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<dl>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at
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the end of a preprocessor directive.</dt>
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<dd>This option, which defaults to on, enables warnings about extra tokens at
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the end of preprocessor directives. For example:</p>
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<pre>
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test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
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#endif bad
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^
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</pre>
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<p>These extra tokens are not strictly conforming, and are usually best handled
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by commenting them out.</p>
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<p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>,
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and <a href="">-Wbaz</a>.</p>
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</dd>
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</dl>
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<!-- ======================================================================= -->
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<h2 id="general_features">Language and Target-Independent Features</h2>
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<!-- ======================================================================= -->
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="diagnostics">Controlling Errors and Warnings</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>Clang provides a number of ways to control which code constructs cause it to
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emit errors and warning messages, and how they are displayed to the console.</p>
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<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
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<p>When Clang emits a diagnostic, it includes rich information in the output,
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and gives you fine-grain control over which information is printed. Clang has
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the ability to print this information, and these are the options that control
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it:</p>
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<p>
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<ol>
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<li>A file/line/column indicator that shows exactly where the diagnostic occurs
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in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a
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href="#opt_fshow-source-location">-fshow-source-location</a>].</li>
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<li>A categorization of the diagnostic as a note, warning, error, or fatal
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error.</li>
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<li>A text string that describes what the problem is.</li>
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<li>An option that indicates how to control the diagnostic (for diagnostics that
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support it) [<a
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href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li>
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<li>The line of source code that the issue occurs on, along with a caret and
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ranges that indicate the important locations [<a
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href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li>
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<li>"FixIt" information, which is a concise explanation of how to fix the
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problem (when Clang is certain it knows) [<a
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href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li>
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<li>A machine-parsable representation of the ranges involved (off by
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default) [<a
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href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
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</ol></p>
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<p>For more information please see <a href="#cl_diag_formatting">Formatting of
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Diagnostics</a>.</p>
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<h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
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<p>All diagnostics are mapped into one of these 5 classes:</p>
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<p>
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<ul>
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<li>Ignored</li>
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<li>Note</li>
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<li>Warning</li>
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<li>Error</li>
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<li>Fatal</li>
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</ul></p>
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<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line Flags</h4>
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<p>-W flags, -pedantic, etc</p>
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<h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4>
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<p>Clang can also control what diagnostics are enabled through the use of
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pragmas in the source code. This is useful for turning off specific warnings
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in a section of source code. Clang supports GCC's pragma for compatibility
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with existing source code, as well as several extensions. </p>
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<p>The pragma may control any warning that can be used from the command line.
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Warnings may be set to ignored, warning, error, or fatal. The following
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example code will tell Clang or GCC to ignore the -Wall warnings:</p>
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<pre>
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#pragma GCC diagnostic ignored "-Wall"
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</pre>
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<p>In addition to all of the functionality of provided by GCC's pragma, Clang
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also allows you to push and pop the current warning state. This is particularly
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useful when writing a header file that will be compiled by other people, because
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you don't know what warning flags they build with.</p>
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<p>In the below example
|
|
-Wmultichar is ignored for only a single line of code, after which the
|
|
diagnostics return to whatever state had previously existed.</p>
|
|
|
|
<pre>
|
|
#pragma clang diagnostic push
|
|
#pragma clang diagnostic ignored "-Wmultichar"
|
|
|
|
char b = 'df'; // no warning.
|
|
|
|
#pragma clang diagnostic pop
|
|
</pre>
|
|
|
|
<p>The push and pop pragmas will save and restore the full diagnostic state of
|
|
the compiler, regardless of how it was set. That means that it is possible to
|
|
use push and pop around GCC compatible diagnostics and Clang will push and pop
|
|
them appropriately, while GCC will ignore the pushes and pops as unknown
|
|
pragmas. It should be noted that while Clang supports the GCC pragma, Clang and
|
|
GCC do not support the exact same set of warnings, so even when using GCC
|
|
compatible #pragmas there is no guarantee that they will have identical behaviour
|
|
on both compilers. </p>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="precompiledheaders">Precompiled Headers</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
|
|
headers</a> are a general approach employed by many compilers to reduce
|
|
compilation time. The underlying motivation of the approach is that it is
|
|
common for the same (and often large) header files to be included by
|
|
multiple source files. Consequently, compile times can often be greatly improved
|
|
by caching some of the (redundant) work done by a compiler to process headers.
