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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>
Jeffrey Yasskinc60e13a2011-01-25 20:08:12 +0000[diff] [blame]28<li><a href="#forbid-temporaries-attribute">Attribute to forbid temporaries of a type</a></li>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]29<li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]30 <ul>
31 <li><a href="#cxx_exceptions">C++ exceptions</a></li>
32 <li><a href="#cxx_rtti">C++ RTTI</a></li>
33 </ul>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]34<li><a href="#checking_upcoming_features">Checks for Upcoming Standard Language Features</a></li>
35 <ul>
36 <li><a href="#cxx_attributes">C++0x attributes</a></li>
37 <li><a href="#cxx_decltype">C++0x <tt>decltype()</tt></a></li>
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>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]49 </ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]50<li><a href="#blocks">Blocks</a></li>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]51<li><a href="#overloading-in-c">Function Overloading in C</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]52<li><a href="#builtins">Builtin Functions</a>
53 <ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]54 <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]55 <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]56 </ul>
57</li>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]58<li><a href="#targetspecific">Target-Specific Extensions</a>
59 <ul>
60 <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
61 </ul>
62</li>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]63<li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
64 <ul>
65 <li><a href="#analyzerattributes">Analyzer Attributes</a></li>
66 </ul>
67</li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]68</ul>
69
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]70<!-- ======================================================================= -->
71<h2 id="intro">Introduction</h2>
72<!-- ======================================================================= -->
73
74<p>This document describes the language extensions provided by Clang. In
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]75addition to the language extensions listed here, Clang aims to support a broad
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]76range of GCC extensions. Please see the <a
77href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
78more information on these extensions.</p>
79
80<!-- ======================================================================= -->
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]81<h2 id="feature_check">Feature Checking Macros</h2>
82<!-- ======================================================================= -->
83
84<p>Language extensions can be very useful, but only if you know you can depend
85on them. In order to allow fine-grain features checks, we support two builtin
86function-like macros. This allows you to directly test for a feature in your
87code without having to resort to something like autoconf or fragile "compiler
88version checks".</p>
89
90<!-- ======================================================================= -->
91<h3 id="__has_builtin">__has_builtin</h3>
92<!-- ======================================================================= -->
93
94<p>This function-like macro takes a single identifier argument that is the name
95of a builtin function. It evaluates to 1 if the builtin is supported or 0 if
96not. It can be used like this:</p>
97
98<blockquote>
99<pre>
100#ifndef __has_builtin // Optional of course.
101 #define __has_builtin(x) 0 // Compatibility with non-clang compilers.
102#endif
103
104...
105#if __has_builtin(__builtin_trap)
106 __builtin_trap();
107#else
108 abort();
109#endif
110...
111</pre>
112</blockquote>
113
114
115<!-- ======================================================================= -->
116<h3 id="__has_feature">__has_feature</h3>
117<!-- ======================================================================= -->
118
119<p>This function-like macro takes a single identifier argument that is the name
120of a feature. It evaluates to 1 if the feature is supported or 0 if not. It
121can be used like this:</p>
122
123<blockquote>
124<pre>
125#ifndef __has_feature // Optional of course.
126 #define __has_feature(x) 0 // Compatibility with non-clang compilers.
127#endif
128
129...
130#if __has_feature(attribute_overloadable) || \
131 __has_feature(blocks)
132...
133#endif
134...
135</pre>
136</blockquote>
137
138<p>The feature tag is described along with the language feature below.</p>
139
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]140<!-- ======================================================================= -->
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]141<h3 id="__has_attribute">__has_attribute</h3>
142<!-- ======================================================================= -->
143
144<p>This function-like macro takes a single identifier argument that is the name
145of an attribute. It evaluates to 1 if the attribute is supported or 0 if not. It
146can be used like this:</p>
147
148<blockquote>
149<pre>
150#ifndef __has_attribute // Optional of course.
151 #define __has_attribute(x) 0 // Compatibility with non-clang compilers.
152#endif
153
154...
Anders Carlsson961003d2011-01-24 03:54:51 +0000[diff] [blame]155#if __has_attribute(always_inline)
156#define ALWAYS_INLINE __attribute__((always_inline))
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]157#else
Anders Carlsson961003d2011-01-24 03:54:51 +0000[diff] [blame]158#define ALWAYS_INLINE
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]159#endif
160...
