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<div class="header"><h1 class="heading"><a href="index.html">
<span>Clang 3.3 documentation</span></a></h1>
<h2 class="heading"><span>Clang Language Extensions</span></h2>
</div>
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«  <a href="UsersManual.html">Clang Compiler User’s Manual</a>
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<a class="uplink" href="index.html">Contents</a>
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<a href="ObjectiveCLiterals.html">Objective-C Literals</a>  »
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<div class="content">
<div class="section" id="clang-language-extensions">
<h1>Clang Language Extensions<a class="headerlink" href="#clang-language-extensions" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id1">Introduction</a></li>
<li><a class="reference internal" href="#feature-checking-macros" id="id2">Feature Checking Macros</a></li>
<li><a class="reference internal" href="#include-file-checking-macros" id="id3">Include File Checking Macros</a></li>
<li><a class="reference internal" href="#builtin-macros" id="id4">Builtin Macros</a></li>
<li><a class="reference internal" href="#vectors-and-extended-vectors" id="id5">Vectors and Extended Vectors</a></li>
<li><a class="reference internal" href="#messages-on-deprecated-and-unavailable-attributes" id="id6">Messages on <tt class="docutils literal"><span class="pre">deprecated</span></tt> and <tt class="docutils literal"><span class="pre">unavailable</span></tt> Attributes</a></li>
<li><a class="reference internal" href="#attributes-on-enumerators" id="id7">Attributes on Enumerators</a></li>
<li><a class="reference internal" href="#user-specified-system-frameworks" id="id8">‘User-Specified’ System Frameworks</a></li>
<li><a class="reference internal" href="#availability-attribute" id="id9">Availability attribute</a></li>
<li><a class="reference internal" href="#checks-for-standard-language-features" id="id10">Checks for Standard Language Features</a></li>
<li><a class="reference internal" href="#checks-for-type-traits" id="id11">Checks for Type Traits</a></li>
<li><a class="reference internal" href="#blocks" id="id12">Blocks</a></li>
<li><a class="reference internal" href="#objective-c-features" id="id13">Objective-C Features</a></li>
<li><a class="reference internal" href="#function-overloading-in-c" id="id14">Function Overloading in C</a></li>
<li><a class="reference internal" href="#initializer-lists-for-complex-numbers-in-c" id="id15">Initializer lists for complex numbers in C</a></li>
<li><a class="reference internal" href="#builtin-functions" id="id16">Builtin Functions</a></li>
<li><a class="reference internal" href="#non-standard-c-11-attributes" id="id17">Non-standard C++11 Attributes</a></li>
<li><a class="reference internal" href="#target-specific-extensions" id="id18">Target-Specific Extensions</a></li>
<li><a class="reference internal" href="#extensions-for-static-analysis" id="id19">Extensions for Static Analysis</a></li>
<li><a class="reference internal" href="#extensions-for-dynamic-analysis" id="id20">Extensions for Dynamic Analysis</a></li>
<li><a class="reference internal" href="#thread-safety-annotation-checking" id="id21">Thread-Safety Annotation Checking</a></li>
<li><a class="reference internal" href="#type-safety-checking" id="id22">Type Safety Checking</a></li>
<li><a class="reference internal" href="#format-string-checking" id="id23">Format String Checking</a></li>
</ul>
</div>
<div class="toctree-wrapper compound">
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id1">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>This document describes the language extensions provided by Clang. In addition
to the language extensions listed here, Clang aims to support a broad range of
GCC extensions. Please see the <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for more information on
these extensions.</p>
</div>
<div class="section" id="feature-checking-macros">
<span id="langext-feature-check"></span><h2><a class="toc-backref" href="#id2">Feature Checking Macros</a><a class="headerlink" href="#feature-checking-macros" title="Permalink to this headline">¶</a></h2>
<p>Language extensions can be very useful, but only if you know you can depend on
them. In order to allow fine-grain features checks, we support three builtin
function-like macros. This allows you to directly test for a feature in your
code without having to resort to something like autoconf or fragile “compiler
version checks”.</p>
<div class="section" id="has-builtin">
<h3><tt class="docutils literal"><span class="pre">__has_builtin</span></tt><a class="headerlink" href="#has-builtin" title="Permalink to this headline">¶</a></h3>
<p>This function-like macro takes a single identifier argument that is the name of
a builtin function. It evaluates to 1 if the builtin is supported or 0 if not.
It can be used like this:</p>
<div class="highlight-c++"><pre>#ifndef __has_builtin // Optional of course.
#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_builtin(__builtin_trap)
__builtin_trap();
#else
abort();
#endif
...</pre>
</div>
</div>
<div class="section" id="has-feature-and-has-extension">
<span id="langext-has-feature-has-extension"></span><h3><tt class="docutils literal"><span class="pre">__has_feature</span></tt> and <tt class="docutils literal"><span class="pre">__has_extension</span></tt><a class="headerlink" href="#has-feature-and-has-extension" title="Permalink to this headline">¶</a></h3>
<p>These function-like macros take a single identifier argument that is the name
of a feature. <tt class="docutils literal"><span class="pre">__has_feature</span></tt> evaluates to 1 if the feature is both
supported by Clang and standardized in the current language standard or 0 if
not (but see <a class="reference internal" href="#langext-has-feature-back-compat"><em>below</em></a>), while
<tt class="docutils literal"><span class="pre">__has_extension</span></tt> evaluates to 1 if the feature is supported by Clang in the
current language (either as a language extension or a standard language
feature) or 0 if not. They can be used like this:</p>
<div class="highlight-c++"><pre>#ifndef __has_feature // Optional of course.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif
#ifndef __has_extension
#define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
#endif
...
#if __has_feature(cxx_rvalue_references)
// This code will only be compiled with the -std=c++11 and -std=gnu++11
// options, because rvalue references are only standardized in C++11.
#endif
#if __has_extension(cxx_rvalue_references)
// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
// and -std=gnu++98 options, because rvalue references are supported as a
// language extension in C++98.
#endif</pre>
</div>
<p id="langext-has-feature-back-compat">For backwards compatibility reasons, <tt class="docutils literal"><span class="pre">__has_feature</span></tt> can also be used to test
for support for non-standardized features, i.e. features not prefixed <tt class="docutils literal"><span class="pre">c_</span></tt>,
<tt class="docutils literal"><span class="pre">cxx_</span></tt> or <tt class="docutils literal"><span class="pre">objc_</span></tt>.</p>
<p>Another use of <tt class="docutils literal"><span class="pre">__has_feature</span></tt> is to check for compiler features not related
to the language standard, such as e.g. <a class="reference internal" href="AddressSanitizer.html"><em>AddressSanitizer</em></a>.</p>
<p>If the <tt class="docutils literal"><span class="pre">-pedantic-errors</span></tt> option is given, <tt class="docutils literal"><span class="pre">__has_extension</span></tt> is equivalent
to <tt class="docutils literal"><span class="pre">__has_feature</span></tt>.</p>
<p>The feature tag is described along with the language feature below.</p>
<p>The feature name or extension name can also be specified with a preceding and
following <tt class="docutils literal"><span class="pre">__</span></tt> (double underscore) to avoid interference from a macro with
the same name. For instance, <tt class="docutils literal"><span class="pre">__cxx_rvalue_references__</span></tt> can be used instead
of <tt class="docutils literal"><span class="pre">cxx_rvalue_references</span></tt>.</p>
</div>
<div class="section" id="has-attribute">
<h3><tt class="docutils literal"><span class="pre">__has_attribute</span></tt><a class="headerlink" href="#has-attribute" title="Permalink to this headline">¶</a></h3>
<p>This function-like macro takes a single identifier argument that is the name of
an attribute. It evaluates to 1 if the attribute is supported or 0 if not. It
can be used like this:</p>
<div class="highlight-c++"><pre>#ifndef __has_attribute // Optional of course.
#define __has_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_attribute(always_inline)
#define ALWAYS_INLINE __attribute__((always_inline))
#else
#define ALWAYS_INLINE
#endif
...</pre>
</div>
<p>The attribute name can also be specified with a preceding and following <tt class="docutils literal"><span class="pre">__</span></tt>
(double underscore) to avoid interference from a macro with the same name. For
instance, <tt class="docutils literal"><span class="pre">__always_inline__</span></tt> can be used instead of <tt class="docutils literal"><span class="pre">always_inline</span></tt>.</p>
</div>
</div>
<div class="section" id="include-file-checking-macros">
<h2><a class="toc-backref" href="#id3">Include File Checking Macros</a><a class="headerlink" href="#include-file-checking-macros" title="Permalink to this headline">¶</a></h2>
<p>Not all developments systems have the same include files. The
<a class="reference internal" href="#langext-has-include"><em>__has_include</em></a> and <a class="reference internal" href="#langext-has-include-next"><em>__has_include_next</em></a> macros allow
you to check for the existence of an include file before doing a possibly
failing <tt class="docutils literal"><span class="pre">#include</span></tt> directive. Include file checking macros must be used
as expressions in <tt class="docutils literal"><span class="pre">#if</span></tt> or <tt class="docutils literal"><span class="pre">#elif</span></tt> preprocessing directives.</p>
<div class="section" id="has-include">
<span id="langext-has-include"></span><h3><tt class="docutils literal"><span class="pre">__has_include</span></tt><a class="headerlink" href="#has-include" title="Permalink to this headline">¶</a></h3>
<p>This function-like macro takes a single file name string argument that is the
name of an include file. It evaluates to 1 if the file can be found using the
include paths, or 0 otherwise:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// Note the two possible file name string formats.</span>
<span class="cp">#if __has_include("myinclude.h") && __has_include(<stdint.h>)</span>
<span class="cp"># include "myinclude.h"</span>
<span class="cp">#endif</span>
<span class="c1">// To avoid problem with non-clang compilers not having this macro.</span>
<span class="cp">#if defined(__has_include) && __has_include("myinclude.h")</span>
<span class="cp"># include "myinclude.h"</span>
<span class="cp">#endif</span>
</pre></div>
</div>
<p>To test for this feature, use <tt class="docutils literal"><span class="pre">#if</span> <span class="pre">defined(__has_include)</span></tt>.</p>
</div>
<div class="section" id="has-include-next">
<span id="langext-has-include-next"></span><h3><tt class="docutils literal"><span class="pre">__has_include_next</span></tt><a class="headerlink" href="#has-include-next" title="Permalink to this headline">¶</a></h3>
<p>This function-like macro takes a single file name string argument that is the
name of an include file. It is like <tt class="docutils literal"><span class="pre">__has_include</span></tt> except that it looks for
the second instance of the given file found in the include paths. It evaluates
to 1 if the second instance of the file can be found using the include paths,
or 0 otherwise:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// Note the two possible file name string formats.</span>
<span class="cp">#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)</span>
<span class="cp"># include_next "myinclude.h"</span>
<span class="cp">#endif</span>
<span class="c1">// To avoid problem with non-clang compilers not having this macro.</span>
<span class="cp">#if defined(__has_include_next) && __has_include_next("myinclude.h")</span>
<span class="cp"># include_next "myinclude.h"</span>
<span class="cp">#endif</span>
</pre></div>
</div>
<p>Note that <tt class="docutils literal"><span class="pre">__has_include_next</span></tt>, like the GNU extension <tt class="docutils literal"><span class="pre">#include_next</span></tt>
directive, is intended for use in headers only, and will issue a warning if
used in the top-level compilation file. A warning will also be issued if an
absolute path is used in the file argument.</p>
</div>
<div class="section" id="has-warning">
<h3><tt class="docutils literal"><span class="pre">__has_warning</span></tt><a class="headerlink" href="#has-warning" title="Permalink to this headline">¶</a></h3>
<p>This function-like macro takes a string literal that represents a command line
option for a warning and returns true if that is a valid warning option.</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#if __has_warning("-Wformat")</span>
<span class="p">...</span>
<span class="cp">#endif</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="builtin-macros">
<h2><a class="toc-backref" href="#id4">Builtin Macros</a><a class="headerlink" href="#builtin-macros" title="Permalink to this headline">¶</a></h2>
<dl class="docutils">
<dt><tt class="docutils literal"><span class="pre">__BASE_FILE__</span></tt></dt>
<dd>Defined to a string that contains the name of the main input file passed to
Clang.</dd>
<dt><tt class="docutils literal"><span class="pre">__COUNTER__</span></tt></dt>
<dd>Defined to an integer value that starts at zero and is incremented each time
the <tt class="docutils literal"><span class="pre">__COUNTER__</span></tt> macro is expanded.</dd>
<dt><tt class="docutils literal"><span class="pre">__INCLUDE_LEVEL__</span></tt></dt>
<dd>Defined to an integral value that is the include depth of the file currently
being translated. For the main file, this value is zero.</dd>
<dt><tt class="docutils literal"><span class="pre">__TIMESTAMP__</span></tt></dt>
<dd>Defined to the date and time of the last modification of the current source
file.</dd>
<dt><tt class="docutils literal"><span class="pre">__clang__</span></tt></dt>
<dd>Defined when compiling with Clang</dd>
<dt><tt class="docutils literal"><span class="pre">__clang_major__</span></tt></dt>
<dd>Defined to the major marketing version number of Clang (e.g., the 2 in
2.0.1). Note that marketing version numbers should not be used to check for
language features, as different vendors use different numbering schemes.
