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| |
| <h1>Clang Compiler User's Manual</h1> |
| |
| <ul> |
| <li><a href="#intro">Introduction</a> |
| <ul> |
| <li><a href="#terminology">Terminology</a></li> |
| <li><a href="#basicusage">Basic Usage</a></li> |
| </ul> |
| </li> |
| <li><a href="#commandline">Command Line Options</a> |
| <ul> |
| <li><a href="#cl_diagnostics">Options to Control Error and Warning |
| Messages</a></li> |
| </ul> |
| </li> |
| <li><a href="#general_features">Language and Target-Independent Features</a> |
| <ul> |
| <li><a href="#diagnostics">Controlling Errors and Warnings</a></li> |
| <li><a href="#precompiledheaders">Precompiled Headers</a></li> |
| </ul> |
| </li> |
| <li><a href="#c">C Language Features</a> |
| <ul> |
| <li><a href="#c_ext">Extensions supported by clang</a></li> |
| <li><a href="#c_modes">Differences between various standard modes</a></li> |
| <li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li> |
| <li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li> |
| <li><a href="#c_ms">Microsoft extensions</a></li> |
| </ul> |
| </li> |
| <li><a href="#objc">Objective-C Language Features</a> |
| <ul> |
| <li><a href="#objc_incompatibilities">Intentional Incompatibilities with |
| GCC</a></li> |
| </ul> |
| </li> |
| <li><a href="#cxx">C++ Language Features</a> |
| </li> |
| <li><a href="#objcxx">Objective C++ Language Features</a> |
| </li> |
| <li><a href="#target_features">Target-Specific Features and Limitations</a> |
| <ul> |
| <li><a href="#target_arch">CPU Architectures Features and Limitations</a> |
| <ul> |
| <li><a href="#target_arch_x86">X86</a></li> |
| <li><a href="#target_arch_arm">ARM</a></li> |
| <li><a href="#target_arch_other">Other platforms</a></li> |
| </ul> |
| </li> |
| <li><a href="#target_os">Operating System Features and Limitations</a> |
| <ul> |
| <li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li> |
| <li>Linux, etc.</li> |
| </ul> |
| |
| </li> |
| </ul> |
| </li> |
| </ul> |
| |
| |
| <!-- ======================================================================= --> |
| <h2 id="intro">Introduction</h2> |
| <!-- ======================================================================= --> |
| |
| <p>The Clang Compiler is an open-source compiler for the C family of programming |
| languages, aiming to be the best in class implementation of these languages. |
| Clang builds on the LLVM optimizer and code generator, allowing it to provide |
| high-quality optimization and code generation support for many targets. For |
| more general information, please see the <a href="http://clang.llvm.org">Clang |
| Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p> |
| |
| <p>This document describes important notes about using Clang as a compiler for |
| an end-user, documenting the supported features, command line options, etc. If |
| you are interested in using Clang to build a tool that processes code, please |
| see <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are |
| interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang |
| Static Analyzer</a>, please see its web page.</p> |
| |
| <p>Clang is designed to support the C family of programming languages, which |
| includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a |
| href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many |
| dialects of those. For language-specific information, please see the |
| corresponding language specific section:</p> |
| |
| <ul> |
| <li><a href="#c">C Language</a>: K&R C, ANSI C89, ISO C90, ISO C94 |
| (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li> |
| <li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus |
| variants depending on base language.</li> |
| <li><a href="#cxx">C++ Language Features</a></li> |
| <li><a href="#objcxx">Objective C++ Language</a></li> |
| </ul> |
| |
| <p>In addition to these base languages and their dialects, Clang supports a |
| broad variety of language extensions, which are documented in the corresponding |
| language section. These extensions are provided to be compatible with the GCC, |
| Microsoft, and other popular compilers as well as to improve functionality |
| through Clang-specific features. The Clang driver and language features are |
| intentionally designed to be as compatible with the GNU GCC compiler as |
| reasonably possible, easing migration from GCC to Clang. In most cases, code |
| "just works".</p> |
| |
| <p>In addition to language specific features, Clang has a variety of features |
| that depend on what CPU architecture or operating system is being compiled for. |
| Please see the <a href="target_features">Target-Specific Features and |
| Limitations</a> section for more details.</p> |
| |
| <p>The rest of the introduction introduces some basic <a |
| href="#terminology">compiler terminology</a> that is used throughout this manual |
| and contains a basic <a href="#basicusage">introduction to using Clang</a> |
| as a command line compiler.</p> |
| |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| <h3 id="terminology">Terminology</h3> |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| |
| <p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic, |
| optimizer</p> |
| |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| <h3 id="basicusage">Basic Usage</h3> |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| |
| <p>Intro to how to use a C compiler for newbies.</p> |
| <p> |
| compile + link |
| |
| compile then link |
| |
| debug info |
| |
| enabling optimizations |
| |
| picking a language to use, defaults to C99 by default. Autosenses based on |
| extension. |
| |
| using a makefile |
| </p> |
| |
| |
| <!-- ======================================================================= --> |
| <h2 id="commandline">Command Line Options</h2> |
| <!-- ======================================================================= --> |
| |
| <p> |
| This section is generally an index into other sections. It does not go into |
| depth on the ones that are covered by other sections. However, the first part |
| introduces the language selection and other high level options like -c, -g, etc. |
| </p> |
| |
| |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| <h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3> |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| |
| <p><b>-Werror</b>: Turn warnings into errors.</p> |
| <p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p> |
| <p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is |
| specified.</p> |
| <p><b>-Wfoo</b>: Enable warning foo</p> |
| <p><b>-Wno-foo</b>: Disable warning foo</p> |
| <p><b>-w</b>: Disable all warnings.</p> |
| <p><b>-pedantic</b>: Warn on language extensions.</p> |
| <p><b>-pedantic-errors</b>: Error on language extensions.</p> |
