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1003 lines
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HTML
1003 lines
42 KiB
HTML
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<title>Clang Compiler User's Manual</title>
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<body>
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<!--#include virtual="../menu.html.incl"-->
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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_categories">Diagnostic Categories</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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<li><a href="#analyzer_diagnositics">Controlling Static Analyzer Diagnostics</a></li>
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</ul>
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<li><a href="#precompiledheaders">Precompiled Headers</a></li>
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<li><a href="#codegen">Controlling Code Generation</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="#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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<p><b>-ferror-limit=123</b>: Stop emitting diagnostics after 123 errors have
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been produced. The default is 20, and the error limit can be disabled with
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-ferror-limit=0.</p>
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<p><b>-ftemplate-backtrace-limit=123</b>: Only emit up to 123 template instantiation notes within the template instantiation backtrace for a single warning or error. The default is 10, and the limit can be disabled with -ftemplate-backtrace-limit=0.</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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<b><font color="black">test.c:28:8: <font color="magenta">warning</font>: extra tokens at end of #endif directive [-Wextra-tokens]</font></b>
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#endif bad
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<font color="green">^</font>
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<font color="green">//</font>
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</pre>
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</dd>
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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fcolor_diagnostics"><b>-f[no-]color-diagnostics</b>: </dt>
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<dd>This option, which defaults to on when a color-capable terminal is
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detected, controls whether or not Clang prints diagnostics in color.
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When this option is enabled, Clang will use colors to highlight
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specific parts of the diagnostic, e.g.,
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<pre>
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<test.c:2:8: 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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</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-show-category"><b>-fdiagnostics-show-category=none/id/name</b>:
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Enable printing category information in diagnostic line.</dt>
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<dd>This option, which defaults to "none",
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controls whether or not Clang prints the category associated with a diagnostic
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when emitting it. Each diagnostic may or many not have an associated category,
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if it has one, it is listed in the diagnostic categorization field of the
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diagnostic line (in the []'s).</p>
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<p>For example, a format string warning will produce these three renditions
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based on the setting of this option:</p>
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<pre>
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t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
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t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,1</b>]
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t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,Format String</b>]
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</pre>
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<p>This category can be used by clients that want to group diagnostics by
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category, so it should be a high level category. We want dozens of these, not
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hundreds or thousands of them.</p>
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</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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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_fdiagnostics-parseable-fixits">
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<b>-fdiagnostics-parseable-fixits</b>:
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Print Fix-Its in a machine parseable form.</dt>
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<dd><p>This option makes Clang print available Fix-Its in a machine parseable format at the end of diagnostics. The following example illustrates the format:</p>
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<pre>
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fix-it:"t.cpp":{7:25-7:29}:"Gamma"
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</pre>
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<p>The range printed is a half-open range, so in this example the characters at column 25 up to but not including column 29 on line 7 in t.cpp should be replaced with the string "Gamma". Either the range or the replacement string may be empty (representing strict insertions and strict erasures, respectively). Both the file name and the insertion string escape backslash (as "\\"), tabs (as "\t"), newlines (as "\n"), double quotes(as "\"") and non-printable characters (as octal "\xxx").</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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<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
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<dt id="opt_Wambiguous-member-template"><b>-Wambiguous-member-template</b>:
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Warn about unqualified uses of a member template whose name resolves
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to another template at the location of the use.</dt>
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<dd>This option, which defaults to on, enables a warning in the
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following code:</p>
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<pre>
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template<typename T> struct set{};
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template<typename T> struct trait { typedef const T& type; };
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struct Value {
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template<typename T> void set(typename trait<T>::type value) {}
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};
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void foo() {
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Value v;
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v.set<double>(3.2);
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}
|
|
</pre>
|
|
|
|
<p>C++ [basic.lookup.classref] requires this to be an error, but,
|
|
because it's hard to work around, Clang downgrades it to a warning as
|
|
an extension.</p>
|
|
</dd>
|
|
|
|
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
|
|
<dt id="opt_Wbind-to-temporary-copy"><b>-Wbind-to-temporary-copy</b>: Warn about
|
|
an unusable copy constructor when binding a reference to a temporary.</dt>
|
|
<dd>This option, which defaults to on, enables warnings about binding a
|
|
reference to a temporary when the temporary doesn't have a usable copy
|
|
constructor. For example:</p>
|
|
|
|
<pre>
|
|
struct NonCopyable {
|
|
NonCopyable();
|
|
private:
|
|
NonCopyable(const NonCopyable&);
|
|
};
|
|
void foo(const NonCopyable&);
|
|
void bar() {
|
|
foo(NonCopyable()); // Disallowed in C++98; allowed in C++0x.