161</pre>
162</blockquote>
163
164<!-- ======================================================================= -->
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]165<h2 id="has_include">Include File Checking Macros</h2>
166<!-- ======================================================================= -->
167
168<p>Not all developments systems have the same include files.
169The <a href="#__has_include">__has_include</a> and
170<a href="#__has_include_next">__has_include_next</a> macros allow you to
171check for the existence of an include file before doing
172a possibly failing #include directive.</p>
173
174<!-- ======================================================================= -->
175<h3 id="__has_include">__has_include</h3>
176<!-- ======================================================================= -->
177
178<p>This function-like macro takes a single file name string argument that
179is the name of an include file. It evaluates to 1 if the file can
180be found using the include paths, or 0 otherwise:</p>
181
182<blockquote>
183<pre>
184// Note the two possible file name string formats.
185#if __has_include("myinclude.h") && __has_include(&lt;stdint.h&gt;)
186# include "myinclude.h"
187#endif
188
189// To avoid problem with non-clang compilers not having this macro.
190#if defined(__has_include) && __has_include("myinclude.h")
191# include "myinclude.h"
192#endif
193</pre>
194</blockquote>
195
196<p>To test for this feature, use #if defined(__has_include).</p>
197
198<!-- ======================================================================= -->
199<h3 id="__has_include_next">__has_include_next</h3>
200<!-- ======================================================================= -->
201
202<p>This function-like macro takes a single file name string argument that
203is the name of an include file. It is like __has_include except that it
204looks for the second instance of the given file found in the include
205paths. It evaluates to 1 if the second instance of the file can
206be found using the include paths, or 0 otherwise:</p>
207
208<blockquote>
209<pre>
210// Note the two possible file name string formats.
211#if __has_include_next("myinclude.h") && __has_include_next(&lt;stdint.h&gt;)
212# include_next "myinclude.h"
213#endif
214
215// To avoid problem with non-clang compilers not having this macro.
216#if defined(__has_include_next) && __has_include_next("myinclude.h")
217# include_next "myinclude.h"
218#endif
219</pre>
220</blockquote>
221
222<p>Note that __has_include_next, like the GNU extension
223#include_next directive, is intended for use in headers only,
224and will issue a warning if used in the top-level compilation
225file. A warning will also be issued if an absolute path
226is used in the file argument.</p>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]227
228<!-- ======================================================================= -->
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]229<h2 id="builtinmacros">Builtin Macros</h2>
230<!-- ======================================================================= -->
231
Douglas Gregor4290fbd2010-04-30 02:51:06 +0000[diff] [blame]232<dl>
233 <dt><code>__BASE_FILE__</code></dt>
234 <dd>Defined to a string that contains the name of the main input
235 file passed to Clang.</dd>
236
237 <dt><code>__COUNTER__</code></dt>
238 <dd>Defined to an integer value that starts at zero and is
239 incremented each time the <code>__COUNTER__</code> macro is
240 expanded.</dd>
241
242 <dt><code>__INCLUDE_LEVEL__</code></dt>
243 <dd>Defined to an integral value that is the include depth of the
244 file currently being translated. For the main file, this value is
245 zero.</dd>
246
247 <dt><code>__TIMESTAMP__</code></dt>
248 <dd>Defined to the date and time of the last modification of the
249 current source file.</dd>
250
251 <dt><code>__clang__</code></dt>
252 <dd>Defined when compiling with Clang</dd>
253
254 <dt><code>__clang_major__</code></dt>
255 <dd>Defined to the major version number of Clang (e.g., the 2 in
256 2.0.1).</dd>
257
258 <dt><code>__clang_minor__</code></dt>
259 <dd>Defined to the minor version number of Clang (e.g., the 0 in
260 2.0.1).</dd>
261
262 <dt><code>__clang_patchlevel__</code></dt>
263 <dd>Defined to the patch level of Clang (e.g., the 1 in 2.0.1).</dd>
264
265 <dt><code>__clang_version__</code></dt>
266 <dd>Defined to a string that captures the Clang version, including