Instead, use the <a class="reference internal" href="#langext-feature-check"><em>Feature Checking Macros</em></a>.</dd>
<dt><tt class="docutils literal"><span class="pre">__clang_minor__</span></tt></dt>
<dd>Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note
that marketing version numbers should not be used to check for language
features, as different vendors use different numbering schemes. Instead, use
the <a class="reference internal" href="#langext-feature-check"><em>Feature Checking Macros</em></a>.</dd>
<dt><tt class="docutils literal"><span class="pre">__clang_patchlevel__</span></tt></dt>
<dd>Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).</dd>
<dt><tt class="docutils literal"><span class="pre">__clang_version__</span></tt></dt>
<dd>Defined to a string that captures the Clang marketing version, including the
Subversion tag or revision number, e.g., “<tt class="docutils literal"><span class="pre">1.5</span> <span class="pre">(trunk</span> <span class="pre">102332)</span></tt>”.</dd>
</dl>
</div>
<div class="section" id="vectors-and-extended-vectors">
<span id="langext-vectors"></span><h2><a class="toc-backref" href="#id5">Vectors and Extended Vectors</a><a class="headerlink" href="#vectors-and-extended-vectors" title="Permalink to this headline">¶</a></h2>
<p>Supports the GCC, OpenCL, AltiVec and NEON vector extensions.</p>
<p>OpenCL vector types are created using <tt class="docutils literal"><span class="pre">ext_vector_type</span></tt> attribute. It
support for <tt class="docutils literal"><span class="pre">V.xyzw</span></tt> syntax and other tidbits as seen in OpenCL. An example
is:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">typedef</span> <span class="kt">float</span> <span class="n">float4</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">ext_vector_type</span><span class="p">(</span><span class="mi">4</span><span class="p">)));</span>
<span class="k">typedef</span> <span class="kt">float</span> <span class="n">float2</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">ext_vector_type</span><span class="p">(</span><span class="mi">2</span><span class="p">)));</span>
<span class="n">float4</span> <span class="nf">foo</span><span class="p">(</span><span class="n">float2</span> <span class="n">a</span><span class="p">,</span> <span class="n">float2</span> <span class="n">b</span><span class="p">)</span> <span class="p">{</span>
<span class="n">float4</span> <span class="n">c</span><span class="p">;</span>
<span class="n">c</span><span class="p">.</span><span class="n">xz</span> <span class="o">=</span> <span class="n">a</span><span class="p">;</span>
<span class="n">c</span><span class="p">.</span><span class="n">yw</span> <span class="o">=</span> <span class="n">b</span><span class="p">;</span>
<span class="k">return</span> <span class="n">c</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_extension(attribute_ext_vector_type)</span></tt>.</p>
<p>Giving <tt class="docutils literal"><span class="pre">-faltivec</span></tt> option to clang enables support for AltiVec vector syntax
and functions. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">vector</span> <span class="kt">float</span> <span class="nf">foo</span><span class="p">(</span><span class="n">vector</span> <span class="kt">int</span> <span class="n">a</span><span class="p">)</span> <span class="p">{</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">b</span><span class="p">;</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">vec_add</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">a</span><span class="p">)</span> <span class="o">+</span> <span class="n">a</span><span class="p">;</span>
<span class="k">return</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">float</span><span class="p">)</span><span class="n">b</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>NEON vector types are created using <tt class="docutils literal"><span class="pre">neon_vector_type</span></tt> and
<tt class="docutils literal"><span class="pre">neon_polyvector_type</span></tt> attributes. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">typedef</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">neon_vector_type</span><span class="p">(</span><span class="mi">8</span><span class="p">)))</span> <span class="kt">int8_t</span> <span class="kt">int8x8_t</span><span class="p">;</span>
<span class="k">typedef</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">neon_polyvector_type</span><span class="p">(</span><span class="mi">16</span><span class="p">)))</span> <span class="kt">poly8_t</span> <span class="kt">poly8x16_t</span><span class="p">;</span>
<span class="kt">int8x8_t</span> <span class="nf">foo</span><span class="p">(</span><span class="kt">int8x8_t</span> <span class="n">a</span><span class="p">)</span> <span class="p">{</span>
<span class="kt">int8x8_t</span> <span class="n">v</span><span class="p">;</span>
<span class="n">v</span> <span class="o">=</span> <span class="n">a</span><span class="p">;</span>
<span class="k">return</span> <span class="n">v</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="section" id="vector-literals">
<h3>Vector Literals<a class="headerlink" href="#vector-literals" title="Permalink to this headline">¶</a></h3>
<p>Vector literals can be used to create vectors from a set of scalars, or
vectors. Either parentheses or braces form can be used. In the parentheses
form the number of literal values specified must be one, i.e. referring to a
scalar value, or must match the size of the vector type being created. If a
single scalar literal value is specified, the scalar literal value will be
replicated to all the components of the vector type. In the brackets form any
number of literals can be specified. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">typedef</span> <span class="kt">int</span> <span class="n">v4si</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">__vector_size__</span><span class="p">(</span><span class="mi">16</span><span class="p">)));</span>
<span class="k">typedef</span> <span class="kt">float</span> <span class="n">float4</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">ext_vector_type</span><span class="p">(</span><span class="mi">4</span><span class="p">)));</span>
<span class="k">typedef</span> <span class="kt">float</span> <span class="n">float2</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">ext_vector_type</span><span class="p">(</span><span class="mi">2</span><span class="p">)));</span>
<span class="n">v4si</span> <span class="n">vsi</span> <span class="o">=</span> <span class="p">(</span><span class="n">v4si</span><span class="p">){</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">};</span>
<span class="n">float4</span> <span class="n">vf</span> <span class="o">=</span> <span class="p">(</span><span class="n">float4</span><span class="p">)(</span><span class="mf">1.0f</span><span class="p">,</span> <span class="mf">2.0f</span><span class="p">,</span> <span class="mf">3.0f</span><span class="p">,</span> <span class="mf">4.0f</span><span class="p">);</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">vi1</span> <span class="o">=</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">int</span><span class="p">)(</span><span class="mi">1</span><span class="p">);</span> <span class="c1">// vi1 will be (1, 1, 1, 1).</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">vi2</span> <span class="o">=</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">int</span><span class="p">){</span><span class="mi">1</span><span class="p">};</span> <span class="c1">// vi2 will be (1, 0, 0, 0).</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">vi3</span> <span class="o">=</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">int</span><span class="p">)(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">);</span> <span class="c1">// error</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">vi4</span> <span class="o">=</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">int</span><span class="p">){</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">};</span> <span class="c1">// vi4 will be (1, 2, 0, 0).</span>
<span class="n">vector</span> <span class="kt">int</span> <span class="n">vi5</span> <span class="o">=</span> <span class="p">(</span><span class="n">vector</span> <span class="kt">int</span><span class="p">)(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">);</span>
<span class="n">float4</span> <span class="n">vf</span> <span class="o">=</span> <span class="p">(</span><span class="n">float4</span><span class="p">)((</span><span class="n">float2</span><span class="p">)(</span><span class="mf">1.0f</span><span class="p">,</span> <span class="mf">2.0f</span><span class="p">),</span> <span class="p">(</span><span class="n">float2</span><span class="p">)(</span><span class="mf">3.0f</span><span class="p">,</span> <span class="mf">4.0f</span><span class="p">));</span>
</pre></div>
</div>
</div>
<div class="section" id="vector-operations">
<h3>Vector Operations<a class="headerlink" href="#vector-operations" title="Permalink to this headline">¶</a></h3>
<p>The table below shows the support for each operation by vector extension. A
dash indicates that an operation is not accepted according to a corresponding
specification.</p>
<table border="1" class="docutils">
<colgroup>
<col width="60%" />
<col width="12%" />
<col width="14%" />
<col width="6%" />
<col width="8%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Opeator</th>
<th class="head">OpenCL</th>
<th class="head">AltiVec</th>
<th class="head">GCC</th>
<th class="head">NEON</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>[]</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-odd"><td>unary operators +, –</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-even"><td>++, – –</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-odd"><td>+,–,*,/,%</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-even"><td>bitwise operators &,|,^,~</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-odd"><td>>>,<<</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
</tr>
<tr class="row-even"><td>!, &&, ||</td>
<td>no</td>
<td>–</td>
<td>–</td>
<td>–</td>
</tr>
<tr class="row-odd"><td>==, !=, >, <, >=, <=</td>
<td>yes</td>
<td>yes</td>
<td>–</td>
<td>–</td>
</tr>
<tr class="row-even"><td>=</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
</tr>
<tr class="row-odd"><td>:?</td>
<td>yes</td>
<td>–</td>
<td>–</td>
<td>–</td>
</tr>
<tr class="row-even"><td>sizeof</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
<td>yes</td>
</tr>
</tbody>
</table>
<p>See also <a class="reference internal" href="#langext-builtin-shufflevector"><em>__builtin_shufflevector</em></a>.</p>
</div>
</div>
<div class="section" id="messages-on-deprecated-and-unavailable-attributes">
<h2><a class="toc-backref" href="#id6">Messages on <tt class="docutils literal"><span class="pre">deprecated</span></tt> and <tt class="docutils literal"><span class="pre">unavailable</span></tt> Attributes</a><a class="headerlink" href="#messages-on-deprecated-and-unavailable-attributes" title="Permalink to this headline">¶</a></h2>
<p>An optional string message can be added to the <tt class="docutils literal"><span class="pre">deprecated</span></tt> and
<tt class="docutils literal"><span class="pre">unavailable</span></tt> attributes. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">explode</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">deprecated</span><span class="p">(</span><span class="s">"extremely unsafe, use 'combust' instead!!!"</span><span class="p">)));</span>
</pre></div>
</div>
<p>If the deprecated or unavailable declaration is used, the message will be
incorporated into the appropriate diagnostic:</p>
<div class="highlight-c++"><pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
[-Wdeprecated-declarations]
explode();
^</pre>
</div>
<p>Query for this feature with
<tt class="docutils literal"><span class="pre">__has_extension(attribute_deprecated_with_message)</span></tt> and
<tt class="docutils literal"><span class="pre">__has_extension(attribute_unavailable_with_message)</span></tt>.</p>
</div>
<div class="section" id="attributes-on-enumerators">
<h2><a class="toc-backref" href="#id7">Attributes on Enumerators</a><a class="headerlink" href="#attributes-on-enumerators" title="Permalink to this headline">¶</a></h2>
<p>Clang allows attributes to be written on individual enumerators. This allows
enumerators to be deprecated, made unavailable, etc. The attribute must appear
after the enumerator name and before any initializer, like so:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">enum</span> <span class="n">OperationMode</span> <span class="p">{</span>
<span class="n">OM_Invalid</span><span class="p">,</span>
<span class="n">OM_Normal</span><span class="p">,</span>
<span class="n">OM_Terrified</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">deprecated</span><span class="p">)),</span>
<span class="n">OM_AbortOnError</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">deprecated</span><span class="p">))</span> <span class="o">=</span> <span class="mi">4</span>
<span class="p">};</span>
</pre></div>
</div>
<p>Attributes on the <tt class="docutils literal"><span class="pre">enum</span></tt> declaration do not apply to individual enumerators.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_extension(enumerator_attributes)</span></tt>.</p>
</div>
<div class="section" id="user-specified-system-frameworks">
<h2><a class="toc-backref" href="#id8">‘User-Specified’ System Frameworks</a><a class="headerlink" href="#user-specified-system-frameworks" title="Permalink to this headline">¶</a></h2>
<p>Clang provides a mechanism by which frameworks can be built in such a way that
they will always be treated as being “system frameworks”, even if they are not
present in a system framework directory. This can be useful to system
framework developers who want to be able to test building other applications
with development builds of their framework, including the manner in which the
compiler changes warning behavior for system headers.</p>
<p>Framework developers can opt-in to this mechanism by creating a
“<tt class="docutils literal"><span class="pre">.system_framework</span></tt>” file at the top-level of their framework. That is, the
framework should have contents like:</p>
<div class="highlight-none"><div class="highlight"><pre>.../TestFramework.framework
.../TestFramework.framework/.system_framework
.../TestFramework.framework/Headers
.../TestFramework.framework/Headers/TestFramework.h
...
</pre></div>
</div>
<p>Clang will treat the presence of this file as an indicator that the framework
should be treated as a system framework, regardless of how it was found in the
framework search path. For consistency, we recommend that such files never be
included in installed versions of the framework.</p>
</div>
<div class="section" id="availability-attribute">
<h2><a class="toc-backref" href="#id9">Availability attribute</a><a class="headerlink" href="#availability-attribute" title="Permalink to this headline">¶</a></h2>
<p>Clang introduces the <tt class="docutils literal"><span class="pre">availability</span></tt> attribute, which can be placed on
declarations to describe the lifecycle of that declaration relative to
operating system versions. Consider the function declaration for a
hypothetical function <tt class="docutils literal"><span class="pre">f</span></tt>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">f</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.4</span><span class="p">,</span><span class="n">deprecated</span><span class="o">=</span><span class="mf">10.6</span><span class="p">,</span><span class="n">obsoleted</span><span class="o">=</span><span class="mf">10.7</span><span class="p">)));</span>
</pre></div>
</div>
<p>The availability attribute states that <tt class="docutils literal"><span class="pre">f</span></tt> was introduced in Mac OS X 10.4,
deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information
is used by Clang to determine when it is safe to use <tt class="docutils literal"><span class="pre">f</span></tt>: for example, if
Clang is instructed to compile code for Mac OS X 10.5, a call to <tt class="docutils literal"><span class="pre">f()</span></tt>
succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call
succeeds but Clang emits a warning specifying that the function is deprecated.
Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call
fails because <tt class="docutils literal"><span class="pre">f()</span></tt> is no longer available.</p>
<p>The availability attribute is a comma-separated list starting with the
platform name and then including clauses specifying important milestones in the
declaration’s lifetime (in any order) along with additional information. Those
clauses can be:</p>
<dl class="docutils">
<dt>introduced=<em>version</em></dt>
<dd>The first version in which this declaration was introduced.</dd>
<dt>deprecated=<em>version</em></dt>
<dd>The first version in which this declaration was deprecated, meaning that
users should migrate away from this API.</dd>
<dt>obsoleted=<em>version</em></dt>
<dd>The first version in which this declaration was obsoleted, meaning that it
was removed completely and can no longer be used.</dd>
<dt>unavailable</dt>
<dd>This declaration is never available on this platform.</dd>
<dt>message=<em>string-literal</em></dt>
<dd>Additional message text that Clang will provide when emitting a warning or
error about use of a deprecated or obsoleted declaration. Useful to direct
users to replacement APIs.</dd>
</dl>
<p>Multiple availability attributes can be placed on a declaration, which may
correspond to different platforms. Only the availability attribute with the
platform corresponding to the target platform will be used; any others will be
ignored. If no availability attribute specifies availability for the current
target platform, the availability attributes are ignored. Supported platforms
are:</p>
<dl class="docutils">
<dt><tt class="docutils literal"><span class="pre">ios</span></tt></dt>
<dd>Apple’s iOS operating system. The minimum deployment target is specified by
the <tt class="docutils literal"><span class="pre">-mios-version-min=*version*</span></tt> or <tt class="docutils literal"><span class="pre">-miphoneos-version-min=*version*</span></tt>
command-line arguments.</dd>
<dt><tt class="docutils literal"><span class="pre">macosx</span></tt></dt>
<dd>Apple’s Mac OS X operating system. The minimum deployment target is
specified by the <tt class="docutils literal"><span class="pre">-mmacosx-version-min=*version*</span></tt> command-line argument.</dd>
</dl>
<p>A declaration can be used even when deploying back to a platform version prior
to when the declaration was introduced. When this happens, the declaration is
<a class="reference external" href="https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html">weakly linked</a>,
as if the <tt class="docutils literal"><span class="pre">weak_import</span></tt> attribute were added to the declaration. A
weakly-linked declaration may or may not be present a run-time, and a program
can determine whether the declaration is present by checking whether the
address of that declaration is non-NULL.</p>
<p>If there are multiple declarations of the same entity, the availability
attributes must either match on a per-platform basis or later
declarations must not have availability attributes for that
platform. For example:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.4</span><span class="p">)));</span>
<span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.4</span><span class="p">)));</span> <span class="c1">// okay, matches</span>
<span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">ios</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">4.0</span><span class="p">)));</span> <span class="c1">// okay, adds a new platform</span>
<span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">void</span><span class="p">);</span> <span class="c1">// okay, inherits both macosx and ios availability from above.</span>
<span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.5</span><span class="p">)));</span> <span class="c1">// error: mismatch</span>
</pre></div>
</div>
<p>When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">method</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.4</span><span class="p">)));</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">method2</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.4</span><span class="p">)));</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span> : <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">method</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.3</span><span class="p">)));</span> <span class="c1">// okay: method moved into base class later</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">method</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">availability</span><span class="p">(</span><span class="n">macosx</span><span class="p">,</span><span class="n">introduced</span><span class="o">=</span><span class="mf">10.5</span><span class="p">)));</span> <span class="c1">// error: this method was available via the base class in 10.4</span>
<span class="k">@end</span>
</pre></div>
</div>
</div>
<div class="section" id="checks-for-standard-language-features">
<h2><a class="toc-backref" href="#id10">Checks for Standard Language Features</a><a class="headerlink" href="#checks-for-standard-language-features" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="docutils literal"><span class="pre">__has_feature</span></tt> macro can be used to query if certain standard language
features are enabled. The <tt class="docutils literal"><span class="pre">__has_extension</span></tt> macro can be used to query if
language features are available as an extension when compiling for a standard
which does not provide them. The features which can be tested are listed here.</p>
<div class="section" id="c-98">
<h3>C++98<a class="headerlink" href="#c-98" title="Permalink to this headline">¶</a></h3>
<p>The features listed below are part of the C++98 standard. These features are
enabled by default when compiling C++ code.</p>
<div class="section" id="c-exceptions">
<h4>C++ exceptions<a class="headerlink" href="#c-exceptions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_exceptions)</span></tt> to determine if C++ exceptions have been
enabled. For example, compiling code with <tt class="docutils literal"><span class="pre">-fno-exceptions</span></tt> disables C++
exceptions.</p>
</div>
<div class="section" id="c-rtti">
<h4>C++ RTTI<a class="headerlink" href="#c-rtti" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_rtti)</span></tt> to determine if C++ RTTI has been enabled. For
example, compiling code with <tt class="docutils literal"><span class="pre">-fno-rtti</span></tt> disables the use of RTTI.</p>
</div>
</div>
<div class="section" id="c-11">
<h3>C++11<a class="headerlink" href="#c-11" title="Permalink to this headline">¶</a></h3>
<p>The features listed below are part of the C++11 standard. As a result, all
these features are enabled with the <tt class="docutils literal"><span class="pre">-std=c++11</span></tt> or <tt class="docutils literal"><span class="pre">-std=gnu++11</span></tt> option
when compiling C++ code.</p>
<div class="section" id="c-11-sfinae-includes-access-control">
<h4>C++11 SFINAE includes access control<a class="headerlink" href="#c-11-sfinae-includes-access-control" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_access_control_sfinae)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_access_control_sfinae)</span></tt> to determine whether
access-control errors (e.g., calling a private constructor) are considered to
be template argument deduction errors (aka SFINAE errors), per <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170">C++ DR1170</a>.</p>
</div>
<div class="section" id="c-11-alias-templates">
<h4>C++11 alias templates<a class="headerlink" href="#c-11-alias-templates" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_alias_templates)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_alias_templates)</span></tt> to determine if support for C++11’s
alias declarations and alias templates is enabled.</p>
</div>
<div class="section" id="c-11-alignment-specifiers">
<h4>C++11 alignment specifiers<a class="headerlink" href="#c-11-alignment-specifiers" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_alignas)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_alignas)</span></tt> to
determine if support for alignment specifiers using <tt class="docutils literal"><span class="pre">alignas</span></tt> is enabled.</p>
</div>
<div class="section" id="c-11-attributes">
<h4>C++11 attributes<a class="headerlink" href="#c-11-attributes" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_attributes)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_attributes)</span></tt> to
determine if support for attribute parsing with C++11’s square bracket notation
is enabled.</p>
</div>
<div class="section" id="c-11-generalized-constant-expressions">
<h4>C++11 generalized constant expressions<a class="headerlink" href="#c-11-generalized-constant-expressions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_constexpr)</span></tt> to determine if support for generalized
constant expressions (e.g., <tt class="docutils literal"><span class="pre">constexpr</span></tt>) is enabled.</p>
</div>
<div class="section" id="c-11-decltype">
<h4>C++11 <tt class="docutils literal"><span class="pre">decltype()</span></tt><a class="headerlink" href="#c-11-decltype" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_decltype)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_decltype)</span></tt> to
determine if support for the <tt class="docutils literal"><span class="pre">decltype()</span></tt> specifier is enabled. C++11’s
<tt class="docutils literal"><span class="pre">decltype</span></tt> does not require type-completeness of a function call expression.
Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_decltype_incomplete_return_types)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_decltype_incomplete_return_types)</span></tt> to determine if
support for this feature is enabled.</p>
</div>
<div class="section" id="c-11-default-template-arguments-in-function-templates">
<h4>C++11 default template arguments in function templates<a class="headerlink" href="#c-11-default-template-arguments-in-function-templates" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_default_function_template_args)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_default_function_template_args)</span></tt> to determine if support
for default template arguments in function templates is enabled.</p>
</div>
<div class="section" id="c-11-defaulted-functions">
<h4>C++11 <tt class="docutils literal"><span class="pre">default</span></tt>ed functions<a class="headerlink" href="#c-11-defaulted-functions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_defaulted_functions)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_defaulted_functions)</span></tt> to determine if support for
defaulted function definitions (with <tt class="docutils literal"><span class="pre">=</span> <span class="pre">default</span></tt>) is enabled.</p>
</div>
<div class="section" id="c-11-delegating-constructors">
<h4>C++11 delegating constructors<a class="headerlink" href="#c-11-delegating-constructors" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_delegating_constructors)</span></tt> to determine if support for
delegating constructors is enabled.</p>
</div>
<div class="section" id="c-11-deleted-functions">
<h4>C++11 <tt class="docutils literal"><span class="pre">deleted</span></tt> functions<a class="headerlink" href="#c-11-deleted-functions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_deleted_functions)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_deleted_functions)</span></tt> to determine if support for deleted
function definitions (with <tt class="docutils literal"><span class="pre">=</span> <span class="pre">delete</span></tt>) is enabled.</p>
</div>
<div class="section" id="c-11-explicit-conversion-functions">
<h4>C++11 explicit conversion functions<a class="headerlink" href="#c-11-explicit-conversion-functions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_explicit_conversions)</span></tt> to determine if support for
<tt class="docutils literal"><span class="pre">explicit</span></tt> conversion functions is enabled.</p>
</div>
<div class="section" id="c-11-generalized-initializers">
<h4>C++11 generalized initializers<a class="headerlink" href="#c-11-generalized-initializers" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_generalized_initializers)</span></tt> to determine if support for
generalized initializers (using braced lists and <tt class="docutils literal"><span class="pre">std::initializer_list</span></tt>) is
enabled.</p>
</div>
<div class="section" id="c-11-implicit-move-constructors-assignment-operators">
<h4>C++11 implicit move constructors/assignment operators<a class="headerlink" href="#c-11-implicit-move-constructors-assignment-operators" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_implicit_moves)</span></tt> to determine if Clang will implicitly
generate move constructors and move assignment operators where needed.</p>
</div>
<div class="section" id="c-11-inheriting-constructors">
<h4>C++11 inheriting constructors<a class="headerlink" href="#c-11-inheriting-constructors" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_inheriting_constructors)</span></tt> to determine if support for
inheriting constructors is enabled.</p>
</div>
<div class="section" id="c-11-inline-namespaces">
<h4>C++11 inline namespaces<a class="headerlink" href="#c-11-inline-namespaces" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_inline_namespaces)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_inline_namespaces)</span></tt> to determine if support for inline
namespaces is enabled.</p>
</div>
<div class="section" id="c-11-lambdas">
<h4>C++11 lambdas<a class="headerlink" href="#c-11-lambdas" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_lambdas)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_lambdas)</span></tt> to
determine if support for lambdas is enabled.</p>
</div>
<div class="section" id="c-11-local-and-unnamed-types-as-template-arguments">
<h4>C++11 local and unnamed types as template arguments<a class="headerlink" href="#c-11-local-and-unnamed-types-as-template-arguments" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_local_type_template_args)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_local_type_template_args)</span></tt> to determine if support for
local and unnamed types as template arguments is enabled.</p>
</div>
<div class="section" id="c-11-noexcept">
<h4>C++11 noexcept<a class="headerlink" href="#c-11-noexcept" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_noexcept)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_noexcept)</span></tt> to
determine if support for noexcept exception specifications is enabled.</p>
</div>
<div class="section" id="c-11-in-class-non-static-data-member-initialization">
<h4>C++11 in-class non-static data member initialization<a class="headerlink" href="#c-11-in-class-non-static-data-member-initialization" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_nonstatic_member_init)</span></tt> to determine whether in-class
initialization of non-static data members is enabled.</p>
</div>
<div class="section" id="c-11-nullptr">
<h4>C++11 <tt class="docutils literal"><span class="pre">nullptr</span></tt><a class="headerlink" href="#c-11-nullptr" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_nullptr)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_nullptr)</span></tt> to
determine if support for <tt class="docutils literal"><span class="pre">nullptr</span></tt> is enabled.</p>
</div>
<div class="section" id="c-11-override-control">
<h4>C++11 <tt class="docutils literal"><span class="pre">override</span> <span class="pre">control</span></tt><a class="headerlink" href="#c-11-override-control" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_override_control)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_override_control)</span></tt> to determine if support for the
override control keywords is enabled.</p>
</div>
<div class="section" id="c-11-reference-qualified-functions">
<h4>C++11 reference-qualified functions<a class="headerlink" href="#c-11-reference-qualified-functions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_reference_qualified_functions)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_reference_qualified_functions)</span></tt> to determine if support
for reference-qualified functions (e.g., member functions with <tt class="docutils literal"><span class="pre">&</span></tt> or <tt class="docutils literal"><span class="pre">&&</span></tt>
applied to <tt class="docutils literal"><span class="pre">*this</span></tt>) is enabled.</p>
</div>
<div class="section" id="c-11-range-based-for-loop">
<h4>C++11 range-based <tt class="docutils literal"><span class="pre">for</span></tt> loop<a class="headerlink" href="#c-11-range-based-for-loop" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_range_for)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_range_for)</span></tt> to
determine if support for the range-based for loop is enabled.</p>
</div>
<div class="section" id="c-11-raw-string-literals">
<h4>C++11 raw string literals<a class="headerlink" href="#c-11-raw-string-literals" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_raw_string_literals)</span></tt> to determine if support for raw
string literals (e.g., <tt class="docutils literal"><span class="pre">R"x(foo\bar)x"</span></tt>) is enabled.</p>
</div>
<div class="section" id="c-11-rvalue-references">
<h4>C++11 rvalue references<a class="headerlink" href="#c-11-rvalue-references" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_rvalue_references)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_rvalue_references)</span></tt> to determine if support for rvalue
references is enabled.</p>
</div>
<div class="section" id="c-11-static-assert">
<h4>C++11 <tt class="docutils literal"><span class="pre">static_assert()</span></tt><a class="headerlink" href="#c-11-static-assert" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_static_assert)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_static_assert)</span></tt> to determine if support for compile-time
assertions using <tt class="docutils literal"><span class="pre">static_assert</span></tt> is enabled.</p>
</div>
<div class="section" id="c-11-thread-local">
<h4>C++11 <tt class="docutils literal"><span class="pre">thread_local</span></tt><a class="headerlink" href="#c-11-thread-local" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_thread_local)</span></tt> to determine if support for
<tt class="docutils literal"><span class="pre">thread_local</span></tt> variables is enabled.</p>
</div>
<div class="section" id="c-11-type-inference">
<h4>C++11 type inference<a class="headerlink" href="#c-11-type-inference" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_auto_type)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(cxx_auto_type)</span></tt> to
determine C++11 type inference is supported using the <tt class="docutils literal"><span class="pre">auto</span></tt> specifier. If
this is disabled, <tt class="docutils literal"><span class="pre">auto</span></tt> will instead be a storage class specifier, as in C
or C++98.</p>
</div>
<div class="section" id="c-11-strongly-typed-enumerations">
<h4>C++11 strongly typed enumerations<a class="headerlink" href="#c-11-strongly-typed-enumerations" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_strong_enums)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_strong_enums)</span></tt> to determine if support for strongly
typed, scoped enumerations is enabled.</p>
</div>
<div class="section" id="c-11-trailing-return-type">
<h4>C++11 trailing return type<a class="headerlink" href="#c-11-trailing-return-type" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_trailing_return)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_trailing_return)</span></tt> to determine if support for the
alternate function declaration syntax with trailing return type is enabled.</p>
</div>
<div class="section" id="c-11-unicode-string-literals">
<h4>C++11 Unicode string literals<a class="headerlink" href="#c-11-unicode-string-literals" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_unicode_literals)</span></tt> to determine if support for Unicode
string literals is enabled.</p>
</div>
<div class="section" id="c-11-unrestricted-unions">
<h4>C++11 unrestricted unions<a class="headerlink" href="#c-11-unrestricted-unions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_unrestricted_unions)</span></tt> to determine if support for
unrestricted unions is enabled.</p>
</div>
<div class="section" id="c-11-user-defined-literals">
<h4>C++11 user-defined literals<a class="headerlink" href="#c-11-user-defined-literals" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_user_literals)</span></tt> to determine if support for
user-defined literals is enabled.</p>
</div>
<div class="section" id="c-11-variadic-templates">
<h4>C++11 variadic templates<a class="headerlink" href="#c-11-variadic-templates" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_variadic_templates)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_variadic_templates)</span></tt> to determine if support for
variadic templates is enabled.</p>
</div>
</div>
<div class="section" id="c-1y">
<h3>C++1y<a class="headerlink" href="#c-1y" title="Permalink to this headline">¶</a></h3>
<p>The features listed below are part of the committee draft for the C++1y
standard. As a result, all these features are enabled with the <tt class="docutils literal"><span class="pre">-std=c++1y</span></tt>
or <tt class="docutils literal"><span class="pre">-std=gnu++1y</span></tt> option when compiling C++ code.</p>
<div class="section" id="c-1y-binary-literals">
<h4>C++1y binary literals<a class="headerlink" href="#c-1y-binary-literals" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_binary_literals)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_binary_literals)</span></tt> to determine whether
binary literals (for instance, <tt class="docutils literal"><span class="pre">0b10010</span></tt>) are recognized. Clang supports this
feature as an extension in all language modes.</p>
</div>
<div class="section" id="c-1y-contextual-conversions">
<h4>C++1y contextual conversions<a class="headerlink" href="#c-1y-contextual-conversions" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_contextual_conversions)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_contextual_conversions)</span></tt> to determine if the C++1y rules
are used when performing an implicit conversion for an array bound in a
<em>new-expression</em>, the operand of a <em>delete-expression</em>, an integral constant
expression, or a condition in a <tt class="docutils literal"><span class="pre">switch</span></tt> statement. Clang does not yet
support this feature.</p>
</div>
<div class="section" id="c-1y-decltype-auto">
<h4>C++1y decltype(auto)<a class="headerlink" href="#c-1y-decltype-auto" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_decltype_auto)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_decltype_auto)</span></tt> to determine if support
for the <tt class="docutils literal"><span class="pre">decltype(auto)</span></tt> placeholder type is enabled.</p>
</div>
<div class="section" id="c-1y-default-initializers-for-aggregates">
<h4>C++1y default initializers for aggregates<a class="headerlink" href="#c-1y-default-initializers-for-aggregates" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_aggregate_nsdmi)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_aggregate_nsdmi)</span></tt> to determine if support
for default initializers in aggregate members is enabled.</p>
</div>
<div class="section" id="c-1y-generalized-lambda-capture">
<h4>C++1y generalized lambda capture<a class="headerlink" href="#c-1y-generalized-lambda-capture" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_generalized_capture)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_generalized_capture</span></tt> to determine if support for
generalized lambda captures is enabled
(for instance, <tt class="docutils literal"><span class="pre">[n(0)]</span> <span class="pre">{</span> <span class="pre">return</span> <span class="pre">++n;</span> <span class="pre">}</span></tt>).
