forked from OSchip/llvm-project
Add some text to the user manual describing the current state of C
support in clang. llvm-svn: 70314
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clang/docs
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@ -38,10 +38,11 @@ td {
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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="#pragmas">Pragmas Supported</a></li> -->
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<li><a href="#c_extensions">C Extensions Supported</a></li>
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<li><a href="#c_incompatibilities">Intentional Incompatibilities with
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GCC</a></li>
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<li><a href="#c_ext">Extensions supported by clang</a></li>
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<li><a href="#c_modes">Differences between various standard modes</a></li>
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<li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li>
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<li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li>
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<li><a href="#c_ms">Microsoft extensions</a></li>
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</ul>
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</li>
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<li><a href="#objc">Objective-C Language Features</a>
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@ -482,15 +483,134 @@ and not specified on the command line using <tt>-include</tt>.</p>
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<h2 id="c">C Language Features</h2>
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<!-- ======================================================================= -->
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<p>The support for standard C in clang is feature-complete except for the C99
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floating-point pragmas.</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="c_incompatibilities">Intentional Incompatibilities with GCC</h3>
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<h3 id="c_ext">Extensions supported by clang</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>No VLAs in structs.</p>
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<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="c_modes">Differences between various standard modes</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>clang supports the -std option, which changes what language mode clang uses.
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The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
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for those modes. If no -std option is specified, clang defaults to gnu99 mode.
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</p>
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<p>Differences between all c* and gnu* modes:</p>
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<ul>
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<li>c* modes define "__STRICT_ANSI__".</li>
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<li>Target-specific defines not prefixed by underscores, like "linux", are defined
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in gnu* modes.</li>
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<li>Trigraphs default to being off in gnu* modes; they can be enabled by the
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-trigraphs option.</li>
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<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
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variants "__asm__" and "__typeof__" are recognized in all modes.</li>
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<li>Some warnings are different.</li>
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</ul>
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<p>Differences between *89 and *99 modes:</p>
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<ul>
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<li>The *99 modes default to implementing "inline" as specified in C99, while
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the *89 modes implement the GNU version. This can be overridden for individual
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functions with the __gnu_inline__ attribute.</li>
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<li>Digraphs are enabled in the *99 modes.</li>
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<li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
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statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
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<li>__STDC_VERSION__ is not defined in *89 modes.</li>
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<li>"inline" and "restrict" are not recognized as keywords in c89 mode.</li>
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<li>Commas are allowed in integer constant expressions in *99 modes.</li>
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<li>Arrays which are not lvalues are not implicitly promoted to pointers in
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*89 modes.</li>
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<li>Constructs like "&*X" are always allowed in *99 modes.</li>
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<li>Some warnings are different.</li>
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</ul>
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<p>c94 mode is identical to c89 mode except that digraphs are enabled in
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c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>clang tries to be compatible with gcc as much as possible, but some gcc
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extensions are not implemented yet:</p>
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<ul>
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<li>clang does not support __label__
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
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a relatively small feature, so it is likely to be implemented relatively
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soon.</li>
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<li>clang does not support attributes on function pointers
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is
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a relatively important feature, so it is likely to be implemented relatively
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soon.</li>
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<li>clang does not support #pragma weak
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 3679</a>). Due to
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the uses described in the bug, this is likely to be implemented at some
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point, at least partially.</li>
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<li>clang does not support #pragma align
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 3811</a>). This is a
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relatively small feature, so it is likely to be implemented relatively
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soon.</li>
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<li>clang does not implement overloads for the __sync_* builtins
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 3824</a>). The
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builtins only currently work with 32-bit types. This is a relatively
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small feature, so it is likely to be implemented relatively soon.</li>
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<li>clang does not support code generation for variables pinned to registers
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(<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>). This
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is a relatively small feature, so it is likely to be implemented relatively
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soon.</li>
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<li>clang does not support decimal floating point types (_Decimal32 and
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friends) or fixed-point types (_Fract and friends); nobody has expressed
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interest in these features yet, so it's hard to say when they will be
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implemented.</li>
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<li>clang does not support nested functions; this is a complex feature which
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is infrequently used, so it is unlikely to be implemented anytime soon.</li>
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<li>clang does not support __builtin_apply and friends; this extension requires
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complex code generator support that does not currently exist in LLVM, and there
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is very little demand, so it is unlikely to be implemented anytime soon.</li>
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</ul>
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<p>This is not a complete list; if you find an unsupported extension
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missing from this list, please send an e-mail to cfe-dev. This list
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currently excludes C++; see <a href="#cxx">C++ Language Features</a>.
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Also, this list does not include bugs in mostly-implemented features; please
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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">
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bug tracker</a> for known existing bugs (FIXME: Is there a section for
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bug-reporting guidelines somewhere?).</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>clang does not support the gcc extension that allows variable-length arrays
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in structures. This is for a few of reasons: one, it is tricky
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to implement, two, the extension is completely undocumented, and three, the
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extension appears to be very rarely used.</p>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<h3 id="c_ms">Microsoft extensions</h3>
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<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
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<p>clang has some experimental support for extensions from
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Microsoft Visual C++; to enable it, use the -fms-extensions command-line
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option. Eventually, this will be the default for Windows targets.
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These extensions are not anywhere near complete, so please do not
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file bugs; patches are welcome, though.</p>
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<!-- ======================================================================= -->
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<h2 id="objc">Objective-C Language Features</h2>
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