267 the Subversion tag or revision number, e.g., "1.5 (trunk
268 102332)".</dd>
269</dl>
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]270
271<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]272<h2 id="vectors">Vectors and Extended Vectors</h2>
273<!-- ======================================================================= -->
274
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]275<p>Supports the GCC vector extensions, plus some stuff like V[1].</p>
276
277<p>Also supports <tt>ext_vector</tt>, which additionally support for V.xyzw
278syntax and other tidbits as seen in OpenCL. An example is:</p>
279
280<blockquote>
281<pre>
282typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
283typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
284
285float4 foo(float2 a, float2 b) {
286 float4 c;
287 c.xz = a;
288 c.yw = b;
289 return c;
290}
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]291</pre>
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]292</blockquote>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]293
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]294<p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>
295
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]296<p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
297
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]298<!-- ======================================================================= -->
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]299<h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]300<!-- ======================================================================= -->
301
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]302<p>An optional string message can be added to the <tt>deprecated</tt>
303and <tt>unavailable</tt> attributes. For example:</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]304
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]305<blockquote>
Chris Lattner4836d6a2010-11-09 19:43:35 +0000[diff] [blame]306<pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame]307</blockquote>
308
309<p>If the deprecated or unavailable declaration is used, the message
310will be incorporated into the appropriate diagnostic:</p>
311
312<blockquote>
Chris Lattner4836d6a2010-11-09 19:43:35 +0000[diff] [blame]313<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]314 explode();
315 ^</pre>
316</blockquote>
317
318<p>Query for this feature
319with <tt>__has_feature(attribute_deprecated_with_message)</tt>
320and <tt>__has_feature(attribute_unavailable_with_message)</tt>.</p>
321
322<!-- ======================================================================= -->
323<h2 id="attributes-on-enumerators">Attributes on Enumerators</h2>
324<!-- ======================================================================= -->
325
326<p>Clang allows attributes to be written on individual enumerators.
327This allows enumerators to be deprecated, made unavailable, etc. The
328attribute must appear after the enumerator name and before any
329initializer, like so:</p>
330
331<blockquote>
332<pre>enum OperationMode {
333 OM_Invalid,
334 OM_Normal,
335 OM_Terrified __attribute__((deprecated)),
336 OM_AbortOnError __attribute__((deprecated)) = 4
337};</pre>
338</blockquote>
339
340<p>Attributes on the <tt>enum</tt> declaration do not apply to
341individual enumerators.</p>
342
343<p>Query for this feature with <tt>__has_feature(enumerator_attributes)</tt>.</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]344
345<!-- ======================================================================= -->
Jeffrey Yasskinc60e13a2011-01-25 20:08:12 +0000[diff] [blame]346<h2 id="forbid-temporaries-attribute">Attribute to forbid temporaries of a type</h2>
347<!-- ======================================================================= -->
348
349<p>Clang provides a <tt>forbid_temporaries</tt> attribute to forbid
350temporaries of a particular type.</p>
351
352<blockquote>
353<pre>class __attribute__((forbid_temporaries)) scoped_lock {
354 ...
355};</pre>
356</blockquote>
357
358<p>This prevents mistakes like</p>
359
360<blockquote>
361<pre>void foo() {
362 scoped_lock(my_mutex);
363 // Forgot the local variable name, so destructor runs here.
364 code_that_needs_lock_held();
365 // User expects destructor to run here.
366};</pre>
367</blockquote>
368
369<p>Query for this feature with <tt>__has_attribute(forbid_temporaries)</tt>.