Clang does not yet support this feature.</p>
</div>
<div class="section" id="c-1y-generic-lambdas">
<h4>C++1y generic lambdas<a class="headerlink" href="#c-1y-generic-lambdas" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_generic_lambda)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_generic_lambda)</span></tt> to determine if support for generic
(polymorphic) lambdas is enabled
(for instance, <tt class="docutils literal"><span class="pre">[]</span> <span class="pre">(auto</span> <span class="pre">x)</span> <span class="pre">{</span> <span class="pre">return</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">1;</span> <span class="pre">}</span></tt>).
Clang does not yet support this feature.</p>
</div>
<div class="section" id="c-1y-relaxed-constexpr">
<h4>C++1y relaxed constexpr<a class="headerlink" href="#c-1y-relaxed-constexpr" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_relaxed_constexpr)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_relaxed_constexpr)</span></tt> to determine if variable
declarations, local variable modification, and control flow constructs
are permitted in <tt class="docutils literal"><span class="pre">constexpr</span></tt> functions.
Clang’s implementation of this feature is incomplete.</p>
</div>
<div class="section" id="c-1y-return-type-deduction">
<h4>C++1y return type deduction<a class="headerlink" href="#c-1y-return-type-deduction" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_return_type_deduction)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_return_type_deduction)</span></tt> to determine if support
for return type deduction for functions (using <tt class="docutils literal"><span class="pre">auto</span></tt> as a return type)
is enabled.
Clang’s implementation of this feature is incomplete.</p>
</div>
<div class="section" id="c-1y-runtime-sized-arrays">
<h4>C++1y runtime-sized arrays<a class="headerlink" href="#c-1y-runtime-sized-arrays" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_runtime_array)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_runtime_array)</span></tt> to determine if support
for arrays of runtime bound (a restricted form of variable-length arrays)
is enabled.
Clang’s implementation of this feature is incomplete.</p>
</div>
<div class="section" id="c-1y-variable-templates">
<h4>C++1y variable templates<a class="headerlink" href="#c-1y-variable-templates" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(cxx_variable_templates)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(cxx_variable_templates)</span></tt> to determine if support for
templated variable declarations is enabled.
Clang does not yet support this feature.</p>
</div>
</div>
<div class="section" id="c11">
<h3>C11<a class="headerlink" href="#c11" title="Permalink to this headline">¶</a></h3>
<p>The features listed below are part of the C11 standard. As a result, all these
features are enabled with the <tt class="docutils literal"><span class="pre">-std=c11</span></tt> or <tt class="docutils literal"><span class="pre">-std=gnu11</span></tt> option when
compiling C code. Additionally, because these features are all
backward-compatible, they are available as extensions in all language modes.</p>
<div class="section" id="c11-alignment-specifiers">
<h4>C11 alignment specifiers<a class="headerlink" href="#c11-alignment-specifiers" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(c_alignas)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(c_alignas)</span></tt> to determine
if support for alignment specifiers using <tt class="docutils literal"><span class="pre">_Alignas</span></tt> is enabled.</p>
</div>
<div class="section" id="c11-atomic-operations">
<h4>C11 atomic operations<a class="headerlink" href="#c11-atomic-operations" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(c_atomic)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(c_atomic)</span></tt> to determine
if support for atomic types using <tt class="docutils literal"><span class="pre">_Atomic</span></tt> is enabled. Clang also provides
<a class="reference internal" href="#langext-c11-atomic"><em>a set of builtins</em></a> which can be used to implement
the <tt class="docutils literal"><span class="pre"><stdatomic.h></span></tt> operations on <tt class="docutils literal"><span class="pre">_Atomic</span></tt> types.</p>
</div>
<div class="section" id="c11-generic-selections">
<h4>C11 generic selections<a class="headerlink" href="#c11-generic-selections" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(c_generic_selections)</span></tt> or
<tt class="docutils literal"><span class="pre">__has_extension(c_generic_selections)</span></tt> to determine if support for generic
selections is enabled.</p>
<p>As an extension, the C11 generic selection expression is available in all
languages supported by Clang. The syntax is the same as that given in the C11
standard.</p>
<p>In C, type compatibility is decided according to the rules given in the
appropriate standard, but in C++, which lacks the type compatibility rules used
in C, types are considered compatible only if they are equivalent.</p>
</div>
<div class="section" id="c11-static-assert">
<h4>C11 <tt class="docutils literal"><span class="pre">_Static_assert()</span></tt><a class="headerlink" href="#c11-static-assert" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(c_static_assert)</span></tt> or <tt class="docutils literal"><span class="pre">__has_extension(c_static_assert)</span></tt>
to determine if support for compile-time assertions using <tt class="docutils literal"><span class="pre">_Static_assert</span></tt> is
enabled.</p>
</div>
<div class="section" id="c11-thread-local">
<h4>C11 <tt class="docutils literal"><span class="pre">_Thread_local</span></tt><a class="headerlink" href="#c11-thread-local" title="Permalink to this headline">¶</a></h4>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(c_thread_local)</span></tt> to determine if support for
<tt class="docutils literal"><span class="pre">_Thread_local</span></tt> variables is enabled.</p>
</div>
</div>
</div>
<div class="section" id="checks-for-type-traits">
<h2><a class="toc-backref" href="#id11">Checks for Type Traits</a><a class="headerlink" href="#checks-for-type-traits" title="Permalink to this headline">¶</a></h2>
<p>Clang supports the <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html">GNU C++ type traits</a> and a subset of the
<a class="reference external" href="http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx">Microsoft Visual C++ Type traits</a>. For each
supported type trait <tt class="docutils literal"><span class="pre">__X</span></tt>, <tt class="docutils literal"><span class="pre">__has_extension(X)</span></tt> indicates the presence of
the type trait. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#if __has_extension(is_convertible_to)</span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span> <span class="n">From</span><span class="p">,</span> <span class="k">typename</span> <span class="n">To</span><span class="o">></span>
<span class="k">struct</span> <span class="n">is_convertible_to</span> <span class="p">{</span>
<span class="k">static</span> <span class="k">const</span> <span class="kt">bool</span> <span class="n">value</span> <span class="o">=</span> <span class="n">__is_convertible_to</span><span class="p">(</span><span class="n">From</span><span class="p">,</span> <span class="n">To</span><span class="p">);</span>
<span class="p">};</span>
<span class="cp">#else</span>
<span class="c1">// Emulate type trait</span>
<span class="cp">#endif</span>
</pre></div>
</div>
<p>The following type traits are supported by Clang:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">__has_nothrow_assign</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_nothrow_copy</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_nothrow_constructor</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_trivial_assign</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_trivial_copy</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_trivial_constructor</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_trivial_destructor</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__has_virtual_destructor</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_abstract</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_base_of</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_class</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_convertible_to</span></tt> (Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_empty</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_enum</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_interface_class</span></tt> (Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_pod</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_polymorphic</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_union</span></tt> (GNU, Microsoft)</li>
<li><tt class="docutils literal"><span class="pre">__is_literal(type)</span></tt>: Determines whether the given type is a literal type</li>
<li><tt class="docutils literal"><span class="pre">__is_final</span></tt>: Determines whether the given type is declared with a
<tt class="docutils literal"><span class="pre">final</span></tt> class-virt-specifier.</li>
<li><tt class="docutils literal"><span class="pre">__underlying_type(type)</span></tt>: Retrieves the underlying type for a given
<tt class="docutils literal"><span class="pre">enum</span></tt> type. This trait is required to implement the C++11 standard
library.</li>
<li><tt class="docutils literal"><span class="pre">__is_trivially_assignable(totype,</span> <span class="pre">fromtype)</span></tt>: Determines whether a value
of type <tt class="docutils literal"><span class="pre">totype</span></tt> can be assigned to from a value of type <tt class="docutils literal"><span class="pre">fromtype</span></tt> such
that no non-trivial functions are called as part of that assignment. This
trait is required to implement the C++11 standard library.</li>
<li><tt class="docutils literal"><span class="pre">__is_trivially_constructible(type,</span> <span class="pre">argtypes...)</span></tt>: Determines whether a
value of type <tt class="docutils literal"><span class="pre">type</span></tt> can be direct-initialized with arguments of types
<tt class="docutils literal"><span class="pre">argtypes...</span></tt> such that no non-trivial functions are called as part of
that initialization. This trait is required to implement the C++11 standard
library.</li>
</ul>
</div>
<div class="section" id="blocks">
<h2><a class="toc-backref" href="#id12">Blocks</a><a class="headerlink" href="#blocks" title="Permalink to this headline">¶</a></h2>
<p>The syntax and high level language feature description is in
<a class="reference internal" href="BlockLanguageSpec.html"><em>BlockLanguageSpec</em></a>. Implementation and ABI details for
the clang implementation are in <a class="reference internal" href="Block-ABI-Apple.html"><em>Block-ABI-Apple</em></a>.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_extension(blocks)</span></tt>.</p>
</div>
<div class="section" id="objective-c-features">
<h2><a class="toc-backref" href="#id13">Objective-C Features</a><a class="headerlink" href="#objective-c-features" title="Permalink to this headline">¶</a></h2>
<div class="section" id="related-result-types">
<h3>Related result types<a class="headerlink" href="#related-result-types" title="Permalink to this headline">¶</a></h3>
<p>According to Cocoa conventions, Objective-C methods with certain names
(“<tt class="docutils literal"><span class="pre">init</span></tt>”, “<tt class="docutils literal"><span class="pre">alloc</span></tt>”, etc.) always return objects that are an instance of
the receiving class’s type. Such methods are said to have a “related result
type”, meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class. For example, given the
following classes:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">NSObject</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">alloc</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">id</span><span class="p">)</span><span class="nf">init</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">NSArray</span> : <span class="nc">NSObject</span>
<span class="k">@end</span>
</pre></div>
</div>
<p>and this common initialization pattern</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="n">NSArray</span> <span class="o">*</span><span class="n">array</span> <span class="o">=</span> <span class="p">[[</span><span class="n">NSArray</span> <span class="n">alloc</span><span class="p">]</span> <span class="n">init</span><span class="p">];</span>
</pre></div>
</div>
<p>the type of the expression <tt class="docutils literal"><span class="pre">[NSArray</span> <span class="pre">alloc]</span></tt> is <tt class="docutils literal"><span class="pre">NSArray*</span></tt> because
<tt class="docutils literal"><span class="pre">alloc</span></tt> implicitly has a related result type. Similarly, the type of the
expression <tt class="docutils literal"><span class="pre">[[NSArray</span> <span class="pre">alloc]</span> <span class="pre">init]</span></tt> is <tt class="docutils literal"><span class="pre">NSArray*</span></tt>, since <tt class="docutils literal"><span class="pre">init</span></tt> has a
related result type and its receiver is known to have the type <tt class="docutils literal"><span class="pre">NSArray</span> <span class="pre">*</span></tt>.
If neither <tt class="docutils literal"><span class="pre">alloc</span></tt> nor <tt class="docutils literal"><span class="pre">init</span></tt> had a related result type, the expressions
would have had type <tt class="docutils literal"><span class="pre">id</span></tt>, as declared in the method signature.</p>
<p>A method with a related result type can be declared by using the type
<tt class="docutils literal"><span class="pre">instancetype</span></tt> as its result type. <tt class="docutils literal"><span class="pre">instancetype</span></tt> is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">A</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span><span class="nf">constructAnA</span><span class="p">;</span>
<span class="k">@end</span>
</pre></div>
</div>
<p>The related result type can also be inferred for some methods. To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., “<tt class="docutils literal"><span class="pre">init</span></tt>” in “<tt class="docutils literal"><span class="pre">initWithObjects</span></tt>”) is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:</p>
<ul class="simple">
<li>the first word is “<tt class="docutils literal"><span class="pre">alloc</span></tt>” or “<tt class="docutils literal"><span class="pre">new</span></tt>”, and the method is a class method,
or</li>
<li>the first word is “<tt class="docutils literal"><span class="pre">autorelease</span></tt>”, “<tt class="docutils literal"><span class="pre">init</span></tt>”, “<tt class="docutils literal"><span class="pre">retain</span></tt>”, or “<tt class="docutils literal"><span class="pre">self</span></tt>”,
and the method is an instance method.</li>
</ul>
<p>If a method with a related result type is overridden by a subclass method, the
subclass method must also return a type that is compatible with the subclass
type. For example:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">NSString</span> : <span class="nc">NSObject</span>
<span class="k">-</span> <span class="p">(</span><span class="n">NSUnrelated</span> <span class="o">*</span><span class="p">)</span><span class="nf">init</span><span class="p">;</span> <span class="c1">// incorrect usage: NSUnrelated is not NSString or a superclass of NSString</span>
<span class="k">@end</span>
</pre></div>
</div>
<p>Related result types only affect the type of a message send or property access
via the given method. In all other respects, a method with a related result
type is treated the same way as method that returns <tt class="docutils literal"><span class="pre">id</span></tt>.</p>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(objc_instancetype)</span></tt> to determine whether the
<tt class="docutils literal"><span class="pre">instancetype</span></tt> contextual keyword is available.</p>
</div>
<div class="section" id="automatic-reference-counting">
<h3>Automatic reference counting<a class="headerlink" href="#automatic-reference-counting" title="Permalink to this headline">¶</a></h3>
<p>Clang provides support for <a class="reference internal" href="AutomaticReferenceCounting.html"><em>automated reference counting</em></a> in Objective-C, which eliminates the need
for manual <tt class="docutils literal"><span class="pre">retain</span></tt>/<tt class="docutils literal"><span class="pre">release</span></tt>/<tt class="docutils literal"><span class="pre">autorelease</span></tt> message sends. There are two
feature macros associated with automatic reference counting:
<tt class="docutils literal"><span class="pre">__has_feature(objc_arc)</span></tt> indicates the availability of automated reference
counting in general, while <tt class="docutils literal"><span class="pre">__has_feature(objc_arc_weak)</span></tt> indicates that
automated reference counting also includes support for <tt class="docutils literal"><span class="pre">__weak</span></tt> pointers to
Objective-C objects.</p>
</div>
<div class="section" id="enumerations-with-a-fixed-underlying-type">
<span id="objc-fixed-enum"></span><h3>Enumerations with a fixed underlying type<a class="headerlink" href="#enumerations-with-a-fixed-underlying-type" title="Permalink to this headline">¶</a></h3>
<p>Clang provides support for C++11 enumerations with a fixed underlying type
within Objective-C. For example, one can write an enumeration type as:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">typedef</span> <span class="k">enum</span> <span class="o">:</span> <span class="kt">unsigned</span> <span class="kt">char</span> <span class="p">{</span> <span class="n">Red</span><span class="p">,</span> <span class="n">Green</span><span class="p">,</span> <span class="n">Blue</span> <span class="p">}</span> <span class="n">Color</span><span class="p">;</span>
</pre></div>
</div>
<p>This specifies that the underlying type, which is used to store the enumeration
value, is <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span></tt>.</p>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(objc_fixed_enum)</span></tt> to determine whether support for fixed
underlying types is available in Objective-C.</p>
</div>
<div class="section" id="interoperability-with-c-11-lambdas">
<h3>Interoperability with C++11 lambdas<a class="headerlink" href="#interoperability-with-c-11-lambdas" title="Permalink to this headline">¶</a></h3>
<p>Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
permitting a lambda to be implicitly converted to a block pointer with the
corresponding signature. For example, consider an API such as <tt class="docutils literal"><span class="pre">NSArray</span></tt>‘s
array-sorting method:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">-</span> <span class="p">(</span><span class="n">NSArray</span> <span class="o">*</span><span class="p">)</span><span class="nf">sortedArrayUsingComparator:</span><span class="p">(</span><span class="n">NSComparator</span><span class="p">)</span><span class="nv">cmptr</span><span class="p">;</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">NSComparator</span></tt> is simply a typedef for the block pointer <tt class="docutils literal"><span class="pre">NSComparisonResult</span>
<span class="pre">(^)(id,</span> <span class="pre">id)</span></tt>, and parameters of this type are generally provided with block
literals as arguments. However, one can also use a C++11 lambda so long as it
provides the same signature (in this case, accepting two parameters of type
<tt class="docutils literal"><span class="pre">id</span></tt> and returning an <tt class="docutils literal"><span class="pre">NSComparisonResult</span></tt>):</p>
<div class="highlight-objc"><pre>NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
@"String 02"];
const NSStringCompareOptions comparisonOptions
= NSCaseInsensitiveSearch | NSNumericSearch |
NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSArray *sorted
= [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
NSRange string1Range = NSMakeRange(0, [s1 length]);
return [s1 compare:s2 options:comparisonOptions
range:string1Range locale:currentLocale];
}];
NSLog(@"sorted: %@", sorted);</pre>
</div>
<p>This code relies on an implicit conversion from the type of the lambda
expression (an unnamed, local class type called the <em>closure type</em>) to the
corresponding block pointer type. The conversion itself is expressed by a
conversion operator in that closure type that produces a block pointer with the
same signature as the lambda itself, e.g.,</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="n">operator</span> <span class="n">NSComparisonResult</span> <span class="p">(</span><span class="o">^</span><span class="p">)(</span><span class="kt">id</span><span class="p">,</span> <span class="kt">id</span><span class="p">)()</span> <span class="k">const</span><span class="p">;</span>
</pre></div>
</div>
<p>This conversion function returns a new block that simply forwards the two
parameters to the lambda object (which it captures by copy), then returns the
result. The returned block is first copied (with <tt class="docutils literal"><span class="pre">Block_copy</span></tt>) and then
autoreleased. As an optimization, if a lambda expression is immediately
converted to a block pointer (as in the first example, above), then the block
is not copied and autoreleased: rather, it is given the same lifetime as a
block literal written at that point in the program, which avoids the overhead
of copying a block to the heap in the common case.</p>
<p>The conversion from a lambda to a block pointer is only available in
Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory
management (autorelease).</p>
</div>
<div class="section" id="object-literals-and-subscripting">
<h3>Object Literals and Subscripting<a class="headerlink" href="#object-literals-and-subscripting" title="Permalink to this headline">¶</a></h3>
<p>Clang provides support for <a class="reference internal" href="ObjectiveCLiterals.html"><em>Object Literals and Subscripting</em></a> in Objective-C, which simplifies common Objective-C
programming patterns, makes programs more concise, and improves the safety of
container creation. There are several feature macros associated with object
literals and subscripting: <tt class="docutils literal"><span class="pre">__has_feature(objc_array_literals)</span></tt> tests the
availability of array literals; <tt class="docutils literal"><span class="pre">__has_feature(objc_dictionary_literals)</span></tt>
tests the availability of dictionary literals;
<tt class="docutils literal"><span class="pre">__has_feature(objc_subscripting)</span></tt> tests the availability of object
subscripting.</p>
</div>
<div class="section" id="objective-c-autosynthesis-of-properties">
<h3>Objective-C Autosynthesis of Properties<a class="headerlink" href="#objective-c-autosynthesis-of-properties" title="Permalink to this headline">¶</a></h3>
<p>Clang provides support for autosynthesis of declared properties. Using this
feature, clang provides default synthesis of those properties not declared
@dynamic and not having user provided backing getter and setter methods.