| <p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p> |
| |
| <!-- ================================================= --> |
| <h4 id="cl_diag_formatting">Formatting of Diagnostics</h4> |
| <!-- ================================================= --> |
| |
| <p>Clang aims to produce beautiful diagnostics by default, particularly for new |
| users that first come to Clang. However, different people have different |
| preferences, and sometimes Clang is driven by another program that wants to |
| parse simple and consistent output, not a person. For these cases, Clang |
| provides a wide range of options to control the exact output format of the |
| diagnostics that it generates.</p> |
| |
| <dl> |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in |
| diagnostic.</dt> |
| <dd>This option, which defaults to on, controls whether or not Clang prints the |
| column number of a diagnostic. For example, when this is enabled, Clang will |
| print something like:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| // |
| </pre> |
| |
| <p>When this is disabled, Clang will print "test.c:28: warning..." with no |
| column number.</p> |
| </dd> |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print |
| source file/line/column information in diagnostic.</dt> |
| <dd>This option, which defaults to on, controls whether or not Clang prints the |
| filename, line number and column number of a diagnostic. For example, |
| when this is enabled, Clang will print something like:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| // |
| </pre> |
| |
| <p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p> |
| </dd> |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source |
| line and ranges from source code in diagnostic.</dt> |
| <dd>This option, which defaults to on, controls whether or not Clang prints the |
| source line, source ranges, and caret when emitting a diagnostic. For example, |
| when this is enabled, Clang will print something like:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| // |
| </pre> |
| |
| <p>When this is disabled, Clang will just print:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| </pre> |
| |
| </dd> |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>: |
| Enable <tt>[-Woption]</tt> information in diagnostic line.</dt> |
| <dd>This option, which defaults to on, |
| controls whether or not Clang prints the associated <A |
| href="#cl_diag_warning_groups">warning group</a> option name when outputting |
| a warning diagnostic. For example, in this output:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| // |
| </pre> |
| |
| <p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing |
| the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the |
| diagnostic. This information tells you the flag needed to enable or disable the |
| diagnostic, either from the command line or through <a |
| href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd> |
| |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>: |
| Enable "FixIt" information in the diagnostics output.</dt> |
| <dd>This option, which defaults to on, controls whether or not Clang prints the |
| information on how to fix a specific diagnostic underneath it when it knows. |
| For example, in this output:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| // |
| </pre> |
| |
| <p>Passing <b>-fno-diagnostics-fixit-info</b> will prevent Clang from printing |
| the "//" line at the end of the message. This information is useful for users |
| who may not understand what is wrong, but can be confusing for machine |
| parsing.</p> |
| </dd> |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_fdiagnostics-print-source-range-info"> |
| <b>-f[no-]diagnostics-print-source-range-info</b>: |
| Print machine parsable information about source ranges.</dt> |
| <dd>This option, which defaults to off, controls whether or not Clang prints |
| information about source ranges in a machine parsable format after the |
| file/line/column number information. The information is a simple sequence of |
| brace enclosed ranges, where each range lists the start and end line/column |
| locations. For example, in this output:</p> |
| |
| <pre> |
| exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float') |
| P = (P-42) + Gamma*4; |
| ~~~~~~ ^ ~~~~~~~ |
| </pre> |
| |
| <p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p> |
| </dd> |
| |
| |
| </dl> |
| |
| |
| |
| |
| <!-- ===================================================== --> |
| <h4 id="cl_diag_warning_groups">Individual Warning Groups</h4> |
| <!-- ===================================================== --> |
| |
| <p>TODO: Generate this from tblgen. Define one anchor per warning group.</p> |
| |
| |
| <dl> |
| |
| |
| <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| <dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at |
| the end of a preprocessor directive.</dt> |
| <dd>This option, which defaults to on, enables warnings about extra tokens at |
| the end of preprocessor directives. For example:</p> |
| |
| <pre> |
| test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| #endif bad |
| ^ |
| </pre> |
| |
| <p>These extra tokens are not strictly conforming, and are usually best handled |
| by commenting them out.</p> |
| |
| <p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>, |
| and <a href="">-Wbaz</a>.</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>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>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>Controlling Which Diagnostics Clang Generates</h4> |
| |
| <p>mappings: ignore, note, warning, error, fatal</p> |
| |
| <p> |
| The two major classes are control from the command line and control via pragmas |
| in your code.</p> |
| |
| |
| <p>-W flags, -pedantic, etc</p> |
| |
| <p>pragma GCC diagnostic</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.cth |
| $ 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> |
| |
| |
| <!-- ======================================================================= --> |
| <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> |
| <li>Some warnings are different.</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> |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| |
| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <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> |
| |
| <p>clang does not support multiple alternative constraints in inline asm; this |
| is an extremely obscure feature which would be complicated to implement |
| correctly.</p> |
| |
| <li>clang does not support __builtin_apply and friends; this extension is |
| extremely obscure and difficult to implement reliably.</li> |
| |
| <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| <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> |