370Use <tt>-Wno-forbid-temporaries</tt> to disable the resulting warning.</p>
371
372<!-- ======================================================================= -->
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]373<h2 id="checking_language_features">Checks for Standard Language Features</h2>
374<!-- ======================================================================= -->
375
376<p>The <tt>__has_feature</tt> macro can be used to query if certain standard language features are
377enabled. Those features are listed here.</p>
378
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]379<h3 id="cxx_exceptions">C++ exceptions</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]380
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]381<p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
382example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]383
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]384<h3 id="cxx_rtti">C++ RTTI</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]385
Ted Kremenek0eb95602009-12-03 02:06:43 +0000[diff] [blame]386<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]387compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]388
389<!-- ======================================================================= -->
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]390<h2 id="checking_upcoming_features">Checks for Upcoming Standard Language Features</h2>
391<!-- ======================================================================= -->
392
393<p>The <tt>__has_feature</tt> macro can be used to query if certain upcoming
394standard language features are enabled. Those features are listed here.</p>
395
396<p>Currently, all features listed here are slated for inclusion in the upcoming
397C++0x standard. As a result, all the features that clang supports are enabled
398with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
399not yet implemented will be noted.</p>
400
401<h3 id="cxx_decltype">C++0x <tt>decltype()</tt></h3>
402
403<p>Use <tt>__has_feature(cxx_decltype)</tt> to determine if support for the
404<tt>decltype()</tt> specifier is enabled.</p>
405
406<h3 id="cxx_attributes">C++0x attributes</h3>
407
408<p>Use <tt>__has_feature(cxx_attributes)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]409attribute parsing with C++0x's square bracket notation is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]410
411<h3 id="cxx_deleted_functions">C++0x deleted functions</tt></h3>
412
413<p>Use <tt>__has_feature(cxx_deleted_functions)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]414deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]415
Douglas Gregor9cc90a32010-01-13 16:27:49 +0000[diff] [blame]416<h3 id="cxx_lambdas">C++0x lambdas</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]417
418<p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]419lambdas is enabled. clang does not currently implement this feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]420
421<h3 id="cxx_nullptr">C++0x <tt>nullptr</tt></h3>
422
423<p>Use <tt>__has_feature(cxx_nullptr)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]424<tt>nullptr</tt> is enabled. clang does not yet fully implement this
425feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]426
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame^]427<h3 id="cxx_reference_qualified_functions">C++0x reference-qualified functions</h3>
428<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>
429
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]430<h3 id="cxx_rvalue_references">C++0x rvalue references</tt></h3>
431
432<p>Use <tt>__has_feature(cxx_rvalue_references)</tt> to determine if support for
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame^]433rvalue references is enabled. </p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]434
435<h3 id="cxx_static_assert">C++0x <tt>static_assert()</tt></h3>
436
437<p>Use <tt>__has_feature(cxx_static_assert)</tt> to determine if support for
438compile-time assertions using <tt>static_assert</tt> is enabled.</p>
439
440<h3 id="cxx_auto_type">C++0x type inference</h3>
441
442<p>Use <tt>__has_feature(cxx_auto_type)</tt> to determine C++0x type inference
443is supported using the <tt>auto</tt> specifier. If this is disabled,
Douglas Gregor56209ff2011-01-26 21:25:54 +0000[diff] [blame^]444<tt>auto</tt> will instead be a storage class specifier, as in C or C++98.
445Clang does not currently implement this feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]446
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]447<h3 id="cxx_variadic_templates">C++0x variadic templates</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]448
449<p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
Douglas Gregor83d77812011-01-19 23:15:20 +0000[diff] [blame]450for variadic templates is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]451
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]452<h3 id="cxx_inline_namespaces">C++0x inline namespaces</h3>
453
454<p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> to determine if support for
455inline namespaces is enabled.</p>
456
Douglas Gregordab60ad2010-10-01 18:44:50 +0000[diff] [blame]457<h3 id="cxx_trailing_return">C++0x trailing return type</h3>
458
459<p>Use <tt>__has_feature(cxx_trailing_return)</tt> to determine if support for
460the alternate function declaration syntax with trailing return type is enabled.</p>
461
Douglas Gregor1274ccd2010-10-08 23:50:27 +0000[diff] [blame]462<h3 id="cxx_strong_enums">C++0x strongly typed enumerations</h3>
463
464<p>Use <tt>__has_feature(cxx_strong_enums)</tt> to determine if support for
465strongly typed, scoped enumerations is enabled.</p>
466
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]467<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]468<h2 id="blocks">Blocks</h2>
469<!-- ======================================================================= -->
470
Chris Lattnera7dbdf52009-03-09 07:03:22 +0000[diff] [blame]471<p>The syntax and high level language feature description is in <a
472href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
473details for the clang implementation are in <a
Chris Lattner5d7650b2010-03-16 21:43:03 +0000[diff] [blame]474href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]475
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]476
477<p>Query for this feature with __has_feature(blocks).</p>
478
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]479<!-- ======================================================================= -->
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]480<h2 id="overloading-in-c">Function Overloading in C</h2>
481<!-- ======================================================================= -->
482