|
|
}
|
|
</pre>
|
|
<pre>
|
|
struct NonCopyable2 {
|
|
NonCopyable2();
|
|
NonCopyable2(NonCopyable2&);
|
|
};
|
|
void foo(const NonCopyable2&);
|
|
void bar() {
|
|
foo(NonCopyable2()); // Disallowed in C++98; allowed in C++0x.
|
|
}
|
|
</pre>
|
|
|
|
<p>Note that if <tt>NonCopyable2::NonCopyable2()</tt> has a default
|
|
argument whose instantiation produces a compile error, that error will
|
|
still be a hard error in C++98 mode even if this warning is turned
|
|
off.</p>
|
|
|
|
</dd>
|
|
|
|
</dl>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="general_features">Language and Target-Independent Features</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="diagnostics">Controlling Errors and Warnings</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>Clang provides a number of ways to control which code constructs cause it to
|
|
emit errors and warning messages, and how they are displayed to the console.</p>
|
|
|
|
<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
|
|
|
|
<p>When Clang emits a diagnostic, it includes rich information in the output,
|
|
and gives you fine-grain control over which information is printed. Clang has
|
|
the ability to print this information, and these are the options that control
|
|
it:</p>
|
|
|
|
<p>
|
|
<ol>
|
|
<li>A file/line/column indicator that shows exactly where the diagnostic occurs
|
|
in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a
|
|
href="#opt_fshow-source-location">-fshow-source-location</a>].</li>
|
|
<li>A categorization of the diagnostic as a note, warning, error, or fatal
|
|
error.</li>
|
|
<li>A text string that describes what the problem is.</li>
|
|
<li>An option that indicates how to control the diagnostic (for diagnostics that
|
|
support it) [<a
|
|
href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li>
|
|
<li>A <a href="#diagnostics_categories">high-level category</a> for the
|
|
diagnostic for clients that want to group diagnostics by class (for
|
|
diagnostics that support it) [<a
|
|
href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a>].</li>
|
|
<li>The line of source code that the issue occurs on, along with a caret and
|
|
ranges that indicate the important locations [<a
|
|
href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li>
|
|
<li>"FixIt" information, which is a concise explanation of how to fix the
|
|
problem (when Clang is certain it knows) [<a
|
|
href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li>
|
|
<li>A machine-parsable representation of the ranges involved (off by
|
|
default) [<a
|
|
href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
|
|
</ol></p>
|
|
|
|
<p>For more information please see <a href="#cl_diag_formatting">Formatting of
|
|
Diagnostics</a>.</p>
|
|
|
|
|
|
<h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
|
|
|
|
<p>All diagnostics are mapped into one of these 5 classes:</p>
|
|
|
|
<p>
|
|
<ul>
|
|
<li>Ignored</li>
|
|
<li>Note</li>
|
|
<li>Warning</li>
|
|
<li>Error</li>
|
|
<li>Fatal</li>
|
|
</ul></p>
|
|
|
|
<h4 id="diagnostics_categories">Diagnostic Categories</h4>
|
|
|
|
<p>Though not shown by default, diagnostics may each be associated with a
|
|
high-level category. This category is intended to make it possible to triage
|
|
builds that produce a large number of errors or warnings in a grouped way.
|
|
</p>
|
|
|
|
<p>Categories are not shown by default, but they can be turned on with the
|
|
<a href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a> option.
|
|
When set to "<tt>name</tt>", the category is printed textually in the diagnostic
|
|
output. When it is set to "<tt>id</tt>", a category number is printed. The
|
|
mapping of category names to category id's can be obtained by running '<tt>clang
|
|
--print-diagnostic-categories</tt>'.
|
|
</p>
|
|
|
|
<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line
|
|
Flags</h4>
|
|
|
|
<p>-W flags, -pedantic, etc</p>
|
|
|
|
<h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4>
|
|
|
|
<p>Clang can also control what diagnostics are enabled through the use of
|
|
pragmas in the source code. This is useful for turning off specific warnings
|
|
in a section of source code. Clang supports GCC's pragma for compatibility
|
|
with existing source code, as well as several extensions. </p>
|
|
|
|
<p>The pragma may control any warning that can be used from the command line.
