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17
18<h1>Clang Language Extensions</h1>
19
20<ul>
21<li><a href="#intro">Introduction</a></li>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]22<li><a href="#feature_check">Feature Checking Macros</a></li>
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]23<li><a href="#has_include">Include File Checking Macros</a></li>
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]24<li><a href="#builtinmacros">Builtin Macros</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]25<li><a href="#vectors">Vectors and Extended Vectors</a></li>
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]26<li><a href="#deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> attributes</a></li>
27<li><a href="#attributes-on-enumerators">Attributes on enumerators</a></li>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]28<li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]29 <ul>
30 <li><a href="#cxx_exceptions">C++ exceptions</a></li>
31 <li><a href="#cxx_rtti">C++ RTTI</a></li>
32 </ul>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]33<li><a href="#checking_upcoming_features">Checks for Upcoming Standard Language Features</a></li>
34 <ul>
35 <li><a href="#cxx_attributes">C++0x attributes</a></li>
36 <li><a href="#cxx_decltype">C++0x <tt>decltype()</tt></a></li>
Douglas Gregor07508002011-02-05 20:35:30 +0000[diff] [blame]37 <li><a href="#cxx_default_function_template_args">C++0x default template arguments in function templates</a></li>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]38 <li><a href="#cxx_deleted_functions">C++0x deleted functions</a></li>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]39 <li><a href="#cxx_lambdas">C++0x lambdas</a></li>
40 <li><a href="#cxx_nullptr">C++0x nullptr</a></li>
41 <li><a href="#cxx_rvalue_references">C++0x rvalue references</a></li>
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame]42 <li><a href="#cxx_reference_qualified_functions">C++0x reference-qualified functions</a></li>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]43 <li><a href="#cxx_static_assert">C++0x <tt>static_assert()</tt></a></li>
44 <li><a href="#cxx_auto_type">C++0x type inference</a></li>
45 <li><a href="#cxx_variadic_templates">C++0x variadic templates</a></li>
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]46 <li><a href="#cxx_inline_namespaces">C++0x inline namespaces</a></li>
Douglas Gregor1274ccd2010-10-08 23:50:27 +0000[diff] [blame]47 <li><a href="#cxx_strong_enums">C++0x strongly-typed enumerations</a></li>
Douglas Gregordab60ad2010-10-01 18:44:50 +0000[diff] [blame]48 <li><a href="#cxx_trailing_return">C++0x trailing return type</a></li>
Sebastian Redl4561ecd2011-03-15 21:17:12 +0000[diff] [blame^]49 <li><a href="#cxx_noexcept">C++0x noexcept specification</a></li>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]50 </ul>
Douglas Gregorafdf1372011-02-03 21:57:35 +0000[diff] [blame]51<li><a href="#checking_type_traits">Checks for Type Traits</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]52<li><a href="#blocks">Blocks</a></li>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]53<li><a href="#overloading-in-c">Function Overloading in C</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]54<li><a href="#builtins">Builtin Functions</a>
55 <ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]56 <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]57 <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
Douglas Gregorafdf1372011-02-03 21:57:35 +0000[diff] [blame]58 </ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]59</li>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]60<li><a href="#targetspecific">Target-Specific Extensions</a>
61 <ul>
62 <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
63 </ul>
64</li>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]65<li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
66 <ul>
67 <li><a href="#analyzerattributes">Analyzer Attributes</a></li>
68 </ul>
69</li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]70</ul>
71
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]72<!-- ======================================================================= -->
73<h2 id="intro">Introduction</h2>
74<!-- ======================================================================= -->
75
76<p>This document describes the language extensions provided by Clang. In
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]77addition to the language extensions listed here, Clang aims to support a broad
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]78range of GCC extensions. Please see the <a
79href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
80more information on these extensions.</p>
81
82<!-- ======================================================================= -->
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]83<h2 id="feature_check">Feature Checking Macros</h2>
84<!-- ======================================================================= -->
85
86<p>Language extensions can be very useful, but only if you know you can depend
87on them. In order to allow fine-grain features checks, we support two builtin
88function-like macros. This allows you to directly test for a feature in your
89code without having to resort to something like autoconf or fragile "compiler
90version checks".</p>
91
92<!-- ======================================================================= -->
93<h3 id="__has_builtin">__has_builtin</h3>
94<!-- ======================================================================= -->
95
96<p>This function-like macro takes a single identifier argument that is the name
97of a builtin function. It evaluates to 1 if the builtin is supported or 0 if
98not. It can be used like this:</p>
99
100<blockquote>
101<pre>
102#ifndef __has_builtin // Optional of course.