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]483<p>Clang provides support for C++ function overloading in C. Function
484overloading in C is introduced using the <tt>overloadable</tt> attribute. For
485example, one might provide several overloaded versions of a <tt>tgsin</tt>
486function that invokes the appropriate standard function computing the sine of a
487value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
488precision:</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]489
490<blockquote>
491<pre>
492#include &lt;math.h&gt;
493float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
494double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
495long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); }
496</pre>
497</blockquote>
498
499<p>Given these declarations, one can call <tt>tgsin</tt> with a
500<tt>float</tt> value to receive a <tt>float</tt> result, with a
501<tt>double</tt> to receive a <tt>double</tt> result, etc. Function
502overloading in C follows the rules of C++ function overloading to pick
503the best overload given the call arguments, with a few C-specific
504semantics:</p>
505<ul>
506 <li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long
507 double</tt> is ranked as a floating-point promotion (per C99) rather
508 than as a floating-point conversion (as in C++).</li>
509
510 <li>A conversion from a pointer of type <tt>T*</tt> to a pointer of type
511 <tt>U*</tt> is considered a pointer conversion (with conversion
512 rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li>
513
514 <li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt>
515 is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This
516 conversion is given "conversion" rank.</li>
517</ul>
518
519<p>The declaration of <tt>overloadable</tt> functions is restricted to
520function declarations and definitions. Most importantly, if any
521function with a given name is given the <tt>overloadable</tt>
522attribute, then all function declarations and definitions with that
523name (and in that scope) must have the <tt>overloadable</tt>
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]524attribute. This rule even applies to redeclarations of functions whose original
525declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]526
527<blockquote>
528<pre>
529int f(int) __attribute__((overloadable));
530float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
531
532int g(int) __attribute__((overloadable));
533int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
534</pre>
535</blockquote>
536
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]537<p>Functions marked <tt>overloadable</tt> must have
538prototypes. Therefore, the following code is ill-formed:</p>
539
540<blockquote>
541<pre>
542int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
543</pre>
544</blockquote>
545
546<p>However, <tt>overloadable</tt> functions are allowed to use a
547ellipsis 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>
548
549<blockquote>
550<pre>
Chris Lattner02246802009-02-18 22:27:46 +0000[diff] [blame]551void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]552</pre>
553</blockquote>
554
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]555<p>Functions declared with the <tt>overloadable</tt> attribute have
556their names mangled according to the same rules as C++ function
557names. For example, the three <tt>tgsin</tt> functions in our
558motivating example get the mangled names <tt>_Z5tgsinf</tt>,
Chris Lattner71b48d62010-11-28 18:19:13 +0000[diff] [blame]559<tt>_Z5tgsind</tt>, and <tt>_Z5tgsine</tt>, respectively. There are two
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]560caveats to this use of name mangling:</p>
561
562<ul>
563
564 <li>Future versions of Clang may change the name mangling of
565 functions overloaded in C, so you should not depend on an specific
566 mangling. To be completely safe, we strongly urge the use of
567 <tt>static inline</tt> with <tt>overloadable</tt> functions.</li>
568
569 <li>The <tt>overloadable</tt> attribute has almost no meaning when
570 used in C++, because names will already be mangled and functions are
571 already overloadable. However, when an <tt>overloadable</tt>
572 function occurs within an <tt>extern "C"</tt> linkage specification,
573 it's name <i>will</i> be mangled in the same way as it would in
574 C.</li>
575</ul>
576
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]577<p>Query for this feature with __has_feature(attribute_overloadable).</p>
578
579
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]580<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]581<h2 id="builtins">Builtin Functions</h2>
582<!-- ======================================================================= -->
583
584<p>Clang supports a number of builtin library functions with the same syntax as
585GCC, including things like <tt>__builtin_nan</tt>,
586<tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
587<tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</tt>, etc. In
588addition to the GCC builtins, Clang supports a number of builtins that GCC does
589not, which are listed here.</p>
590
591<p>Please note that Clang does not and will not support all of the GCC builtins
592for vector operations. Instead of using builtins, you should use the functions
593defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
594define portable wrappers for these. Many of the Clang versions of these
595functions are implemented directly in terms of <a href="#vectors">extended
596vector support</a> instead of builtins, in order to reduce the number of
597builtins that we need to implement.</p>
598
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]599<!-- ======================================================================= -->
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]600<h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]601<!-- ======================================================================= -->
602
Chris Lattneraad826b2009-09-16 18:56:12 +0000[diff] [blame]603<p><tt>__builtin_shufflevector</tt> is used to express generic vector
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]604permutation/shuffle/swizzle operations. This builtin is also very important for
605the implementation of various target-specific header files like
606<tt>&lt;xmmintrin.h&gt;</tt>.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]607</p>
608
609<p><b>Syntax:</b></p>
610
611<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]612__builtin_shufflevector(vec1, vec2, index1, index2, ...)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]613</pre>
614
615<p><b>Examples:</b></p>
616
617<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]618 // Identity operation - return 4-element vector V1.