<tt class="docutils literal"><span class="pre">__has_feature(objc_default_synthesize_properties)</span></tt> checks for availability
of this feature in version of clang being used.</p>
</div>
<div class="section" id="the-objc-method-family-attribute">
<span id="langext-objc-method-family"></span><h3>The <tt class="docutils literal"><span class="pre">objc_method_family</span></tt> attribute<a class="headerlink" href="#the-objc-method-family-attribute" title="Permalink to this headline">¶</a></h3>
<p>Many methods in Objective-C have conventional meanings determined by their
selectors. It is sometimes useful to be able to mark a method as having a
particular conventional meaning despite not having the right selector, or as
not having the conventional meaning that its selector would suggest. For these
use cases, we provide an attribute to specifically describe the “method family”
that a method belongs to.</p>
<p><strong>Usage</strong>: <tt class="docutils literal"><span class="pre">__attribute__((objc_method_family(X)))</span></tt>, where <tt class="docutils literal"><span class="pre">X</span></tt> is one of
<tt class="docutils literal"><span class="pre">none</span></tt>, <tt class="docutils literal"><span class="pre">alloc</span></tt>, <tt class="docutils literal"><span class="pre">copy</span></tt>, <tt class="docutils literal"><span class="pre">init</span></tt>, <tt class="docutils literal"><span class="pre">mutableCopy</span></tt>, or <tt class="docutils literal"><span class="pre">new</span></tt>. This
attribute can only be placed at the end of a method declaration:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">-</span> <span class="p">(</span><span class="n">NSString</span> <span class="o">*</span><span class="p">)</span><span class="nf">initMyStringValue</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">objc_method_family</span><span class="p">(</span><span class="n">none</span><span class="p">)));</span>
</pre></div>
</div>
<p>Users who do not wish to change the conventional meaning of a method, and who
merely want to document its non-standard retain and release semantics, should
use the <a class="reference internal" href="#langext-objc-retain-release"><em>retaining behavior attributes</em></a>
described below.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_attribute(objc_method_family)</span></tt>.</p>
</div>
<div class="section" id="objective-c-retaining-behavior-attributes">
<span id="langext-objc-retain-release"></span><h3>Objective-C retaining behavior attributes<a class="headerlink" href="#objective-c-retaining-behavior-attributes" title="Permalink to this headline">¶</a></h3>
<p>In Objective-C, functions and methods are generally assumed to follow the
<a class="reference external" href="http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html">Cocoa Memory Management</a>
conventions for ownership of object arguments and
return values. However, there are exceptions, and so Clang provides attributes
to allow these exceptions to be documented. This are used by ARC and the
<a class="reference external" href="http://clang-analyzer.llvm.org">static analyzer</a> Some exceptions may be
better described using the <a class="reference internal" href="#langext-objc-method-family"><em>objc_method_family</em></a> attribute instead.</p>
<p><strong>Usage</strong>: The <tt class="docutils literal"><span class="pre">ns_returns_retained</span></tt>, <tt class="docutils literal"><span class="pre">ns_returns_not_retained</span></tt>,
<tt class="docutils literal"><span class="pre">ns_returns_autoreleased</span></tt>, <tt class="docutils literal"><span class="pre">cf_returns_retained</span></tt>, and
<tt class="docutils literal"><span class="pre">cf_returns_not_retained</span></tt> attributes can be placed on methods and functions
that return Objective-C or CoreFoundation objects. They are commonly placed at
the end of a function prototype or method declaration:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="kt">id</span> <span class="nf">foo</span><span class="p">()</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">ns_returns_retained</span><span class="p">));</span>
<span class="k">-</span> <span class="p">(</span><span class="n">NSString</span> <span class="o">*</span><span class="p">)</span><span class="nf">bar:</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="nv">x</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">ns_returns_retained</span><span class="p">));</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">*_returns_retained</span></tt> attributes specify that the returned object has a +1
retain count. The <tt class="docutils literal"><span class="pre">*_returns_not_retained</span></tt> attributes specify that the return
object has a +0 retain count, even if the normal convention for its selector
would be +1. <tt class="docutils literal"><span class="pre">ns_returns_autoreleased</span></tt> specifies that the returned object is
+0, but is guaranteed to live at least as long as the next flush of an
autorelease pool.</p>
<p><strong>Usage</strong>: The <tt class="docutils literal"><span class="pre">ns_consumed</span></tt> and <tt class="docutils literal"><span class="pre">cf_consumed</span></tt> attributes can be placed on
an parameter declaration; they specify that the argument is expected to have a
+1 retain count, which will be balanced in some way by the function or method.
The <tt class="docutils literal"><span class="pre">ns_consumes_self</span></tt> attribute can only be placed on an Objective-C
method; it specifies that the method expects its <tt class="docutils literal"><span class="pre">self</span></tt> parameter to have a
+1 retain count, which it will balance in some way.</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">foo</span><span class="p">(</span><span class="n">__attribute__</span><span class="p">((</span><span class="n">ns_consumed</span><span class="p">))</span> <span class="n">NSString</span> <span class="o">*</span><span class="n">string</span><span class="p">);</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">bar</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">ns_consumes_self</span><span class="p">));</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">baz:</span><span class="p">(</span><span class="kt">id</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">ns_consumed</span><span class="p">))</span> <span class="n">x</span><span class="p">;</span>
</pre></div>
</div>
<p>Further examples of these attributes are available in the static analyzer’s <a class="reference external" href="http://clang-analyzer.llvm.org/annotations.html#cocoa_mem">list of annotations for analysis</a>.</p>
<p>Query for these features with <tt class="docutils literal"><span class="pre">__has_attribute(ns_consumed)</span></tt>,
<tt class="docutils literal"><span class="pre">__has_attribute(ns_returns_retained)</span></tt>, etc.</p>
</div>
</div>
<div class="section" id="function-overloading-in-c">
<h2><a class="toc-backref" href="#id14">Function Overloading in C</a><a class="headerlink" href="#function-overloading-in-c" title="Permalink to this headline">¶</a></h2>
<p>Clang provides support for C++ function overloading in C. Function overloading
in C is introduced using the <tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute. For example, one
might provide several overloaded versions of a <tt class="docutils literal"><span class="pre">tgsin</span></tt> function that invokes
the appropriate standard function computing the sine of a value with <tt class="docutils literal"><span class="pre">float</span></tt>,
<tt class="docutils literal"><span class="pre">double</span></tt>, or <tt class="docutils literal"><span class="pre">long</span> <span class="pre">double</span></tt> precision:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="cp">#include <math.h></span>
<span class="kt">float</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">))</span> <span class="n">tgsin</span><span class="p">(</span><span class="kt">float</span> <span class="n">x</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">sinf</span><span class="p">(</span><span class="n">x</span><span class="p">);</span> <span class="p">}</span>
<span class="kt">double</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">))</span> <span class="n">tgsin</span><span class="p">(</span><span class="kt">double</span> <span class="n">x</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">sin</span><span class="p">(</span><span class="n">x</span><span class="p">);</span> <span class="p">}</span>
<span class="kt">long</span> <span class="kt">double</span> <span class="nf">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">))</span> <span class="n">tgsin</span><span class="p">(</span><span class="kt">long</span> <span class="kt">double</span> <span class="n">x</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">sinl</span><span class="p">(</span><span class="n">x</span><span class="p">);</span> <span class="p">}</span>
</pre></div>
</div>
<p>Given these declarations, one can call <tt class="docutils literal"><span class="pre">tgsin</span></tt> with a <tt class="docutils literal"><span class="pre">float</span></tt> value to
receive a <tt class="docutils literal"><span class="pre">float</span></tt> result, with a <tt class="docutils literal"><span class="pre">double</span></tt> to receive a <tt class="docutils literal"><span class="pre">double</span></tt> result,
etc. Function overloading in C follows the rules of C++ function overloading
to pick the best overload given the call arguments, with a few C-specific
semantics:</p>
<ul class="simple">
<li>Conversion from <tt class="docutils literal"><span class="pre">float</span></tt> or <tt class="docutils literal"><span class="pre">double</span></tt> to <tt class="docutils literal"><span class="pre">long</span> <span class="pre">double</span></tt> is ranked as a
floating-point promotion (per C99) rather than as a floating-point conversion
(as in C++).</li>
<li>A conversion from a pointer of type <tt class="docutils literal"><span class="pre">T*</span></tt> to a pointer of type <tt class="docutils literal"><span class="pre">U*</span></tt> is
considered a pointer conversion (with conversion rank) if <tt class="docutils literal"><span class="pre">T</span></tt> and <tt class="docutils literal"><span class="pre">U</span></tt> are
compatible types.</li>
<li>A conversion from type <tt class="docutils literal"><span class="pre">T</span></tt> to a value of type <tt class="docutils literal"><span class="pre">U</span></tt> is permitted if <tt class="docutils literal"><span class="pre">T</span></tt>
and <tt class="docutils literal"><span class="pre">U</span></tt> are compatible types. This conversion is given “conversion” rank.</li>
</ul>
<p>The declaration of <tt class="docutils literal"><span class="pre">overloadable</span></tt> functions is restricted to function
declarations and definitions. Most importantly, if any function with a given
name is given the <tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute, then all function declarations
and definitions with that name (and in that scope) must have the
<tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute. This rule even applies to redeclarations of
functions whose original declaration had the <tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute, e.g.,</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">f</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">));</span>
<span class="kt">float</span> <span class="nf">f</span><span class="p">(</span><span class="kt">float</span><span class="p">);</span> <span class="c1">// error: declaration of "f" must have the "overloadable" attribute</span>
<span class="kt">int</span> <span class="nf">g</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">));</span>
<span class="kt">int</span> <span class="nf">g</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span> <span class="c1">// error: redeclaration of "g" must also have the "overloadable" attribute</span>
</pre></div>
</div>
<p>Functions marked <tt class="docutils literal"><span class="pre">overloadable</span></tt> must have prototypes. Therefore, the
following code is ill-formed:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">h</span><span class="p">()</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">));</span> <span class="c1">// error: h does not have a prototype</span>
</pre></div>
</div>
<p>However, <tt class="docutils literal"><span class="pre">overloadable</span></tt> functions are allowed to use a ellipsis even if there
are no named parameters (as is permitted in C++). This feature is particularly
useful when combined with the <tt class="docutils literal"><span class="pre">unavailable</span></tt> attribute:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">honeypot</span><span class="p">(...)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">overloadable</span><span class="p">,</span> <span class="n">unavailable</span><span class="p">));</span> <span class="c1">// calling me is an error</span>
</pre></div>
</div>
<p>Functions declared with the <tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute have their names mangled
according to the same rules as C++ function names. For example, the three
<tt class="docutils literal"><span class="pre">tgsin</span></tt> functions in our motivating example get the mangled names
<tt class="docutils literal"><span class="pre">_Z5tgsinf</span></tt>, <tt class="docutils literal"><span class="pre">_Z5tgsind</span></tt>, and <tt class="docutils literal"><span class="pre">_Z5tgsine</span></tt>, respectively. There are two
caveats to this use of name mangling:</p>
<ul class="simple">
<li>Future versions of Clang may change the name mangling of functions overloaded
in C, so you should not depend on an specific mangling. To be completely
safe, we strongly urge the use of <tt class="docutils literal"><span class="pre">static</span> <span class="pre">inline</span></tt> with <tt class="docutils literal"><span class="pre">overloadable</span></tt>
functions.</li>
<li>The <tt class="docutils literal"><span class="pre">overloadable</span></tt> attribute has almost no meaning when used in C++,
because names will already be mangled and functions are already overloadable.