|
|
Warnings may be set to ignored, warning, error, or fatal. The following
|
|
example code will tell Clang or GCC to ignore the -Wall warnings:</p>
|
|
|
|
<pre>
|
|
#pragma GCC diagnostic ignored "-Wall"
|
|
</pre>
|
|
|
|
<p>In addition to all of the functionality of provided by GCC's pragma, Clang
|
|
also allows you to push and pop the current warning state. This is particularly
|
|
useful when writing a header file that will be compiled by other people, because
|
|
you don't know what warning flags they build with.</p>
|
|
|
|
<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>
|
|
|
|
<h4 id="analyzer_diagnositics">Controlling Static Analyzer Diagnostics</h4>
|
|
|
|
<p>While not strictly part of the compiler, the diagnostics from Clang's <a
|
|
href="http://clang-analyzer.llvm.org">static analyzer</a> can also be influenced
|
|
by the user via changes to the source code. This can be done in two ways:
|
|
|
|
<ul>
|
|
|
|
<li id="analyzer_annotations"><b>Annotations</b>: The static analyzer recognizes various GCC-style
|
|
attributes (e.g., <tt>__attribute__((nonnull)))</tt>) that can either suppress
|
|
static analyzer warnings or teach the analyzer about code invariants which
|
|
enable it to find more bugs. While many of these attributes are standard GCC
|
|
attributes, additional ones have added to Clang to specifically support the
|
|
static analyzer. Detailed information on these annotations can be found in the
|
|
<a href="http://clang-analyzer.llvm.org/annotations.html">analyzer's
|
|
documentation</a>.</li>
|
|
|
|
<li><b><tt>__clang_analyzer__</tt></b>: When the static analyzer is using Clang
|
|
to parse source files, it implicitly defines the preprocessor macro
|
|
<tt>__clang_analyzer__</tt>. While discouraged, code can use this macro to
|
|
selectively exclude code the analyzer examines. Here is an example:
|
|
|
|
<pre>
|
|
#ifndef __clang_analyzer__
|
|
// Code not to be analyzed
|
|
#endif
|
|
</pre>
|
|
|
|
In general, this usage is discouraged. Instead, we prefer that users file bugs
|
|
against the analyzer when it flags false positives. There is also active
|
|
discussion of allowing users in the future to selectively silence specific
|
|
analyzer warnings (some which can already be done using <a
|
|
href="analyzer_annotations">annotations</a>).</li>
|
|
|
|
</ul>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<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>
|
|
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
<h3 id="codegen">Controlling Code Generation</h3>
|
|
<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
|
|
|
|
<p>Clang provides a number of ways to control code generation. The options are listed below.</p>
|
|
|
|
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
|
|
<dt id="opt_fcatch-undefined-behavior"><b>-fcatch-undefined-behavior</b>: Turn
|
|
on runtime code generation to check for undefined behavior.</dt>
|
|
|
|
<dd>This option, which defaults to off, controls whether or not Clang
|
|
adds runtime checks for undefined runtime behavior. If the check fails,
|
|
<tt>__builtin_trap()</tt> is used to indicate failure.
|
|
The checks are:
|
|
<p>
|
|
<li>Subscripting where the static type of one operand is variable
|
|
which is decayed from an array type and the other operand is
|
|
greater than the size of the array or less than zero.</li>
|
|
<li>Shift operators where the amount shifted is greater or equal to the
|
|
promoted bit-width of the left-hand-side or less than zero.</li>
|
|
<li>If control flow reaches __builtin_unreachable.
|
|
<li>When llvm implements more __builtin_object_size support, reads and
|
|
writes for objects that __builtin_object_size indicates we aren't
|
|
accessing valid memory. Bit-fields and vectors are not yet checked.
|
|
</p>
|
|
</dd>
|
|
|
|
<dt id="opt_fno-assume-sane-operator-new"><b>-fno-assume-sane-operator-new</b>:
|
|
Don't assume that the C++'s new operator is sane.</dt>
|
|
<dd>This option tells the compiler to do not assume that C++'s global new
|
|
operator will always return a pointer that do not
|
|
alias any other pointer when the function returns.</dd>
|
|
|
|
<!-- ======================================================================= -->
|
|
<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 #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 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 allows setting _MCS_VER with -fmsc-version=. It defaults to 1300 which
|
|
is the same as Visual C/C++ 2003. Any number is supported and can greatly affect
|
|
what Windows SDK and c++stdlib headers clang can compile. This option will be
|
|
removed when clang supports the full set of MS extensions required for these
|
|
headers.</li>
|
|
|
|
<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="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 many large C, C++, Objective-C, and Objective-C++ codebases.</p>
|
|
|
|
<!-- ======================== -->
|
|
<h4 id="target_arch_arm">ARM</h4>
|
|
<!-- ======================== -->
|
|
|
|
<p>The support for ARM (specifically ARMv6 and ARMv7) is considered stable on
|
|
Darwin (iOS): it has been tested to correctly compile many large C, C++,
|
|
Objective-C, and Objective-C++ codebases.</p>
|
|
|
|
<p>It is not quite feature complete, the following areas still need work:</p>
|
|
<ul>
|
|
<li>Vector support (NEON in particular) is not yet well tested and may have
|
|
ABI errors.</li>
|
|
<li>Clang only supports a limited number of ARM architectures. It does not yet
|
|
fully support ARMv5, for example.</li>
|
|
</ul>
|
|
|
|
<!-- ======================== -->
|
|
<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 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>
|