619 __builtin_shufflevector(V1, V1, 0, 1, 2, 3)
620
621 // "Splat" element 0 of V1 into a 4-element result.
622 __builtin_shufflevector(V1, V1, 0, 0, 0, 0)
623
624 // Reverse 4-element vector V1.
625 __builtin_shufflevector(V1, V1, 3, 2, 1, 0)
626
627 // Concatenate every other element of 4-element vectors V1 and V2.
628 __builtin_shufflevector(V1, V2, 0, 2, 4, 6)
629
630 // Concatenate every other element of 8-element vectors V1 and V2.
631 __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]632</pre>
633
634<p><b>Description:</b></p>
635
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]636<p>The first two arguments to __builtin_shufflevector are vectors that have the
637same element type. The remaining arguments are a list of integers that specify
638the elements indices of the first two vectors that should be extracted and
639returned in a new vector. These element indices are numbered sequentially
640starting with the first vector, continuing into the second vector. Thus, if
641vec1 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]642</p>
643
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]644<p>The result of __builtin_shufflevector is a vector
645with the same element type as vec1/vec2 but that has an element count equal to
646the number of indices specified.
647</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]648
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]649<p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>
650
651<!-- ======================================================================= -->
652<h3 id="__builtin_unreachable">__builtin_unreachable</h3>
653<!-- ======================================================================= -->
654
655<p><tt>__builtin_unreachable</tt> is used to indicate that a specific point in
656the program cannot be reached, even if the compiler might otherwise think it
657can. This is useful to improve optimization and eliminates certain warnings.
658For example, without the <tt>__builtin_unreachable</tt> in the example below,
659the compiler assumes that the inline asm can fall through and prints a "function
660declared 'noreturn' should not return" warning.
661</p>
662
663<p><b>Syntax:</b></p>
664
665<pre>
666__builtin_unreachable()
667</pre>
668
669<p><b>Example of Use:</b></p>
670
671<pre>
672void myabort(void) __attribute__((noreturn));
673void myabort(void) {
674 asm("int3");
675 __builtin_unreachable();
676}
677</pre>
678
679<p><b>Description:</b></p>
680
681<p>The __builtin_unreachable() builtin has completely undefined behavior. Since
682it has undefined behavior, it is a statement that it is never reached and the
683optimizer can take advantage of this to produce better code. This builtin takes
684no arguments and produces a void result.