However, when an <tt class="docutils literal"><span class="pre">overloadable</span></tt> function occurs within an <tt class="docutils literal"><span class="pre">extern</span> <span class="pre">"C"</span></tt>
linkage specification, it’s name <em>will</em> be mangled in the same way as it
would in C.</li>
</ul>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_extension(attribute_overloadable)</span></tt>.</p>
</div>
<div class="section" id="initializer-lists-for-complex-numbers-in-c">
<h2><a class="toc-backref" href="#id15">Initializer lists for complex numbers in C</a><a class="headerlink" href="#initializer-lists-for-complex-numbers-in-c" title="Permalink to this headline">¶</a></h2>
<p>clang supports an extension which allows the following in C:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#include <math.h></span>
<span class="cp">#include <complex.h></span>
<span class="n">complex</span> <span class="kt">float</span> <span class="n">x</span> <span class="o">=</span> <span class="p">{</span> <span class="mf">1.0f</span><span class="p">,</span> <span class="n">INFINITY</span> <span class="p">};</span> <span class="c1">// Init to (1, Inf)</span>
</pre></div>
</div>
<p>This construct is useful because there is no way to separately initialize the
real and imaginary parts of a complex variable in standard C, given that clang
does not support <tt class="docutils literal"><span class="pre">_Imaginary</span></tt>. (Clang also supports the <tt class="docutils literal"><span class="pre">__real__</span></tt> and
<tt class="docutils literal"><span class="pre">__imag__</span></tt> extensions from gcc, which help in some cases, but are not usable
in static initializers.)</p>
<p>Note that this extension does not allow eliding the braces; the meaning of the
following two lines is different:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">complex</span> <span class="kt">float</span> <span class="n">x</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span> <span class="p">{</span> <span class="mf">1.0f</span><span class="p">,</span> <span class="mf">1.0f</span> <span class="p">}</span> <span class="p">};</span> <span class="c1">// [0] = (1, 1)</span>
<span class="n">complex</span> <span class="kt">float</span> <span class="n">x</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span> <span class="mf">1.0f</span><span class="p">,</span> <span class="mf">1.0f</span> <span class="p">};</span> <span class="c1">// [0] = (1, 0), [1] = (1, 0)</span>
</pre></div>
</div>
<p>This extension also works in C++ mode, as far as that goes, but does not apply
to the C++ <tt class="docutils literal"><span class="pre">std::complex</span></tt>. (In C++11, list initialization allows the same
syntax to be used with <tt class="docutils literal"><span class="pre">std::complex</span></tt> with the same meaning.)</p>
</div>
<div class="section" id="builtin-functions">
<h2><a class="toc-backref" href="#id16">Builtin Functions</a><a class="headerlink" href="#builtin-functions" title="Permalink to this headline">¶</a></h2>
<p>Clang supports a number of builtin library functions with the same syntax as
GCC, including things like <tt class="docutils literal"><span class="pre">__builtin_nan</span></tt>, <tt class="docutils literal"><span class="pre">__builtin_constant_p</span></tt>,
<tt class="docutils literal"><span class="pre">__builtin_choose_expr</span></tt>, <tt class="docutils literal"><span class="pre">__builtin_types_compatible_p</span></tt>,
<tt class="docutils literal"><span class="pre">__sync_fetch_and_add</span></tt>, etc. In addition to the GCC builtins, Clang supports
a number of builtins that GCC does not, which are listed here.</p>
<p>Please note that Clang does not and will not support all of the GCC builtins
for vector operations. Instead of using builtins, you should use the functions
defined in target-specific header files like <tt class="docutils literal"><span class="pre"><xmmintrin.h></span></tt>, which define
portable wrappers for these. Many of the Clang versions of these functions are
implemented directly in terms of <a class="reference internal" href="#langext-vectors"><em>extended vector support</em></a> instead of builtins, in order to reduce the number of
builtins that we need to implement.</p>
<div class="section" id="builtin-readcyclecounter">
<h3><tt class="docutils literal"><span class="pre">__builtin_readcyclecounter</span></tt><a class="headerlink" href="#builtin-readcyclecounter" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">__builtin_readcyclecounter</span></tt> is used to access the cycle counter register (or
a similar low-latency, high-accuracy clock) on those targets that support it.</p>
<p><strong>Syntax</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">__builtin_readcyclecounter</span><span class="p">()</span>
</pre></div>
</div>
<p><strong>Example of Use</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">t0</span> <span class="o">=</span> <span class="n">__builtin_readcyclecounter</span><span class="p">();</span>
<span class="n">do_something</span><span class="p">();</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">t1</span> <span class="o">=</span> <span class="n">__builtin_readcyclecounter</span><span class="p">();</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">cycles_to_do_something</span> <span class="o">=</span> <span class="n">t1</span> <span class="o">-</span> <span class="n">t0</span><span class="p">;</span> <span class="c1">// assuming no overflow</span>
</pre></div>
</div>
<p><strong>Description</strong>:</p>
<p>The <tt class="docutils literal"><span class="pre">__builtin_readcyclecounter()</span></tt> builtin returns the cycle counter value,
which may be either global or process/thread-specific depending on the target.
As the backing counters often overflow quickly (on the order of seconds) this
should only be used for timing small intervals. When not supported by the
target, the return value is always zero. This builtin takes no arguments and
produces an unsigned long long result.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_builtin(__builtin_readcyclecounter)</span></tt>.</p>
</div>
<div class="section" id="builtin-shufflevector">
<span id="langext-builtin-shufflevector"></span><h3><tt class="docutils literal"><span class="pre">__builtin_shufflevector</span></tt><a class="headerlink" href="#builtin-shufflevector" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">__builtin_shufflevector</span></tt> is used to express generic vector
permutation/shuffle/swizzle operations. This builtin is also very important
for the implementation of various target-specific header files like
<tt class="docutils literal"><span class="pre"><xmmintrin.h></span></tt>.</p>
<p><strong>Syntax</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">vec1</span><span class="p">,</span> <span class="n">vec2</span><span class="p">,</span> <span class="n">index1</span><span class="p">,</span> <span class="n">index2</span><span class="p">,</span> <span class="p">...)</span>
</pre></div>
</div>
<p><strong>Examples</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// Identity operation - return 4-element vector V1.</span>
<span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="c1">// "Splat" element 0 of V1 into a 4-element result.</span>
<span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="c1">// Reverse 4-element vector V1.</span>
<span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V1</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="c1">// Concatenate every other element of 4-element vectors V1 and V2.</span>
<span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V2</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span>
<span class="c1">// Concatenate every other element of 8-element vectors V1 and V2.</span>
<span class="n">__builtin_shufflevector</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V2</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">14</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>Description</strong>:</p>
<p>The first two arguments to <tt class="docutils literal"><span class="pre">__builtin_shufflevector</span></tt> are vectors that have
the same element type. The remaining arguments are a list of integers that
specify the elements indices of the first two vectors that should be extracted
and returned in a new vector. These element indices are numbered sequentially
starting with the first vector, continuing into the second vector. Thus, if
<tt class="docutils literal"><span class="pre">vec1</span></tt> is a 4-element vector, index 5 would refer to the second element of
<tt class="docutils literal"><span class="pre">vec2</span></tt>.</p>
<p>The result of <tt class="docutils literal"><span class="pre">__builtin_shufflevector</span></tt> is a vector with the same element
type as <tt class="docutils literal"><span class="pre">vec1</span></tt>/<tt class="docutils literal"><span class="pre">vec2</span></tt> but that has an element count equal to the number of
indices specified.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_builtin(__builtin_shufflevector)</span></tt>.</p>
</div>
<div class="section" id="builtin-unreachable">
<h3><tt class="docutils literal"><span class="pre">__builtin_unreachable</span></tt><a class="headerlink" href="#builtin-unreachable" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">__builtin_unreachable</span></tt> is used to indicate that a specific point in the
program cannot be reached, even if the compiler might otherwise think it can.
This is useful to improve optimization and eliminates certain warnings. For
example, without the <tt class="docutils literal"><span class="pre">__builtin_unreachable</span></tt> in the example below, the
compiler assumes that the inline asm can fall through and prints a “function
declared ‘<tt class="docutils literal"><span class="pre">noreturn</span></tt>‘ should not return” warning.</p>
<p><strong>Syntax</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">__builtin_unreachable</span><span class="p">()</span>
</pre></div>
</div>
<p><strong>Example of use</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">myabort</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">noreturn</span><span class="p">));</span>
<span class="kt">void</span> <span class="nf">myabort</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span>
<span class="k">asm</span><span class="p">(</span><span class="s">"int3"</span><span class="p">);</span>
<span class="n">__builtin_unreachable</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<p><strong>Description</strong>:</p>
<p>The <tt class="docutils literal"><span class="pre">__builtin_unreachable()</span></tt> builtin has completely undefined behavior.
Since it has undefined behavior, it is a statement that it is never reached and
the optimizer can take advantage of this to produce better code. This builtin
takes no arguments and produces a void result.</p>
<p>Query for this feature with <tt class="docutils literal"><span class="pre">__has_builtin(__builtin_unreachable)</span></tt>.</p>
</div>
<div class="section" id="sync-swap">
<h3><tt class="docutils literal"><span class="pre">__sync_swap</span></tt><a class="headerlink" href="#sync-swap" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">__sync_swap</span></tt> is used to atomically swap integers or pointers in memory.</p>
<p><strong>Syntax</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">type</span> <span class="n">__sync_swap</span><span class="p">(</span><span class="n">type</span> <span class="o">*</span><span class="n">ptr</span><span class="p">,</span> <span class="n">type</span> <span class="n">value</span><span class="p">,</span> <span class="p">...)</span>
</pre></div>
</div>
<p><strong>Example of Use</strong>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">int</span> <span class="n">old_value</span> <span class="o">=</span> <span class="n">__sync_swap</span><span class="p">(</span><span class="o">&</span><span class="n">value</span><span class="p">,</span> <span class="n">new_value</span><span class="p">);</span>
</pre></div>
</div>
<p><strong>Description</strong>:</p>
<p>The <tt class="docutils literal"><span class="pre">__sync_swap()</span></tt> builtin extends the existing <tt class="docutils literal"><span class="pre">__sync_*()</span></tt> family of
atomic intrinsics to allow code to atomically swap the current value with the
new value. More importantly, it helps developers write more efficient and
correct code by avoiding expensive loops around
<tt class="docutils literal"><span class="pre">__sync_bool_compare_and_swap()</span></tt> or relying on the platform specific
implementation details of <tt class="docutils literal"><span class="pre">__sync_lock_test_and_set()</span></tt>. The
<tt class="docutils literal"><span class="pre">__sync_swap()</span></tt> builtin is a full barrier.</p>
</div>
<div class="section" id="multiprecision-arithmetic-builtins">
<h3>Multiprecision Arithmetic Builtins<a class="headerlink" href="#multiprecision-arithmetic-builtins" title="Permalink to this headline">¶</a></h3>
<p>Clang provides a set of builtins which expose multiprecision arithmetic in a
manner amenable to C. They all have the following form:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">unsigned</span> <span class="n">x</span> <span class="o">=</span> <span class="p">...,</span> <span class="n">y</span> <span class="o">=</span> <span class="p">...,</span> <span class="n">carryin</span> <span class="o">=</span> <span class="p">...,</span> <span class="n">carryout</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">sum</span> <span class="o">=</span> <span class="n">__builtin_addc</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">carryin</span><span class="p">,</span> <span class="o">&</span><span class="n">carryout</span><span class="p">);</span>
</pre></div>
</div>
<p>Thus one can form a multiprecision addition chain in the following manner:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">unsigned</span> <span class="o">*</span><span class="n">x</span><span class="p">,</span> <span class="o">*</span><span class="n">y</span><span class="p">,</span> <span class="o">*</span><span class="n">z</span><span class="p">,</span> <span class="n">carryin</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">carryout</span><span class="p">;</span>
<span class="n">z</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">__builtin_addc</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">y</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">carryin</span><span class="p">,</span> <span class="o">&</span><span class="n">carryout</span><span class="p">);</span>
<span class="n">carryin</span> <span class="o">=</span> <span class="n">carryout</span><span class="p">;</span>
<span class="n">z</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">__builtin_addc</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="n">y</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="n">carryin</span><span class="p">,</span> <span class="o">&</span><span class="n">carryout</span><span class="p">);</span>
<span class="n">carryin</span> <span class="o">=</span> <span class="n">carryout</span><span class="p">;</span>
<span class="n">z</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">__builtin_addc</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="mi">2</span><span class="p">],</span> <span class="n">y</span><span class="p">[</span><span class="mi">2</span><span class="p">],</span> <span class="n">carryin</span><span class="p">,</span> <span class="o">&</span><span class="n">carryout</span><span class="p">);</span>
<span class="n">carryin</span> <span class="o">=</span> <span class="n">carryout</span><span class="p">;</span>
<span class="n">z</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">__builtin_addc</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="mi">3</span><span class="p">],</span> <span class="n">y</span><span class="p">[</span><span class="mi">3</span><span class="p">],</span> <span class="n">carryin</span><span class="p">,</span> <span class="o">&</span><span class="n">carryout</span><span class="p">);</span>
</pre></div>
</div>
<p>The complete list of builtins are:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">unsigned</span> <span class="kt">short</span> <span class="nf">__builtin_addcs</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="nf">__builtin_addc</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="nf">__builtin_addcl</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="nf">__builtin_addcll</span><span class="p">(</span><span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="kt">short</span> <span class="nf">__builtin_subcs</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">short</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="nf">__builtin_subc</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="nf">__builtin_subcl</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="nf">__builtin_subcll</span><span class="p">(</span><span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">x</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">y</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="n">carryin</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="kt">long</span> <span class="kt">long</span> <span class="o">*</span><span class="n">carryout</span><span class="p">);</span>
</pre></div>
</div>
</div>
<div class="section" id="c11-atomic-builtins">
<span id="langext-c11-atomic"></span><h3>__c11_atomic builtins<a class="headerlink" href="#c11-atomic-builtins" title="Permalink to this headline">¶</a></h3>
<p>Clang provides a set of builtins which are intended to be used to implement
C11’s <tt class="docutils literal"><span class="pre"><stdatomic.h></span></tt> header. These builtins provide the semantics of the
<tt class="docutils literal"><span class="pre">_explicit</span></tt> form of the corresponding C11 operation, and are named with a
<tt class="docutils literal"><span class="pre">__c11_</span></tt> prefix. The supported operations are:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">__c11_atomic_init</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_thread_fence</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_signal_fence</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_is_lock_free</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_store</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_load</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_exchange</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_compare_exchange_strong</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_compare_exchange_weak</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_fetch_add</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_fetch_sub</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_fetch_and</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_fetch_or</span></tt></li>
<li><tt class="docutils literal"><span class="pre">__c11_atomic_fetch_xor</span></tt></li>
</ul>
</div>
</div>
<div class="section" id="non-standard-c-11-attributes">
<h2><a class="toc-backref" href="#id17">Non-standard C++11 Attributes</a><a class="headerlink" href="#non-standard-c-11-attributes" title="Permalink to this headline">¶</a></h2>
<p>Clang’s non-standard C++11 attributes live in the <tt class="docutils literal"><span class="pre">clang</span></tt> attribute
namespace.</p>
<div class="section" id="the-clang-fallthrough-attribute">
<h3>The <tt class="docutils literal"><span class="pre">clang::fallthrough</span></tt> attribute<a class="headerlink" href="#the-clang-fallthrough-attribute" title="Permalink to this headline">¶</a></h3>
<p>The <tt class="docutils literal"><span class="pre">clang::fallthrough</span></tt> attribute is used along with the
<tt class="docutils literal"><span class="pre">-Wimplicit-fallthrough</span></tt> argument to annotate intentional fall-through
between switch labels. It can only be applied to a null statement placed at a
point of execution between any statement and the next switch label. It is
common to mark these places with a specific comment, but this attribute is
meant to replace comments with a more strict annotation, which can be checked
by the compiler. This attribute doesn’t change semantics of the code and can
be used wherever an intended fall-through occurs. It is designed to mimic
control-flow statements like <tt class="docutils literal"><span class="pre">break;</span></tt>, so it can be placed in most places
where <tt class="docutils literal"><span class="pre">break;</span></tt> can, but only if there are no statements on the execution path
between it and the next switch label.</p>
<p>Here is an example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// compile with -Wimplicit-fallthrough</span>
<span class="k">switch</span> <span class="p">(</span><span class="n">n</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="mi">22</span>:
<span class="k">case</span> <span class="mi">33</span>: <span class="c1">// no warning: no statements between case labels</span>
<span class="n">f</span><span class="p">();</span>