103 #define __has_builtin(x) 0 // Compatibility with non-clang compilers.
104#endif
105
106...
107#if __has_builtin(__builtin_trap)
108 __builtin_trap();
109#else
110 abort();
111#endif
112...
113</pre>
114</blockquote>
115
116
117<!-- ======================================================================= -->
118<h3 id="__has_feature">__has_feature</h3>
119<!-- ======================================================================= -->
120
121<p>This function-like macro takes a single identifier argument that is the name
122of a feature. It evaluates to 1 if the feature is supported or 0 if not. It
123can be used like this:</p>
124
125<blockquote>
126<pre>
127#ifndef __has_feature // Optional of course.
128 #define __has_feature(x) 0 // Compatibility with non-clang compilers.
129#endif
130
131...
132#if __has_feature(attribute_overloadable) || \
133 __has_feature(blocks)
134...
135#endif
136...
137</pre>
138</blockquote>
139
140<p>The feature tag is described along with the language feature below.</p>
141
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]142<!-- ======================================================================= -->
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]143<h3 id="__has_attribute">__has_attribute</h3>
144<!-- ======================================================================= -->
145
146<p>This function-like macro takes a single identifier argument that is the name
147of an attribute. It evaluates to 1 if the attribute is supported or 0 if not. It
148can be used like this:</p>
149
150<blockquote>
151<pre>
152#ifndef __has_attribute // Optional of course.
153 #define __has_attribute(x) 0 // Compatibility with non-clang compilers.
154#endif
155
156...
Anders Carlsson961003d2011-01-24 03:54:51 +0000[diff] [blame]157#if __has_attribute(always_inline)
158#define ALWAYS_INLINE __attribute__((always_inline))
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]159#else
Anders Carlsson961003d2011-01-24 03:54:51 +0000[diff] [blame]160#define ALWAYS_INLINE
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]161#endif
162...
163</pre>
164</blockquote>
165
166<!-- ======================================================================= -->
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]167<h2 id="has_include">Include File Checking Macros</h2>
168<!-- ======================================================================= -->
169
170<p>Not all developments systems have the same include files.
171The <a href="#__has_include">__has_include</a> and
172<a href="#__has_include_next">__has_include_next</a> macros allow you to
173check for the existence of an include file before doing
174a possibly failing #include directive.</p>
175
176<!-- ======================================================================= -->
177<h3 id="__has_include">__has_include</h3>
178<!-- ======================================================================= -->
179
180<p>This function-like macro takes a single file name string argument that
181is the name of an include file. It evaluates to 1 if the file can
182be found using the include paths, or 0 otherwise:</p>
183
184<blockquote>
185<pre>
186// Note the two possible file name string formats.
187#if __has_include("myinclude.h") && __has_include(&lt;stdint.h&gt;)
188# include "myinclude.h"
189#endif
190
191// To avoid problem with non-clang compilers not having this macro.
192#if defined(__has_include) && __has_include("myinclude.h")
193# include "myinclude.h"
194#endif
195</pre>
196</blockquote>
197
198<p>To test for this feature, use #if defined(__has_include).</p>
199
200<!-- ======================================================================= -->
201<h3 id="__has_include_next">__has_include_next</h3>
202<!-- ======================================================================= -->
203
204<p>This function-like macro takes a single file name string argument that
205is the name of an include file. It is like __has_include except that it
206looks for the second instance of the given file found in the include
207paths. It evaluates to 1 if the second instance of the file can
208be found using the include paths, or 0 otherwise:</p>
209
210<blockquote>
211<pre>
212// Note the two possible file name string formats.
213#if __has_include_next("myinclude.h") && __has_include_next(&lt;stdint.h&gt;)
214# include_next "myinclude.h"
215#endif
216
217// To avoid problem with non-clang compilers not having this macro.