|
|
Precompiled header files, which represent one of many ways to implement
|
|
this optimization, are literally files that represent an on-disk cache that
|
|
contains the vital information necessary to reduce some of the work
|
|
needed to process a corresponding header file. While details of precompiled
|
|
headers vary between compilers, precompiled headers have been shown to be a
|
|
highly effective at speeding up program compilation on systems with very large
|
|
system headers (e.g., Mac OS/X).</p>
|
|
|
|
<h4>Generating a PCH File</h4>
|
|
|
|
<p>To generate a PCH file using Clang, one invokes Clang with
|
|
the <b><tt>-x <i><language></i>-header</tt></b> option. This mirrors the
|
|
interface in GCC for generating PCH files:</p>
|
|
|
|
<pre>
|
|
$ gcc -x c-header test.h -o test.h.gch
|
|
$ clang -x c-header test.h -o test.h.pch
|
|
</pre>
|
|
|
|
<h4>Using a PCH File</h4>
|
|
|
|
<p>A PCH file can then be used as a prefix header when a
|
|
<b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p>
|
|
|
|
<pre>
|
|
$ clang -include test.h test.c -o test
|
|
</pre>
|
|
|
|
<p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt>
|
|
is available; if so, the contents of <tt>test.h</tt> (and the files it includes)
|
|
will be processed from the PCH file. Otherwise, Clang falls back to
|
|
directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
|
|
GCC.</p>
|
|
|
|
<p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files
|
|
for headers that are directly included within a source file. For example:</p>
|
|
|
|
<pre>
|
|
$ clang -x c-header test.h -o test.h.pch
|
|
$ cat test.c
|
|
#include "test.h"
|
|
$ clang test.c -o test
|
|
</pre>
|
|
|
|
<p>In this example, <tt>clang</tt> will not automatically use the PCH file for
|
|
<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
|
|
and not specified on the command line using <tt>-include</tt>.</p>
|
|
|
|
<h4>Relocatable PCH Files</h4>
|
|
<p>It is sometimes necessary to build a precompiled header from headers that
|
|
are not yet in their final, installed locations. For example, one might build a
|
|
precompiled header within the build tree that is then meant to be installed
|
|
alongside the headers. Clang permits the creation of "relocatable" precompiled
|
|
headers, which are built with a given path (into the build directory) and can
|
|
later be used from an installed location.</p>
|
|
|
|
<p>To build a relocatable precompiled header, place your headers into a
|
|
subdirectory whose structure mimics the installed location. For example, if you
|
|
want to build a precompiled header for the header <code>mylib.h</code> that
|
|
will be installed into <code>/usr/include</code>, create a subdirectory
|
|
<code>build/usr/include</code> and place the header <code>mylib.h</code> into
|
|
that subdirectory. If <code>mylib.h</code> depends on other headers, then
|
|
they can be stored within <code>build/usr/include</code> in a way that mimics
|
|
the installed location.</p>
|
|
|
|
<p>Building a relocatable precompiled header requires two additional arguments.
|
|
First, pass the <code>--relocatable-pch</code> flag to indicate that the
|
|
resulting PCH file should be relocatable. Second, pass
|
|
<code>-isysroot /path/to/build</code>, which makes all includes for your
|
|
library relative to the build directory. For example:</p>
|
|
|
|
<pre>
|
|
# clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
|
|
</pre>
|
|
|
|
<p>When loading the relocatable PCH file, the various headers used in the PCH
|
|
file are found from the system header root. For example, <code>mylib.h</code>
|
|
can be found in <code>/usr/include/mylib.h</code>. If the headers are installed
|
|
in some other system root, the <code>-isysroot</code> option can be used provide
|
|
a different system root from which the headers will be based. For example,
|
|
<code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for
|
|
<code>mylib.h</code> in
|
|
<code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p>
|
|
|
|
<p>Relocatable precompiled headers are intended to be used in a limited number
|
|
of cases where the compilation environment is tightly controlled and the
|
|
precompiled header cannot be generated after headers have been installed.
|
|
Relocatable precompiled headers also have some performance impact, because
|
|
the difference in location between the header locations at PCH build time vs.
|
|
at the time of PCH use requires one of the PCH optimizations,
|
|
<code>stat()</code> caching, to be disabled. However, this change is only
|
|
likely to affect PCH files that reference a large number of headers.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="c">C Language Features</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>The support for standard C in clang is feature-complete except for the C99
|
|
floating-point pragmas.</p>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="c_ext">Extensions supported by clang</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="c_modes">Differences between various standard modes</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>clang supports the -std option, which changes what language mode clang uses.
|
|
The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
|
|
for those modes. If no -std option is specified, clang defaults to gnu99 mode.