<span class="k">case</span> <span class="mi">44</span>: <span class="c1">// warning: unannotated fall-through</span>
<span class="n">g</span><span class="p">();</span>
<span class="p">[[</span><span class="n">clang</span><span class="o">::</span><span class="n">fallthrough</span><span class="p">]];</span>
<span class="k">case</span> <span class="mi">55</span>: <span class="c1">// no warning</span>
<span class="k">if</span> <span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="p">{</span>
<span class="n">h</span><span class="p">();</span>
<span class="k">break</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">else</span> <span class="p">{</span>
<span class="n">i</span><span class="p">();</span>
<span class="p">[[</span><span class="n">clang</span><span class="o">::</span><span class="n">fallthrough</span><span class="p">]];</span>
<span class="p">}</span>
<span class="k">case</span> <span class="mi">66</span>: <span class="c1">// no warning</span>
<span class="n">p</span><span class="p">();</span>
<span class="p">[[</span><span class="n">clang</span><span class="o">::</span><span class="n">fallthrough</span><span class="p">]];</span> <span class="c1">// warning: fallthrough annotation does not</span>
<span class="c1">// directly precede case label</span>
<span class="n">q</span><span class="p">();</span>
<span class="k">case</span> <span class="mi">77</span>: <span class="c1">// warning: unannotated fall-through</span>
<span class="n">r</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="gnu-attributes">
<h3><tt class="docutils literal"><span class="pre">gnu::</span></tt> attributes<a class="headerlink" href="#gnu-attributes" title="Permalink to this headline">¶</a></h3>
<p>Clang also supports GCC’s <tt class="docutils literal"><span class="pre">gnu</span></tt> attribute namespace. All GCC attributes which
are accepted with the <tt class="docutils literal"><span class="pre">__attribute__((foo))</span></tt> syntax are also accepted as
<tt class="docutils literal"><span class="pre">[[gnu::foo]]</span></tt>. This only extends to attributes which are specified by GCC
(see the list of <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html">GCC function attributes</a>, <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html">GCC variable
attributes</a>, and
<a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html">GCC type attributes</a>. As with the GCC
implementation, these attributes must appertain to the <em>declarator-id</em> in a
declaration, which means they must go either at the start of the declaration or
immediately after the name being declared.</p>
<p>For example, this applies the GNU <tt class="docutils literal"><span class="pre">unused</span></tt> attribute to <tt class="docutils literal"><span class="pre">a</span></tt> and <tt class="docutils literal"><span class="pre">f</span></tt>, and
also applies the GNU <tt class="docutils literal"><span class="pre">noreturn</span></tt> attribute to <tt class="docutils literal"><span class="pre">f</span></tt>.</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="p">[[</span><span class="n">gnu</span><span class="o">::</span><span class="n">unused</span><span class="p">]]</span> <span class="kt">int</span> <span class="n">a</span><span class="p">,</span> <span class="n">f</span> <span class="p">[[</span><span class="n">gnu</span><span class="o">::</span><span class="n">noreturn</span><span class="p">]]</span> <span class="p">();</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="target-specific-extensions">
<h2><a class="toc-backref" href="#id18">Target-Specific Extensions</a><a class="headerlink" href="#target-specific-extensions" title="Permalink to this headline">¶</a></h2>
<p>Clang supports some language features conditionally on some targets.</p>
<div class="section" id="x86-x86-64-language-extensions">
<h3>X86/X86-64 Language Extensions<a class="headerlink" href="#x86-x86-64-language-extensions" title="Permalink to this headline">¶</a></h3>
<p>The X86 backend has these language extensions:</p>
<div class="section" id="memory-references-off-the-gs-segment">
<h4>Memory references off the GS segment<a class="headerlink" href="#memory-references-off-the-gs-segment" title="Permalink to this headline">¶</a></h4>
<p>Annotating a pointer with address space #256 causes it to be code generated
relative to the X86 GS segment register, and address space #257 causes it to be
relative to the X86 FS segment. Note that this is a very very low-level
feature that should only be used if you know what you’re doing (for example in
an OS kernel).</p>
<p>Here is an example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#define GS_RELATIVE __attribute__((address_space(256)))</span>
<span class="kt">int</span> <span class="nf">foo</span><span class="p">(</span><span class="kt">int</span> <span class="n">GS_RELATIVE</span> <span class="o">*</span><span class="n">P</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="o">*</span><span class="n">P</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Which compiles to (on X86-32):</p>
<div class="highlight-gas"><div class="highlight"><pre><span class="nl">_foo:</span>
<span class="nf">movl</span> <span class="mi">4</span><span class="p">(</span><span class="nv">%esp</span><span class="p">),</span> <span class="nv">%eax</span>
<span class="nf">movl</span> <span class="nv">%gs</span><span class="p">:(</span><span class="nv">%eax</span><span class="p">),</span> <span class="nv">%eax</span>
<span class="nf">ret</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="extensions-for-static-analysis">
<h2><a class="toc-backref" href="#id19">Extensions for Static Analysis</a><a class="headerlink" href="#extensions-for-static-analysis" title="Permalink to this headline">¶</a></h2>
<p>Clang supports additional attributes that are useful for documenting program
invariants and rules for static analysis tools, such as the <a class="reference external" href="http://clang-analyzer.llvm.org/">Clang Static
Analyzer</a>. These attributes are documented
in the analyzer’s <a class="reference external" href="http://clang-analyzer.llvm.org/annotations.html">list of source-level annotations</a>.</p>
</div>
<div class="section" id="extensions-for-dynamic-analysis">
<h2><a class="toc-backref" href="#id20">Extensions for Dynamic Analysis</a><a class="headerlink" href="#extensions-for-dynamic-analysis" title="Permalink to this headline">¶</a></h2>
<div class="section" id="addresssanitizer">
<span id="langext-address-sanitizer"></span><h3>AddressSanitizer<a class="headerlink" href="#addresssanitizer" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(address_sanitizer)</span></tt> to check if the code is being built
with <a class="reference internal" href="AddressSanitizer.html"><em>AddressSanitizer</em></a>.</p>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((no_sanitize_address))</span></tt>
on a function declaration
to specify that address safety instrumentation (e.g. AddressSanitizer) should
not be applied to that function.</p>
</div>
<div class="section" id="threadsanitizer">
<span id="langext-thread-sanitizer"></span><h3>ThreadSanitizer<a class="headerlink" href="#threadsanitizer" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(thread_sanitizer)</span></tt> to check if the code is being built
with <a class="reference internal" href="ThreadSanitizer.html"><em>ThreadSanitizer</em></a>.</p>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((no_sanitize_thread))</span></tt> on a function declaration
to specify that checks for data races on plain (non-atomic) memory accesses
should not be inserted by ThreadSanitizer.
The function may still be instrumented by the tool
to avoid false positives in other places.</p>
</div>
<div class="section" id="memorysanitizer">
<span id="langext-memory-sanitizer"></span><h3>MemorySanitizer<a class="headerlink" href="#memorysanitizer" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__has_feature(memory_sanitizer)</span></tt> to check if the code is being built
with <a class="reference internal" href="MemorySanitizer.html"><em>MemorySanitizer</em></a>.</p>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((no_sanitize_memory))</span></tt> on a function declaration
to specify that checks for uninitialized memory should not be inserted
(e.g. by MemorySanitizer). The function may still be instrumented by the tool
to avoid false positives in other places.</p>
</div>
</div>
<div class="section" id="thread-safety-annotation-checking">
<h2><a class="toc-backref" href="#id21">Thread-Safety Annotation Checking</a><a class="headerlink" href="#thread-safety-annotation-checking" title="Permalink to this headline">¶</a></h2>
<p>Clang supports additional attributes for checking basic locking policies in
multithreaded programs. Clang currently parses the following list of
attributes, although <strong>the implementation for these annotations is currently in
development.</strong> For more details, see the <a class="reference external" href="http://gcc.gnu.org/wiki/ThreadSafetyAnnotation">GCC implementation</a>.</p>
<div class="section" id="no-thread-safety-analysis">
<h3><tt class="docutils literal"><span class="pre">no_thread_safety_analysis</span></tt><a class="headerlink" href="#no-thread-safety-analysis" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((no_thread_safety_analysis))</span></tt> on a function declaration to
specify that the thread safety analysis should not be run on that function.
This attribute provides an escape hatch (e.g. for situations when it is
difficult to annotate the locking policy).</p>
</div>
<div class="section" id="lockable">
<h3><tt class="docutils literal"><span class="pre">lockable</span></tt><a class="headerlink" href="#lockable" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((lockable))</span></tt> on a class definition to specify that it has
a lockable type (e.g. a Mutex class). This annotation is primarily used to
check consistency.</p>
</div>
<div class="section" id="scoped-lockable">
<h3><tt class="docutils literal"><span class="pre">scoped_lockable</span></tt><a class="headerlink" href="#scoped-lockable" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((scoped_lockable))</span></tt> on a class definition to specify that
it has a “scoped” lockable type. Objects of this type will acquire the lock
upon construction and release it upon going out of scope. This annotation is
primarily used to check consistency.</p>
</div>
<div class="section" id="guarded-var">
<h3><tt class="docutils literal"><span class="pre">guarded_var</span></tt><a class="headerlink" href="#guarded-var" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((guarded_var))</span></tt> on a variable declaration to specify that
the variable must be accessed while holding some lock.</p>
</div>
<div class="section" id="pt-guarded-var">
<h3><tt class="docutils literal"><span class="pre">pt_guarded_var</span></tt><a class="headerlink" href="#pt-guarded-var" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((pt_guarded_var))</span></tt> on a pointer declaration to specify
that the pointer must be dereferenced while holding some lock.</p>
</div>
<div class="section" id="guarded-by-l">
<h3><tt class="docutils literal"><span class="pre">guarded_by(l)</span></tt><a class="headerlink" href="#guarded-by-l" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((guarded_by(l)))</span></tt> on a variable declaration to specify
that the variable must be accessed while holding lock <tt class="docutils literal"><span class="pre">l</span></tt>.</p>
</div>
<div class="section" id="pt-guarded-by-l">
<h3><tt class="docutils literal"><span class="pre">pt_guarded_by(l)</span></tt><a class="headerlink" href="#pt-guarded-by-l" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((pt_guarded_by(l)))</span></tt> on a pointer declaration to specify
that the pointer must be dereferenced while holding lock <tt class="docutils literal"><span class="pre">l</span></tt>.</p>
</div>
<div class="section" id="acquired-before">
<h3><tt class="docutils literal"><span class="pre">acquired_before(...)</span></tt><a class="headerlink" href="#acquired-before" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((acquired_before(...)))</span></tt> on a declaration of a lockable
variable to specify that the lock must be acquired before all attribute
arguments. Arguments must be lockable type, and there must be at least one
argument.</p>
</div>
<div class="section" id="acquired-after">
<h3><tt class="docutils literal"><span class="pre">acquired_after(...)</span></tt><a class="headerlink" href="#acquired-after" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((acquired_after(...)))</span></tt> on a declaration of a lockable
variable to specify that the lock must be acquired after all attribute
arguments. Arguments must be lockable type, and there must be at least one
argument.</p>
</div>
<div class="section" id="exclusive-lock-function">
<h3><tt class="docutils literal"><span class="pre">exclusive_lock_function(...)</span></tt><a class="headerlink" href="#exclusive-lock-function" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((exclusive_lock_function(...)))</span></tt> on a function declaration
to specify that the function acquires all listed locks exclusively. This
attribute takes zero or more arguments: either of lockable type or integers
indexing into function parameters of lockable type. If no arguments are given,
the acquired lock is implicitly <tt class="docutils literal"><span class="pre">this</span></tt> of the enclosing object.</p>
</div>
<div class="section" id="shared-lock-function">
<h3><tt class="docutils literal"><span class="pre">shared_lock_function(...)</span></tt><a class="headerlink" href="#shared-lock-function" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((shared_lock_function(...)))</span></tt> on a function declaration to
specify that the function acquires all listed locks, although the locks may be
shared (e.g. read locks). This attribute takes zero or more arguments: either
of lockable type or integers indexing into function parameters of lockable
type. If no arguments are given, the acquired lock is implicitly <tt class="docutils literal"><span class="pre">this</span></tt> of
the enclosing object.</p>
</div>
<div class="section" id="exclusive-trylock-function">
<h3><tt class="docutils literal"><span class="pre">exclusive_trylock_function(...)</span></tt><a class="headerlink" href="#exclusive-trylock-function" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((exclusive_lock_function(...)))</span></tt> on a function declaration
to specify that the function will try (without blocking) to acquire all listed
locks exclusively. This attribute takes one or more arguments. The first
argument is an integer or boolean value specifying the return value of a
successful lock acquisition. The remaining arugments are either of lockable
type or integers indexing into function parameters of lockable type. If only
one argument is given, the acquired lock is implicitly <tt class="docutils literal"><span class="pre">this</span></tt> of the
enclosing object.</p>
</div>
<div class="section" id="shared-trylock-function">
<h3><tt class="docutils literal"><span class="pre">shared_trylock_function(...)</span></tt><a class="headerlink" href="#shared-trylock-function" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((shared_lock_function(...)))</span></tt> on a function declaration to
specify that the function will try (without blocking) to acquire all listed
locks, although the locks may be shared (e.g. read locks). This attribute
takes one or more arguments. The first argument is an integer or boolean value
specifying the return value of a successful lock acquisition. The remaining
arugments are either of lockable type or integers indexing into function
parameters of lockable type. If only one argument is given, the acquired lock
is implicitly <tt class="docutils literal"><span class="pre">this</span></tt> of the enclosing object.</p>
</div>
<div class="section" id="unlock-function">
<h3><tt class="docutils literal"><span class="pre">unlock_function(...)</span></tt><a class="headerlink" href="#unlock-function" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((unlock_function(...)))</span></tt> on a function declaration to
specify that the function release all listed locks. This attribute takes zero
or more arguments: either of lockable type or integers indexing into function
parameters of lockable type. If no arguments are given, the acquired lock is
implicitly <tt class="docutils literal"><span class="pre">this</span></tt> of the enclosing object.</p>
</div>
<div class="section" id="lock-returned-l">
<h3><tt class="docutils literal"><span class="pre">lock_returned(l)</span></tt><a class="headerlink" href="#lock-returned-l" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((lock_returned(l)))</span></tt> on a function declaration to specify
that the function returns lock <tt class="docutils literal"><span class="pre">l</span></tt> (<tt class="docutils literal"><span class="pre">l</span></tt> must be of lockable type). This
annotation is used to aid in resolving lock expressions.</p>
</div>
<div class="section" id="locks-excluded">
<h3><tt class="docutils literal"><span class="pre">locks_excluded(...)</span></tt><a class="headerlink" href="#locks-excluded" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((locks_excluded(...)))</span></tt> on a function declaration to
specify that the function must not be called with the listed locks. Arguments
must be lockable type, and there must be at least one argument.</p>
</div>
<div class="section" id="exclusive-locks-required">
<h3><tt class="docutils literal"><span class="pre">exclusive_locks_required(...)</span></tt><a class="headerlink" href="#exclusive-locks-required" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((exclusive_locks_required(...)))</span></tt> on a function
declaration to specify that the function must be called while holding the
listed exclusive locks. Arguments must be lockable type, and there must be at
least one argument.</p>
</div>
<div class="section" id="shared-locks-required">
<h3><tt class="docutils literal"><span class="pre">shared_locks_required(...)</span></tt><a class="headerlink" href="#shared-locks-required" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((shared_locks_required(...)))</span></tt> on a function declaration
to specify that the function must be called while holding the listed shared
locks. Arguments must be lockable type, and there must be at least one
argument.</p>
</div>
</div>
<div class="section" id="type-safety-checking">
<h2><a class="toc-backref" href="#id22">Type Safety Checking</a><a class="headerlink" href="#type-safety-checking" title="Permalink to this headline">¶</a></h2>
<p>Clang supports additional attributes to enable checking type safety properties
that can’t be enforced by C type system. Usecases include:</p>
<ul class="simple">
<li>MPI library implementations, where these attributes enable checking that
buffer type matches the passed <tt class="docutils literal"><span class="pre">MPI_Datatype</span></tt>;</li>
<li>for HDF5 library there is a similar usecase as MPI;</li>
<li>checking types of variadic functions’ arguments for functions like
<tt class="docutils literal"><span class="pre">fcntl()</span></tt> and <tt class="docutils literal"><span class="pre">ioctl()</span></tt>.</li>
</ul>
<p>You can detect support for these attributes with <tt class="docutils literal"><span class="pre">__has_attribute()</span></tt>. For
example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#if defined(__has_attribute)</span>
<span class="cp"># if __has_attribute(argument_with_type_tag) && \</span>
<span class="cp"> __has_attribute(pointer_with_type_tag) && \</span>
<span class="cp"> __has_attribute(type_tag_for_datatype)</span>
<span class="cp"># define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))</span>
<span class="cm">/* ... other macros ... */</span>
<span class="cp"># endif</span>
<span class="cp">#endif</span>
<span class="cp">#if !defined(ATTR_MPI_PWT)</span>
<span class="cp"># define ATTR_MPI_PWT(buffer_idx, type_idx)</span>
<span class="cp">#endif</span>
<span class="kt">int</span> <span class="nf">MPI_Send</span><span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="n">buf</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">,</span> <span class="n">MPI_Datatype</span> <span class="n">datatype</span> <span class="cm">/*, other args omitted */</span><span class="p">)</span>
<span class="n">ATTR_MPI_PWT</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">);</span>
</pre></div>
</div>
<div class="section" id="argument-with-type-tag">
<h3><tt class="docutils literal"><span class="pre">argument_with_type_tag(...)</span></tt><a class="headerlink" href="#argument-with-type-tag" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((argument_with_type_tag(arg_kind,</span> <span class="pre">arg_idx,</span>
<span class="pre">type_tag_idx)))</span></tt> on a function declaration to specify that the function
accepts a type tag that determines the type of some other argument.