218#if defined(__has_include_next) && __has_include_next("myinclude.h")
219# include_next "myinclude.h"
220#endif
221</pre>
222</blockquote>
223
224<p>Note that __has_include_next, like the GNU extension
225#include_next directive, is intended for use in headers only,
226and will issue a warning if used in the top-level compilation
227file. A warning will also be issued if an absolute path
228is used in the file argument.</p>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]229
230<!-- ======================================================================= -->
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]231<h2 id="builtinmacros">Builtin Macros</h2>
232<!-- ======================================================================= -->
233
Douglas Gregor4290fbd2010-04-30 02:51:06 +0000[diff] [blame]234<dl>
235 <dt><code>__BASE_FILE__</code></dt>
236 <dd>Defined to a string that contains the name of the main input
237 file passed to Clang.</dd>
238
239 <dt><code>__COUNTER__</code></dt>
240 <dd>Defined to an integer value that starts at zero and is
241 incremented each time the <code>__COUNTER__</code> macro is
242 expanded.</dd>
243
244 <dt><code>__INCLUDE_LEVEL__</code></dt>
245 <dd>Defined to an integral value that is the include depth of the
246 file currently being translated. For the main file, this value is
247 zero.</dd>
248
249 <dt><code>__TIMESTAMP__</code></dt>
250 <dd>Defined to the date and time of the last modification of the
251 current source file.</dd>
252
253 <dt><code>__clang__</code></dt>
254 <dd>Defined when compiling with Clang</dd>
255
256 <dt><code>__clang_major__</code></dt>
257 <dd>Defined to the major version number of Clang (e.g., the 2 in
258 2.0.1).</dd>
259
260 <dt><code>__clang_minor__</code></dt>
261 <dd>Defined to the minor version number of Clang (e.g., the 0 in
262 2.0.1).</dd>
263
264 <dt><code>__clang_patchlevel__</code></dt>
265 <dd>Defined to the patch level of Clang (e.g., the 1 in 2.0.1).</dd>
266
267 <dt><code>__clang_version__</code></dt>
268 <dd>Defined to a string that captures the Clang version, including
269 the Subversion tag or revision number, e.g., "1.5 (trunk
270 102332)".</dd>
271</dl>
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]272
273<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]274<h2 id="vectors">Vectors and Extended Vectors</h2>
275<!-- ======================================================================= -->
276
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]277<p>Supports the GCC vector extensions, plus some stuff like V[1].</p>
278
279<p>Also supports <tt>ext_vector</tt>, which additionally support for V.xyzw
280syntax and other tidbits as seen in OpenCL. An example is:</p>
281
282<blockquote>
283<pre>
284typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
285typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
286
287float4 foo(float2 a, float2 b) {
288 float4 c;
289 c.xz = a;
290 c.yw = b;
291 return c;
292}
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]293</pre>
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]294</blockquote>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]295
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]296<p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>
297
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]298<p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
299
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]300<!-- ======================================================================= -->
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]301<h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]302<!-- ======================================================================= -->
303
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]304<p>An optional string message can be added to the <tt>deprecated</tt>
305and <tt>unavailable</tt> attributes. For example:</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]306
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]307<blockquote>
Chris Lattner4836d6a2010-11-09 19:43:35 +0000[diff] [blame]308<pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]309</blockquote>
310
311<p>If the deprecated or unavailable declaration is used, the message
312will be incorporated into the appropriate diagnostic:</p>
313
314<blockquote>
Chris Lattner4836d6a2010-11-09 19:43:35 +0000[diff] [blame]315<pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!! [-Wdeprecated-declarations]
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]316 explode();
317 ^</pre>
318</blockquote>
319
320<p>Query for this feature
321with <tt>__has_feature(attribute_deprecated_with_message)</tt>
322and <tt>__has_feature(attribute_unavailable_with_message)</tt>.</p>
323
324<!-- ======================================================================= -->
325<h2 id="attributes-on-enumerators">Attributes on Enumerators</h2>
326<!-- ======================================================================= -->
327
328<p>Clang allows attributes to be written on individual enumerators.