|
|
</p>
|
|
|
|
<p>Differences between all c* and gnu* modes:</p>
|
|
<ul>
|
|
<li>c* modes define "__STRICT_ANSI__".</li>
|
|
<li>Target-specific defines not prefixed by underscores, like "linux", are
|
|
defined in gnu* modes.</li>
|
|
<li>Trigraphs default to being off in gnu* modes; they can be enabled by the
|
|
-trigraphs option.</li>
|
|
<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
|
|
variants "__asm__" and "__typeof__" are recognized in all modes.</li>
|
|
<li>The Apple "blocks" extension is recognized by default in gnu* modes
|
|
on some platforms; it can be enabled in any mode with the "-fblocks"
|
|
option.</li>
|
|
</ul>
|
|
|
|
<p>Differences between *89 and *99 modes:</p>
|
|
<ul>
|
|
<li>The *99 modes default to implementing "inline" as specified in C99, while
|
|
the *89 modes implement the GNU version. This can be overridden for individual
|
|
functions with the __gnu_inline__ attribute.</li>
|
|
<li>Digraphs are not recognized in c89 mode.</li>
|
|
<li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
|
|
statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
|
|
<li>__STDC_VERSION__ is not defined in *89 modes.</li>
|
|
<li>"inline" is not recognized as a keyword in c89 mode.</li>
|
|
<li>"restrict" is not recognized as a keyword in *89 modes.</li>
|
|
<li>Commas are allowed in integer constant expressions in *99 modes.</li>
|
|
<li>Arrays which are not lvalues are not implicitly promoted to pointers in
|
|
*89 modes.</li>
|
|
<li>Some warnings are different.</li>
|
|
</ul>
|
|
|
|
<p>c94 mode is identical to c89 mode except that digraphs are enabled in
|
|
c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>clang tries to be compatible with gcc as much as possible, but some gcc
|
|
extensions are not implemented yet:</p>
|
|
|
|
<ul>
|
|
<li>clang does not support __label__
|
|
(<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
|
|
a relatively small feature, so it is likely to be implemented relatively
|
|
soon.</li>
|
|
|
|
<li>clang does not support attributes on function pointers
|
|
(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is
|
|
a relatively important feature, so it is likely to be implemented relatively
|
|
soon.</li>
|
|
|
|
<li>clang does not support #pragma weak
|
|
(<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to
|
|
the uses described in the bug, this is likely to be implemented at some
|
|
point, at least partially.</li>
|
|
|
|
<li>clang does not support #pragma align
|
|
(<a href="http://llvm.org/bugs/show_bug.cgi?id=3811">bug 3811</a>). This is a
|
|
relatively small feature, so it is likely to be implemented relatively
|
|
soon.</li>
|
|
|
|
<li>clang does not support code generation for local variables pinned to
|
|
registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>).
|
|
This is a relatively small feature, so it is likely to be implemented
|
|
relatively soon.</li>
|
|
|
|
<li>clang does not support decimal floating point types (_Decimal32 and
|
|
friends) or fixed-point types (_Fract and friends); nobody has expressed
|
|
interest in these features yet, so it's hard to say when they will be
|
|
implemented.</li>
|
|
|
|
<li>clang does not support nested functions; this is a complex feature which
|
|
is infrequently used, so it is unlikely to be implemented anytime soon.</li>
|
|
|
|
<li>clang does not support global register variables, this is unlikely
|
|
to be implemented soon because it requires additional LLVM backend support.
|
|
</li>
|
|
|
|
<li>clang does not support static initialization of flexible array
|
|
members. This appears to be a rarely used extension, but could be
|
|
implemented pending user demand.</li>
|
|
|
|
<li>clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len.
|
|
This is used rarely, but in some potentially interesting places, like the
|
|
glibc headers, so it may be implemented pending user demand. Note that
|
|
because clang pretends to be like GCC 4.2, and this extension was introduced
|
|
in 4.3, the glibc headers will not try to use this extension with clang at
|
|
the moment.</li>
|
|
|
|
<li>clang does not support the gcc extension for forward-declaring function
|
|
parameters; this has not showed up in any real-world code yet, though, so it
|
|
might never be implemented.</li>
|
|
|
|
</ul>
|
|
|
|
<p>This is not a complete list; if you find an unsupported extension
|
|
missing from this list, please send an e-mail to cfe-dev. This list
|
|
currently excludes C++; see <a href="#cxx">C++ Language Features</a>.