<tt class="docutils literal"><span class="pre">arg_kind</span></tt> is an identifier that should be used when annotating all
applicable type tags.</p>
<p>This attribute is primarily useful for checking arguments of variadic functions
(<tt class="docutils literal"><span class="pre">pointer_with_type_tag</span></tt> can be used in most of non-variadic cases).</p>
<p>For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">fcntl</span><span class="p">(</span><span class="kt">int</span> <span class="n">fd</span><span class="p">,</span> <span class="kt">int</span> <span class="n">cmd</span><span class="p">,</span> <span class="p">...)</span>
<span class="n">__attribute__</span><span class="p">((</span> <span class="n">argument_with_type_tag</span><span class="p">(</span><span class="n">fcntl</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">2</span><span class="p">)</span> <span class="p">));</span>
</pre></div>
</div>
</div>
<div class="section" id="pointer-with-type-tag">
<h3><tt class="docutils literal"><span class="pre">pointer_with_type_tag(...)</span></tt><a class="headerlink" href="#pointer-with-type-tag" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">__attribute__((pointer_with_type_tag(ptr_kind,</span> <span class="pre">ptr_idx,</span> <span class="pre">type_tag_idx)))</span></tt>
on a function declaration to specify that the function accepts a type tag that
determines the pointee type of some other pointer argument.</p>
<p>For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">MPI_Send</span><span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="n">buf</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">,</span> <span class="n">MPI_Datatype</span> <span class="n">datatype</span> <span class="cm">/*, other args omitted */</span><span class="p">)</span>
<span class="n">__attribute__</span><span class="p">((</span> <span class="n">pointer_with_type_tag</span><span class="p">(</span><span class="n">mpi</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span> <span class="p">));</span>
</pre></div>
</div>
</div>
<div class="section" id="type-tag-for-datatype">
<h3><tt class="docutils literal"><span class="pre">type_tag_for_datatype(...)</span></tt><a class="headerlink" href="#type-tag-for-datatype" title="Permalink to this headline">¶</a></h3>
<p>Clang supports annotating type tags of two forms.</p>
<ul>
<li><p class="first"><strong>Type tag that is an expression containing a reference to some declared
identifier.</strong> Use <tt class="docutils literal"><span class="pre">__attribute__((type_tag_for_datatype(kind,</span> <span class="pre">type)))</span></tt> on a
declaration with that identifier:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">extern</span> <span class="k">struct</span> <span class="n">mpi_datatype</span> <span class="n">mpi_datatype_int</span>
<span class="nf">__attribute__</span><span class="p">((</span> <span class="n">type_tag_for_datatype</span><span class="p">(</span><span class="n">mpi</span><span class="p">,</span><span class="kt">int</span><span class="p">)</span> <span class="p">));</span>
<span class="cp">#define MPI_INT ((MPI_Datatype) &mpi_datatype_int)</span>
</pre></div>
</div>
</li>
<li><p class="first"><strong>Type tag that is an integral literal.</strong> Introduce a <tt class="docutils literal"><span class="pre">static</span> <span class="pre">const</span></tt>
variable with a corresponding initializer value and attach
<tt class="docutils literal"><span class="pre">__attribute__((type_tag_for_datatype(kind,</span> <span class="pre">type)))</span></tt> on that declaration,
for example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#define MPI_INT ((MPI_Datatype) 42)</span>
<span class="k">static</span> <span class="k">const</span> <span class="n">MPI_Datatype</span> <span class="n">mpi_datatype_int</span>
<span class="n">__attribute__</span><span class="p">((</span> <span class="n">type_tag_for_datatype</span><span class="p">(</span><span class="n">mpi</span><span class="p">,</span><span class="kt">int</span><span class="p">)</span> <span class="p">))</span> <span class="o">=</span> <span class="mi">42</span>
</pre></div>
</div>
</li>
</ul>
<p>The attribute also accepts an optional third argument that determines how the
expression is compared to the type tag. There are two supported flags:</p>
<ul>
<li><p class="first"><tt class="docutils literal"><span class="pre">layout_compatible</span></tt> will cause types to be compared according to
layout-compatibility rules (C++11 [class.mem] p 17, 18). This is
implemented to support annotating types like <tt class="docutils literal"><span class="pre">MPI_DOUBLE_INT</span></tt>.</p>
<p>For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cm">/* In mpi.h */</span>
<span class="k">struct</span> <span class="n">internal_mpi_double_int</span> <span class="p">{</span> <span class="kt">double</span> <span class="n">d</span><span class="p">;</span> <span class="kt">int</span> <span class="n">i</span><span class="p">;</span> <span class="p">};</span>
<span class="k">extern</span> <span class="k">struct</span> <span class="n">mpi_datatype</span> <span class="n">mpi_datatype_double_int</span>
<span class="nf">__attribute__</span><span class="p">((</span> <span class="n">type_tag_for_datatype</span><span class="p">(</span><span class="n">mpi</span><span class="p">,</span> <span class="k">struct</span> <span class="n">internal_mpi_double_int</span><span class="p">,</span> <span class="n">layout_compatible</span><span class="p">)</span> <span class="p">));</span>
<span class="cp">#define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)</span>
<span class="cm">/* In user code */</span>
<span class="k">struct</span> <span class="n">my_pair</span> <span class="p">{</span> <span class="kt">double</span> <span class="n">a</span><span class="p">;</span> <span class="kt">int</span> <span class="n">b</span><span class="p">;</span> <span class="p">};</span>
<span class="k">struct</span> <span class="n">my_pair</span> <span class="o">*</span><span class="n">buffer</span><span class="p">;</span>
<span class="n">MPI_Send</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">MPI_DOUBLE_INT</span> <span class="cm">/*, ... */</span><span class="p">);</span> <span class="c1">// no warning</span>
<span class="k">struct</span> <span class="n">my_int_pair</span> <span class="p">{</span> <span class="kt">int</span> <span class="n">a</span><span class="p">;</span> <span class="kt">int</span> <span class="n">b</span><span class="p">;</span> <span class="p">}</span>
<span class="k">struct</span> <span class="n">my_int_pair</span> <span class="o">*</span><span class="n">buffer2</span><span class="p">;</span>
<span class="n">MPI_Send</span><span class="p">(</span><span class="n">buffer2</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">MPI_DOUBLE_INT</span> <span class="cm">/*, ... */</span><span class="p">);</span> <span class="c1">// warning: actual buffer element</span>
<span class="c1">// type 'struct my_int_pair'</span>
<span class="c1">// doesn't match specified MPI_Datatype</span>
</pre></div>
</div>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">must_be_null</span></tt> specifies that the expression should be a null pointer
constant, for example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cm">/* In mpi.h */</span>
<span class="k">extern</span> <span class="k">struct</span> <span class="n">mpi_datatype</span> <span class="n">mpi_datatype_null</span>
<span class="nf">__attribute__</span><span class="p">((</span> <span class="n">type_tag_for_datatype</span><span class="p">(</span><span class="n">mpi</span><span class="p">,</span> <span class="kt">void</span><span class="p">,</span> <span class="n">must_be_null</span><span class="p">)</span> <span class="p">));</span>
<span class="cp">#define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)</span>
<span class="cm">/* In user code */</span>
<span class="n">MPI_Send</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">MPI_DATATYPE_NULL</span> <span class="cm">/*, ... */</span><span class="p">);</span> <span class="c1">// warning: MPI_DATATYPE_NULL</span>
<span class="c1">// was specified but buffer</span>
<span class="c1">// is not a null pointer</span>
</pre></div>
</div>
</li>
</ul>
</div>
</div>
<div class="section" id="format-string-checking">
<h2><a class="toc-backref" href="#id23">Format String Checking</a><a class="headerlink" href="#format-string-checking" title="Permalink to this headline">¶</a></h2>
<p>Clang supports the <tt class="docutils literal"><span class="pre">format</span></tt> attribute, which indicates that the function
accepts a <tt class="docutils literal"><span class="pre">printf</span></tt> or <tt class="docutils literal"><span class="pre">scanf</span></tt>-like format string and corresponding
arguments or a <tt class="docutils literal"><span class="pre">va_list</span></tt> that contains these arguments.</p>
<p>Please see <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html">GCC documentation about format attribute</a> to find details
about attribute syntax.</p>
<p>Clang implements two kinds of checks with this attribute.</p>
<ol class="arabic">
<li><p class="first">Clang checks that the function with the <tt class="docutils literal"><span class="pre">format</span></tt> attribute is called with
a format string that uses format specifiers that are allowed, and that
arguments match the format string. This is the <tt class="docutils literal"><span class="pre">-Wformat</span></tt> warning, it is
on by default.</p>
</li>
<li><p class="first">Clang checks that the format string argument is a literal string. This is
the <tt class="docutils literal"><span class="pre">-Wformat-nonliteral</span></tt> warning, it is off by default.</p>
<p>Clang implements this mostly the same way as GCC, but there is a difference
for functions that accept a <tt class="docutils literal"><span class="pre">va_list</span></tt> argument (for example, <tt class="docutils literal"><span class="pre">vprintf</span></tt>).
GCC does not emit <tt class="docutils literal"><span class="pre">-Wformat-nonliteral</span></tt> warning for calls to such
fuctions. Clang does not warn if the format string comes from a function
parameter, where the function is annotated with a compatible attribute,
otherwise it warns. For example:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">__attribute__</span><span class="p">((</span><span class="n">__format__</span> <span class="p">(</span><span class="n">__scanf__</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">)))</span>
<span class="kt">void</span> <span class="n">foo</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">s</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf</span><span class="p">,</span> <span class="p">...)</span> <span class="p">{</span>
<span class="kt">va_list</span> <span class="n">ap</span><span class="p">;</span>
<span class="n">va_start</span><span class="p">(</span><span class="n">ap</span><span class="p">,</span> <span class="n">buf</span><span class="p">);</span>
<span class="n">vprintf</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">ap</span><span class="p">);</span> <span class="c1">// warning: format string is not a string literal</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this case we warn because <tt class="docutils literal"><span class="pre">s</span></tt> contains a format string for a
<tt class="docutils literal"><span class="pre">scanf</span></tt>-like function, but it is passed to a <tt class="docutils literal"><span class="pre">printf</span></tt>-like function.</p>
<p>If the attribute is removed, clang still warns, because the format string is
not a string literal.</p>
<p>Another example:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">__attribute__</span><span class="p">((</span><span class="n">__format__</span> <span class="p">(</span><span class="n">__printf__</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">)))</span>
<span class="kt">void</span> <span class="n">foo</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">s</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf</span><span class="p">,</span> <span class="p">...)</span> <span class="p">{</span>
<span class="kt">va_list</span> <span class="n">ap</span><span class="p">;</span>
<span class="n">va_start</span><span class="p">(</span><span class="n">ap</span><span class="p">,</span> <span class="n">buf</span><span class="p">);</span>
<span class="n">vprintf</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">ap</span><span class="p">);</span> <span class="c1">// warning</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this case Clang does not warn because the format string <tt class="docutils literal"><span class="pre">s</span></tt> and
the corresponding arguments are annotated. If the arguments are
incorrect, the caller of <tt class="docutils literal"><span class="pre">foo</span></tt> will receive a warning.</p>
</li>
</ol>
</div>
</div>
</div>
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