329This allows enumerators to be deprecated, made unavailable, etc. The
330attribute must appear after the enumerator name and before any
331initializer, like so:</p>
332
333<blockquote>
334<pre>enum OperationMode {
335 OM_Invalid,
336 OM_Normal,
337 OM_Terrified __attribute__((deprecated)),
338 OM_AbortOnError __attribute__((deprecated)) = 4
339};</pre>
340</blockquote>
341
342<p>Attributes on the <tt>enum</tt> declaration do not apply to
343individual enumerators.</p>
344
345<p>Query for this feature with <tt>__has_feature(enumerator_attributes)</tt>.</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]346
347<!-- ======================================================================= -->
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]348<h2 id="checking_language_features">Checks for Standard Language Features</h2>
349<!-- ======================================================================= -->
350
351<p>The <tt>__has_feature</tt> macro can be used to query if certain standard language features are
352enabled. Those features are listed here.</p>
353
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]354<h3 id="cxx_exceptions">C++ exceptions</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]355
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]356<p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
357example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]358
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]359<h3 id="cxx_rtti">C++ RTTI</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]360
Ted Kremenek0eb95602009-12-03 02:06:43 +0000[diff] [blame]361<p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]362compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]363
364<!-- ======================================================================= -->
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]365<h2 id="checking_upcoming_features">Checks for Upcoming Standard Language Features</h2>
366<!-- ======================================================================= -->
367
368<p>The <tt>__has_feature</tt> macro can be used to query if certain upcoming
369standard language features are enabled. Those features are listed here.</p>
370
371<p>Currently, all features listed here are slated for inclusion in the upcoming
372C++0x standard. As a result, all the features that clang supports are enabled
373with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
374not yet implemented will be noted.</p>
375
376<h3 id="cxx_decltype">C++0x <tt>decltype()</tt></h3>
377
378<p>Use <tt>__has_feature(cxx_decltype)</tt> to determine if support for the
379<tt>decltype()</tt> specifier is enabled.</p>
380
381<h3 id="cxx_attributes">C++0x attributes</h3>
382
383<p>Use <tt>__has_feature(cxx_attributes)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]384attribute parsing with C++0x's square bracket notation is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]385
Douglas Gregor07508002011-02-05 20:35:30 +0000[diff] [blame]386<h3 id="cxx_default_function_template_args">C++0x default template arguments in function templates</h3>
387
388<p>Use <tt>__has_feature(cxx_default_function_template_args)</tt> to determine if support for default template arguments in function templates is enabled.</p>
389
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]390<h3 id="cxx_deleted_functions">C++0x deleted functions</tt></h3>
391
392<p>Use <tt>__has_feature(cxx_deleted_functions)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]393deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]394
Douglas Gregor9cc90a32010-01-13 16:27:49 +0000[diff] [blame]395<h3 id="cxx_lambdas">C++0x lambdas</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]396
397<p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]398lambdas is enabled. clang does not currently implement this feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]399
400<h3 id="cxx_nullptr">C++0x <tt>nullptr</tt></h3>
401
402<p>Use <tt>__has_feature(cxx_nullptr)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]403<tt>nullptr</tt> is enabled. clang does not yet fully implement this
404feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]405
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame]406<h3 id="cxx_reference_qualified_functions">C++0x reference-qualified functions</h3>
407<p>Use <tt>__has_feature(cxx_reference_qualified_functions)</tt> to determine if support for reference-qualified functions (e.g., member functions with <code>&amp;</code> or <code>&amp;&amp;</code> applied to <code>*this</code>) is enabled.</p>
408
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]409<h3 id="cxx_rvalue_references">C++0x rvalue references</tt></h3>
410
411<p>Use <tt>__has_feature(cxx_rvalue_references)</tt> to determine if support for
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame]412rvalue references is enabled. </p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]413
414<h3 id="cxx_static_assert">C++0x <tt>static_assert()</tt></h3>
415
416<p>Use <tt>__has_feature(cxx_static_assert)</tt> to determine if support for
417compile-time assertions using <tt>static_assert</tt> is enabled.</p>
418
419<h3 id="cxx_auto_type">C++0x type inference</h3>
420
421<p>Use <tt>__has_feature(cxx_auto_type)</tt> to determine C++0x type inference
422is supported using the <tt>auto</tt> specifier. If this is disabled,
Richard Smithfd405ef2011-02-23 00:41:16 +0000[diff] [blame]423<tt>auto</tt> will instead be a storage class specifier, as in C or C++98.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]424
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]425<h3 id="cxx_variadic_templates">C++0x variadic templates</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]426