685</p>
686
687<p>Query for this feature with __has_builtin(__builtin_unreachable).</p>
688
689
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]690<!-- ======================================================================= -->
691<h2 id="targetspecific">Target-Specific Extensions</h2>
692<!-- ======================================================================= -->
693
694<p>Clang supports some language features conditionally on some targets.</p>
695
696<!-- ======================================================================= -->
697<h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
698<!-- ======================================================================= -->
699
700<p>The X86 backend has these language extensions:</p>
701
702<!-- ======================================================================= -->
703<h4 id="x86-gs-segment">Memory references off the GS segment</h4>
704<!-- ======================================================================= -->
705
706<p>Annotating a pointer with address space #256 causes it to be code generated
Chris Lattnera021e7c2009-05-05 18:54:47 +0000[diff] [blame]707relative to the X86 GS segment register, and address space #257 causes it to be
708relative to the X86 FS segment. Note that this is a very very low-level
709feature that should only be used if you know what you're doing (for example in
710an OS kernel).</p>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]711
712<p>Here is an example:</p>
713
714<pre>
715#define GS_RELATIVE __attribute__((address_space(256)))
716int foo(int GS_RELATIVE *P) {
717 return *P;
718}
719</pre>
720
721<p>Which compiles to (on X86-32):</p>
722
723<pre>
724_foo:
725 movl 4(%esp), %eax
726 movl %gs:(%eax), %eax
727 ret
728</pre>
729
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]730<!-- ======================================================================= -->
731<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
732<!-- ======================================================================= -->
733
734<p>Clang supports additional attributes that are useful for documenting program
735invariants and rules for static analysis tools. The extensions documented here
736are used by the <a
737href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
738engine</a> that is part of Clang's Analysis library.</p>
739
740<!-- ======================================================================= -->
741<h3 id="analyzerattributes">Analyzer Attributes</h3>
742<!-- ======================================================================= -->
743
744<h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>
745
746<p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
Ted Kremenek4df21142009-04-10 05:04:22 +0000[diff] [blame]747attribute. This attribute, which is typically affixed to a function prototype,
748indicates that a call to a given function never returns. Function prototypes for
749common functions like <tt>exit</tt> are typically annotated with this attribute,
750as well as a variety of common assertion handlers. Users can educate the static
751analyzer about their own custom assertion handles (thus cutting down on false
752positives due to false paths) by marking their own &quot;panic&quot; functions
753with this attribute.</p>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]754
755<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
Nick Lewycky625b5862009-06-14 04:08:08 +0000[diff] [blame]756there are special functions that for all intents and purposes should be
757considered panic functions (i.e., they are only called when an internal program
758error occurs) but may actually return so that the program can fail gracefully.
759The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
760as being interpreted as &quot;no return&quot; functions by the analyzer (thus
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]761pruning bogus paths) but will not affect compilation (as in the case of
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]762<tt>noreturn</tt>).</p>
763
764<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]765same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]766placed at the end of function prototypes:</p>
767
768<pre>
769 void foo() <b>__attribute__((analyzer_noreturn))</b>;
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]770</pre>
771
772<p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>
773
John McCall630b7ae2011-01-25 04:26:21 +0000[diff] [blame]774<h4 id="attr_retain_release">Objective-C retaining behavior attributes</h4>
775
776<p>In Objective-C, functions and methods are generally assumed to take
777and return objects with +0 retain counts, with some exceptions for
778special methods like <tt>+alloc</tt> and <tt>init</tt>. However,
779there are exceptions, and so Clang provides attributes to allow these
780exceptions to be documented, which helps the analyzer find leaks (and
781ignore non-leaks).</p>
782
783<p><b>Usage</b>: The <tt>ns_returns_retained</tt>, <tt>ns_returns_not_retained</tt>,
784<tt>ns_returns_autoreleased</tt>, <tt>cf_returns_retained</tt>,
785and <tt>cf_returns_not_retained</tt> attributes can be placed on
786methods and functions that return Objective-C or CoreFoundation
787objects. They are commonly placed at the end of a function prototype
788or method declaration:</p>
789
790<pre>
791 id foo() <b>__attribute__((ns_returns_retained))</b>;
792
793 - (NSString*) bar: (int) x <b>__attribute__((ns_returns_retained))</b>;
794</pre>
795
796<p>The <tt>*_returns_retained</tt> attributes specify that the
797returned object has a +1 retain count.
798The <tt>*_returns_not_retained</tt> attributes specify that the return
799object has a +0 retain count, even if the normal convention for its
800selector would be +1. <tt>ns_returns_autoreleased</tt> specifies that the
801returned object is +0, but is guaranteed to live at least as long as the
802next flush of an autorelease pool.</p>
803
804<p><b>Usage</b>: The <tt>ns_consumed</tt> and <tt>cf_consumed</tt>
805attributes can be placed on an parameter declaration; they specify
806that the argument is expected to have a +1 retain count, which will be
807balanced in some way by the function or method.
808The <tt>ns_consumes_self</tt> attribute can only be placed on an
809Objective-C method; it specifies that the method expects
810its <tt>self</tt> parameter to have a +1 retain count, which it will
811balance in some way.</p>
812
813<pre>
814 void <b>foo(__attribute__((ns_consumed))</b> NSString *string);
815
816 - (void) bar <b>__attribute__((ns_consumes_self))</b>;
817 - (void) baz: (id) <b>__attribute__((ns_consumed))</b> x;
818</pre>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]819
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]820</div>
821</body>
822</html>