|
|
Also, this list does not include bugs in mostly-implemented features; please
|
|
see the <a href="http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer">
|
|
bug tracker</a> for known existing bugs (FIXME: Is there a section for
|
|
bug-reporting guidelines somewhere?).</p>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<ul>
|
|
|
|
<li>clang does not support the gcc extension that allows variable-length arrays
|
|
in structures. This is for a few of reasons: one, it is tricky
|
|
to implement, two, the extension is completely undocumented, and three, the
|
|
extension appears to be rarely used.</li>
|
|
|
|
<li>clang does not support duplicate definitions of a function where one is
|
|
inline. This complicates clients of the AST which normally can expect there is
|
|
at most one definition for each function. Source code using this feature should
|
|
be changed to define the inline and out-of-line definitions in separate
|
|
translation units.</li>
|
|
|
|
<li>clang does not have an equivalent to gcc's "fold"; this means that
|
|
clang doesn't accept some constructs gcc might accept in contexts where a
|
|
constant expression is required, like "x-x" where x is a variable, or calls
|
|
to C library functions like strlen.</li>
|
|
|
|
<li>clang does not support multiple alternative constraints in inline asm; this
|
|
is an extremely obscure feature which would be complicated to implement
|
|
correctly.</li>
|
|
|
|
<li>clang does not support __builtin_apply and friends; this extension is
|
|
extremely obscure and difficult to implement reliably.</li>
|
|
|
|
</ul>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="c_ms">Microsoft extensions</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>clang has some experimental support for extensions from
|
|
Microsoft Visual C++; to enable it, use the -fms-extensions command-line
|
|
option. This is the default for Windows targets. Note that the
|
|
support is incomplete; enabling Microsoft extensions will silently drop
|
|
certain constructs (including __declspec and Microsoft-style asm statements).
|
|
</p>
|
|
|
|
<li>clang does not support the Microsoft extension where anonymous
|
|
record members can be declared using user defined typedefs.</li>
|
|
|
|
<li>clang supports the Microsoft "#pragma pack" feature for
|
|
controlling record layout. GCC also contains support for this feature,
|
|
however where MSVC and GCC are incompatible clang follows the MSVC
|
|
definition.</li>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="objc">Objective-C Language Features</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="objc_incompatibilities">Intentional Incompatibilities with GCC</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>No cast of super, no lvalue casts.</p>
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="cxx">C++ Language Features</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>At this point, Clang C++ is not generally useful. However, Clang C++ support
|
|
is under active development and is progressing rapidly. Please see the <a
|
|
href="http://clang.llvm.org/cxx_status.html">C++ Status</a> page for details or
|
|
ask on the mailing list about how you can help.</p>
|
|
|
|
<p>Note that the clang driver will refuse to even try to use clang to compile
|
|
C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. If
|
|
you really want to play with Clang's C++ support, please pass that flag. </p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="objcxx">Objective C++ Language Features</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>At this point, Clang C++ support is not generally useful (and therefore,
|
|
neither is Objective-C++). Please see the <a href="#cxx">C++ section</a> for
|
|
more information.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="target_features">Target-Specific Features and Limitations</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="target_arch">CPU Architectures Features and Limitations</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<!-- ======================== -->
|
|
<h4 id="target_arch_x86">X86</h4>
|
|
<!-- ======================== -->
|
|
<p>The support for X86 (both 32-bit and 64-bit) is considered stable
|
|
on Darwin (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to
|
|
correctly compile large C and Objective-C codebases. (FIXME: Anything specific
|
|
we want to say here? Possibly mention some LLVM x86 limitations?)
|
|
|
|
<!-- ======================== -->
|
|
<h4 id="target_arch_arm">ARM</h4>
|
|
<!-- ======================== -->
|
|
ARM support is mostly feature-complete, but still experimental; it hasn't
|
|
undergone significant testing.
|
|
|
|
<!-- ======================== -->
|
|
<h4 id="target_arch_other">Other platforms</h4>
|
|
<!-- ======================== -->
|
|
clang currently contains some support for PPC and Sparc; however, significant
|
|
pieces of code generation are still missing, and they haven't undergone
|
|
significant testing.
|
|
|
|
<p>clang contains some support for the embedded PIC16 processor
|
|
(FIXME: I haven't been keeping track of this; what should this say?).
|
|
|
|
<p>clang contains limited support for the MSP430 embedded processor, but both
|
|
the clang support and the LLVM backend support are highly experimental.
|
|
|
|
<p>Other platforms are completely unsupported at the moment. Adding the
|
|
minimal support needed for parsing and semantic analysis on a new platform
|
|
is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level
|
|
of support is also sufficient for conversion to LLVM IR for simple programs.
|
|
Proper support for conversion to LLVM IR requires adding code to
|
|
lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though.
|
|
Generating assembly requires a suitable LLVM backend.
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="target_os">Operating System Features and Limitations</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<!-- ======================================= -->
|
|
<h4 id="target_os_darwin">Darwin (Mac OS/X)</h4>
|
|
<!-- ======================================= -->
|
|
|
|
<p>No __thread support, 64-bit ObjC support requires SL tools.</p>
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|