427<p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
Douglas Gregor83d77812011-01-19 23:15:20 +0000[diff] [blame]428for variadic templates is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]429
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]430<h3 id="cxx_inline_namespaces">C++0x inline namespaces</h3>
431
432<p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> to determine if support for
433inline namespaces is enabled.</p>
434
Douglas Gregordab60ad2010-10-01 18:44:50 +0000[diff] [blame]435<h3 id="cxx_trailing_return">C++0x trailing return type</h3>
436
437<p>Use <tt>__has_feature(cxx_trailing_return)</tt> to determine if support for
438the alternate function declaration syntax with trailing return type is enabled.</p>
439
Sebastian Redl4561ecd2011-03-15 21:17:12 +0000[diff] [blame^]440<h3 id="cxx_noexcept">C++0x noexcept</h3>
441
442<p>Use <tt>__has_feature(cxx_noexcept)</tt> to determine if support for
443noexcept exception specifications is enabled.</p>
444
Douglas Gregor1274ccd2010-10-08 23:50:27 +0000[diff] [blame]445<h3 id="cxx_strong_enums">C++0x strongly typed enumerations</h3>
446
447<p>Use <tt>__has_feature(cxx_strong_enums)</tt> to determine if support for
448strongly typed, scoped enumerations is enabled.</p>
449
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]450<!-- ======================================================================= -->
Douglas Gregorafdf1372011-02-03 21:57:35 +0000[diff] [blame]451<h2 id="checking_type_traits">Checks for Type Traits</h2>
452<!-- ======================================================================= -->
453
454<p>Clang supports the <a hef="http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html">GNU C++ type traits</a> and a subset of the <a href="http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx">Microsoft Visual C++ Type traits</a>. For each supported type trait <code>__X</code>, <code>__has_feature(X)</code> indicates the presence of the type trait. For example:
455<blockquote>
456<pre>
457#if __has_feature(is_convertible_to)
458template&lt;typename From, typename To&gt;
459struct is_convertible_to {
460 static const bool value = __is_convertible_to(From, To);
461};
462#else
463// Emulate type trait
464#endif
465</pre>
466</blockquote>
467
468<p>The following type traits are supported by Clang:</p>
469<ul>
470 <li><code>__has_nothrow_assign</code> (GNU, Microsoft)</li>
471 <li><code>__has_nothrow_copy</code> (GNU, Microsoft)</li>
472 <li><code>__has_nothrow_constructor</code> (GNU, Microsoft)</li>
473 <li><code>__has_trivial_assign</code> (GNU, Microsoft)</li>
474 <li><code>__has_trivial_copy</code> (GNU, Microsoft)</li>
475 <li><code>__has_trivial_constructor</code> (GNU, Microsoft)</li>
476 <li><code>__has_trivial_destructor</code> (GNU, Microsoft)</li>
477 <li><code>__has_virtual_destructor</code> (GNU, Microsoft)</li>
478 <li><code>__is_abstract</code> (GNU, Microsoft)</li>
479 <li><code>__is_base_of</code> (GNU, Microsoft)</li>
480 <li><code>__is_class</code> (GNU, Microsoft)</li>
481 <li><code>__is_convertible_to</code> (Microsoft)</li>
482 <li><code>__is_empty</code> (GNU, Microsoft)</li>
483 <li><code>__is_enum</code> (GNU, Microsoft)</li>
484 <li><code>__is_pod</code> (GNU, Microsoft)</li>
485 <li><code>__is_polymorphic</code> (GNU, Microsoft)</li>
486 <li><code>__is_union</code> (GNU, Microsoft)</li>
487 <li><code>__is_literal(type)</code>: Determines whether the given type is a literal type</li>
488</ul>
489
490<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]491<h2 id="blocks">Blocks</h2>
492<!-- ======================================================================= -->
493
Chris Lattnera7dbdf52009-03-09 07:03:22 +0000[diff] [blame]494<p>The syntax and high level language feature description is in <a
495href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
496details for the clang implementation are in <a
Chris Lattner5d7650b2010-03-16 21:43:03 +0000[diff] [blame]497href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]498
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]499
500<p>Query for this feature with __has_feature(blocks).</p>
501
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]502<!-- ======================================================================= -->
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]503<h2 id="overloading-in-c">Function Overloading in C</h2>
504<!-- ======================================================================= -->
505
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]506<p>Clang provides support for C++ function overloading in C. Function
507overloading in C is introduced using the <tt>overloadable</tt> attribute. For
508example, one might provide several overloaded versions of a <tt>tgsin</tt>
509function that invokes the appropriate standard function computing the sine of a
510value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
511precision:</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]512
513<blockquote>
514<pre>
515#include &lt;math.h&gt;
516float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
517double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
518long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); }
519</pre>
520</blockquote>
521
522<p>Given these declarations, one can call <tt>tgsin</tt> with a
523<tt>float</tt> value to receive a <tt>float</tt> result, with a
524<tt>double</tt> to receive a <tt>double</tt> result, etc. Function
525overloading in C follows the rules of C++ function overloading to pick
526the best overload given the call arguments, with a few C-specific
527semantics:</p>
528<ul>
529 <li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long
530 double</tt> is ranked as a floating-point promotion (per C99) rather
531 than as a floating-point conversion (as in C++).</li>
532
533 <li>A conversion from a pointer of type <tt>T*</tt> to a pointer of type
534 <tt>U*</tt> is considered a pointer conversion (with conversion
535 rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li>
536
537 <li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt>
538 is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This
539 conversion is given "conversion" rank.</li>
540</ul>
541
542<p>The declaration of <tt>overloadable</tt> functions is restricted to
543function declarations and definitions. Most importantly, if any
544function with a given name is given the <tt>overloadable</tt>
545attribute, then all function declarations and definitions with that
546name (and in that scope) must have the <tt>overloadable</tt>
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]547attribute. This rule even applies to redeclarations of functions whose original
548declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]549
550<blockquote>
551<pre>
552int f(int) __attribute__((overloadable));
553float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
554
555int g(int) __attribute__((overloadable));
556int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
557</pre>
558</blockquote>
559
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]560<p>Functions marked <tt>overloadable</tt> must have
561prototypes. Therefore, the following code is ill-formed:</p>
562
563<blockquote>
564<pre>
565int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
566</pre>
567</blockquote>
568
569<p>However, <tt>overloadable</tt> functions are allowed to use a
570ellipsis even if there are no named parameters (as is permitted in C++). This feature is particularly useful when combined with the <tt>unavailable</tt> attribute:</p>
571
572<blockquote>
573<pre>
Chris Lattner02246802009-02-18 22:27:46 +0000[diff] [blame]574void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]575</pre>
576</blockquote>
577
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]578<p>Functions declared with the <tt>overloadable</tt> attribute have
579their names mangled according to the same rules as C++ function
580names. For example, the three <tt>tgsin</tt> functions in our
581motivating example get the mangled names <tt>_Z5tgsinf</tt>,
Chris Lattner71b48d62010-11-28 18:19:13 +0000[diff] [blame]582<tt>_Z5tgsind</tt>, and <tt>_Z5tgsine</tt>, respectively. There are two
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]583caveats to this use of name mangling:</p>
584
585<ul>
586
587 <li>Future versions of Clang may change the name mangling of
588 functions overloaded in C, so you should not depend on an specific
589 mangling. To be completely safe, we strongly urge the use of
590 <tt>static inline</tt> with <tt>overloadable</tt> functions.</li>
591
592 <li>The <tt>overloadable</tt> attribute has almost no meaning when
593 used in C++, because names will already be mangled and functions are
594 already overloadable. However, when an <tt>overloadable</tt>
595 function occurs within an <tt>extern "C"</tt> linkage specification,
596 it's name <i>will</i> be mangled in the same way as it would in
597 C.</li>
598</ul>
599
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]600<p>Query for this feature with __has_feature(attribute_overloadable).</p>
601
602
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]603<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]604<h2 id="builtins">Builtin Functions</h2>
605<!-- ======================================================================= -->
606
607<p>Clang supports a number of builtin library functions with the same syntax as
608GCC, including things like <tt>__builtin_nan</tt>,
609<tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
610<tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</tt>, etc. In
611addition to the GCC builtins, Clang supports a number of builtins that GCC does
612not, which are listed here.</p>
613
614<p>Please note that Clang does not and will not support all of the GCC builtins
615for vector operations. Instead of using builtins, you should use the functions
616defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
617define portable wrappers for these. Many of the Clang versions of these
618functions are implemented directly in terms of <a href="#vectors">extended
619vector support</a> instead of builtins, in order to reduce the number of
620builtins that we need to implement.</p>
621
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]622<!-- ======================================================================= -->
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]623<h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]624<!-- ======================================================================= -->
625
Chris Lattneraad826b2009-09-16 18:56:12 +0000[diff] [blame]626<p><tt>__builtin_shufflevector</tt> is used to express generic vector
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]627permutation/shuffle/swizzle operations. This builtin is also very important for
628the implementation of various target-specific header files like
629<tt>&lt;xmmintrin.h&gt;</tt>.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]630</p>
631
632<p><b>Syntax:</b></p>
633
634<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]635__builtin_shufflevector(vec1, vec2, index1, index2, ...)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]636</pre>
637
638<p><b>Examples:</b></p>
639
640<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]641 // Identity operation - return 4-element vector V1.
642 __builtin_shufflevector(V1, V1, 0, 1, 2, 3)
643
644 // "Splat" element 0 of V1 into a 4-element result.
645 __builtin_shufflevector(V1, V1, 0, 0, 0, 0)
646
647 // Reverse 4-element vector V1.
648 __builtin_shufflevector(V1, V1, 3, 2, 1, 0)
649
650 // Concatenate every other element of 4-element vectors V1 and V2.
651 __builtin_shufflevector(V1, V2, 0, 2, 4, 6)
652
653 // Concatenate every other element of 8-element vectors V1 and V2.
654 __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]655</pre>
656
657<p><b>Description:</b></p>
658
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]659<p>The first two arguments to __builtin_shufflevector are vectors that have the
660same element type. The remaining arguments are a list of integers that specify
661the elements indices of the first two vectors that should be extracted and
662returned in a new vector. These element indices are numbered sequentially
663starting with the first vector, continuing into the second vector. Thus, if
664vec1 is a 4-element vector, index 5 would refer to the second element of vec2.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]665</p>
666
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]667<p>The result of __builtin_shufflevector is a vector
668with the same element type as vec1/vec2 but that has an element count equal to
669the number of indices specified.
670</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]671
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]672<p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>
673
674<!-- ======================================================================= -->
675<h3 id="__builtin_unreachable">__builtin_unreachable</h3>
676<!-- ======================================================================= -->
677
678<p><tt>__builtin_unreachable</tt> is used to indicate that a specific point in
679the program cannot be reached, even if the compiler might otherwise think it
680can. This is useful to improve optimization and eliminates certain warnings.
681For example, without the <tt>__builtin_unreachable</tt> in the example below,
682the compiler assumes that the inline asm can fall through and prints a "function
683declared 'noreturn' should not return" warning.
684</p>
685
686<p><b>Syntax:</b></p>
687
688<pre>
689__builtin_unreachable()
690</pre>
691
692<p><b>Example of Use:</b></p>
693
694<pre>
695void myabort(void) __attribute__((noreturn));
696void myabort(void) {
697 asm("int3");
698 __builtin_unreachable();
699}
700</pre>
701
702<p><b>Description:</b></p>
703
704<p>The __builtin_unreachable() builtin has completely undefined behavior. Since
705it has undefined behavior, it is a statement that it is never reached and the
706optimizer can take advantage of this to produce better code. This builtin takes
707no arguments and produces a void result.
708</p>
709
710<p>Query for this feature with __has_builtin(__builtin_unreachable).</p>
711
712
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]713<!-- ======================================================================= -->
714<h2 id="targetspecific">Target-Specific Extensions</h2>
715<!-- ======================================================================= -->
716
717<p>Clang supports some language features conditionally on some targets.</p>
718
719<!-- ======================================================================= -->
720<h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
721<!-- ======================================================================= -->
722
723<p>The X86 backend has these language extensions:</p>
724
725<!-- ======================================================================= -->
726<h4 id="x86-gs-segment">Memory references off the GS segment</h4>
727<!-- ======================================================================= -->
728
729<p>Annotating a pointer with address space #256 causes it to be code generated
Chris Lattnera021e7c2009-05-05 18:54:47 +0000[diff] [blame]730relative to the X86 GS segment register, and address space #257 causes it to be
731relative to the X86 FS segment. Note that this is a very very low-level
732feature that should only be used if you know what you're doing (for example in
733an OS kernel).</p>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]734
735<p>Here is an example:</p>
736
737<pre>
738#define GS_RELATIVE __attribute__((address_space(256)))
739int foo(int GS_RELATIVE *P) {
740 return *P;
741}
742</pre>
743
744<p>Which compiles to (on X86-32):</p>
745
746<pre>
747_foo:
748 movl 4(%esp), %eax
749 movl %gs:(%eax), %eax
750 ret
751</pre>
752
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]753<!-- ======================================================================= -->
754<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
755<!-- ======================================================================= -->
756
757<p>Clang supports additional attributes that are useful for documenting program
758invariants and rules for static analysis tools. The extensions documented here
759are used by the <a
760href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
761engine</a> that is part of Clang's Analysis library.</p>
762
763<!-- ======================================================================= -->
764<h3 id="analyzerattributes">Analyzer Attributes</h3>
765<!-- ======================================================================= -->
766
767<h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>
768
769<p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
Ted Kremenek4df21142009-04-10 05:04:22 +0000[diff] [blame]770attribute. This attribute, which is typically affixed to a function prototype,
771indicates that a call to a given function never returns. Function prototypes for
772common functions like <tt>exit</tt> are typically annotated with this attribute,
773as well as a variety of common assertion handlers. Users can educate the static
774analyzer about their own custom assertion handles (thus cutting down on false
775positives due to false paths) by marking their own &quot;panic&quot; functions
776with this attribute.</p>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]777
778<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
Nick Lewycky625b5862009-06-14 04:08:08 +0000[diff] [blame]779there are special functions that for all intents and purposes should be
780considered panic functions (i.e., they are only called when an internal program
781error occurs) but may actually return so that the program can fail gracefully.
782The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
783as being interpreted as &quot;no return&quot; functions by the analyzer (thus
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]784pruning bogus paths) but will not affect compilation (as in the case of
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]785<tt>noreturn</tt>).</p>
786
787<p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]788same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]789placed at the end of function prototypes:</p>
790
791<pre>
792 void foo() <b>__attribute__((analyzer_noreturn))</b>;
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]793</pre>
794
795<p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>
796
John McCall630b7ae2011-01-25 04:26:21 +0000[diff] [blame]797<h4 id="attr_retain_release">Objective-C retaining behavior attributes</h4>
798
799<p>In Objective-C, functions and methods are generally assumed to take
800and return objects with +0 retain counts, with some exceptions for
801special methods like <tt>+alloc</tt> and <tt>init</tt>. However,
802there are exceptions, and so Clang provides attributes to allow these
803exceptions to be documented, which helps the analyzer find leaks (and
804ignore non-leaks).</p>
805
806<p><b>Usage</b>: The <tt>ns_returns_retained</tt>, <tt>ns_returns_not_retained</tt>,
807<tt>ns_returns_autoreleased</tt>, <tt>cf_returns_retained</tt>,
808and <tt>cf_returns_not_retained</tt> attributes can be placed on
809methods and functions that return Objective-C or CoreFoundation
810objects. They are commonly placed at the end of a function prototype
811or method declaration:</p>
812
813<pre>
814 id foo() <b>__attribute__((ns_returns_retained))</b>;
815
816 - (NSString*) bar: (int) x <b>__attribute__((ns_returns_retained))</b>;
817</pre>
818
819<p>The <tt>*_returns_retained</tt> attributes specify that the
820returned object has a +1 retain count.
821The <tt>*_returns_not_retained</tt> attributes specify that the return
822object has a +0 retain count, even if the normal convention for its
823selector would be +1. <tt>ns_returns_autoreleased</tt> specifies that the
824returned object is +0, but is guaranteed to live at least as long as the
825next flush of an autorelease pool.</p>
826
827<p><b>Usage</b>: The <tt>ns_consumed</tt> and <tt>cf_consumed</tt>
828attributes can be placed on an parameter declaration; they specify
829that the argument is expected to have a +1 retain count, which will be
830balanced in some way by the function or method.
831The <tt>ns_consumes_self</tt> attribute can only be placed on an
832Objective-C method; it specifies that the method expects
833its <tt>self</tt> parameter to have a +1 retain count, which it will
834balance in some way.</p>
835
836<pre>
837 void <b>foo(__attribute__((ns_consumed))</b> NSString *string);
838
839 - (void) bar <b>__attribute__((ns_consumes_self))</b>;
840 - (void) baz: (id) <b>__attribute__((ns_consumed))</b> x;
841</pre>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]842
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]843</div>
844</body>
845</html>