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<h1>Clang Language Extensions</h1>
<ul>
<li><a href="#intro">Introduction</a></li>
<li><a href="#feature_check">Feature Checking Macros</a></li>
<li><a href="#has_include">Include File Checking Macros</a></li>
<li><a href="#builtinmacros">Builtin Macros</a></li>
<li><a href="#vectors">Vectors and Extended Vectors</a></li>
<li><a href="#deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> attributes</a></li>
<li><a href="#attributes-on-enumerators">Attributes on enumerators</a></li>
<li><a href="#user_specified_system_framework">'User-Specified' System Frameworks</a></li>
<li><a href="#availability">Availability attribute</a></li>
<li><a href="#checking_language_features">Checks for Standard Language Features</a>
<ul>
<li><a href="#cxx98">C++98</a>
<ul>
<li><a href="#cxx_exceptions">C++ exceptions</a></li>
<li><a href="#cxx_rtti">C++ RTTI</a></li>
</ul></li>
<li><a href="#cxx11">C++11</a>
<ul>
<li><a href="#cxx_access_control_sfinae">C++11 SFINAE includes access control</a></li>
<li><a href="#cxx_alias_templates">C++11 alias templates</a></li>
<li><a href="#cxx_alignas">C++11 alignment specifiers</a></li>
<li><a href="#cxx_attributes">C++11 attributes</a></li>
<li><a href="#cxx_constexpr">C++11 generalized constant expressions</a></li>
<li><a href="#cxx_decltype">C++11 <tt>decltype()</tt></a></li>
<li><a href="#cxx_default_function_template_args">C++11 default template arguments in function templates</a></li>
<li><a href="#cxx_defaulted_functions">C++11 defaulted functions</a></li>
<li><a href="#cxx_delegating_constructor">C++11 delegating constructors</a></li>
<li><a href="#cxx_deleted_functions">C++11 deleted functions</a></li>
<li><a href="#cxx_explicit_conversions">C++11 explicit conversion functions</a></li>
<li><a href="#cxx_generalized_initializers">C++11 generalized initializers</a></li>
<li><a href="#cxx_implicit_moves">C++11 implicit move constructors/assignment operators</a></li>
<li><a href="#cxx_inheriting_constructors">C++11 inheriting constructors</a></li>
<li><a href="#cxx_inline_namespaces">C++11 inline namespaces</a></li>
<li><a href="#cxx_lambdas">C++11 lambdas</a></li>
<li><a href="#cxx_local_type_template_args">C++11 local and unnamed types as template arguments</a></li>
<li><a href="#cxx_noexcept">C++11 noexcept specification</a></li>
<li><a href="#cxx_nonstatic_member_init">C++11 in-class non-static data member initialization</a></li>
<li><a href="#cxx_nullptr">C++11 nullptr</a></li>
<li><a href="#cxx_override_control">C++11 override control</a></li>
<li><a href="#cxx_range_for">C++11 range-based for loop</a></li>
<li><a href="#cxx_raw_string_literals">C++11 raw string literals</a></li>
<li><a href="#cxx_rvalue_references">C++11 rvalue references</a></li>
<li><a href="#cxx_reference_qualified_functions">C++11 reference-qualified functions</a></li>
<li><a href="#cxx_static_assert">C++11 <tt>static_assert()</tt></a></li>
<li><a href="#cxx_auto_type">C++11 type inference</a></li>
<li><a href="#cxx_strong_enums">C++11 strongly-typed enumerations</a></li>
<li><a href="#cxx_trailing_return">C++11 trailing return type</a></li>
<li><a href="#cxx_unicode_literals">C++11 Unicode string literals</a></li>
<li><a href="#cxx_unrestricted_unions">C++11 unrestricted unions</a></li>
<li><a href="#cxx_user_literals">C++11 user-defined literals</a></li>
<li><a href="#cxx_variadic_templates">C++11 variadic templates</a></li>
</ul></li>
<li><a href="#c11">C11</a>
<ul>
<li><a href="#c_alignas">C11 alignment specifiers</a></li>
<li><a href="#c_atomic">C11 atomic operations</a></li>
<li><a href="#c_generic_selections">C11 generic selections</a></li>
<li><a href="#c_static_assert">C11 <tt>_Static_assert()</tt></a></li>
</ul></li>
</ul></li>
<li><a href="#checking_type_traits">Checks for Type Traits</a></li>
<li><a href="#blocks">Blocks</a></li>
<li><a href="#objc_features">Objective-C Features</a>
<ul>
<li><a href="#objc_instancetype">Related result types</a></li>
<li><a href="#objc_arc">Automatic reference counting</a></li>
<li><a href="#objc_fixed_enum">Enumerations with a fixed underlying type</a></li>
<li><a href="#objc_lambdas">Interoperability with C++11 lambdas</a></li>
<li><a href="#object-literals-subscripting">Object Literals and Subscripting</a></li>
</ul>
</li>
<li><a href="#overloading-in-c">Function Overloading in C</a></li>
<li><a href="#complex-list-init">Initializer lists for complex numbers in C</a></li>
<li><a href="#builtins">Builtin Functions</a>
<ul>
<li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
<li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
<li><a href="#__sync_swap">__sync_swap</a></li>
</ul>
</li>
<li><a href="#targetspecific">Target-Specific Extensions</a>
<ul>
<li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
</ul>
</li>
<li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a></li>
<li><a href="#dynamicanalyzerspecific">Dynamic Analysis-Specific Extensions</a>
<ul>
<li><a href="#address_sanitizer">AddressSanitizer</a></li>
</ul>
</li>
<li><a href="#threadsafety">Thread Safety Annotation Checking</a>
<ul>
<li><a href="#ts_noanal"><tt>no_thread_safety_analysis</tt></a></li>
<li><a href="#ts_lockable"><tt>lockable</tt></a></li>
<li><a href="#ts_scopedlockable"><tt>scoped_lockable</tt></a></li>
<li><a href="#ts_guardedvar"><tt>guarded_var</tt></a></li>
<li><a href="#ts_ptguardedvar"><tt>pt_guarded_var</tt></a></li>
<li><a href="#ts_guardedby"><tt>guarded_by(l)</tt></a></li>
<li><a href="#ts_ptguardedby"><tt>pt_guarded_by(l)</tt></a></li>
<li><a href="#ts_acquiredbefore"><tt>acquired_before(...)</tt></a></li>
<li><a href="#ts_acquiredafter"><tt>acquired_after(...)</tt></a></li>
<li><a href="#ts_elf"><tt>exclusive_lock_function(...)</tt></a></li>
<li><a href="#ts_slf"><tt>shared_lock_function(...)</tt></a></li>
<li><a href="#ts_etf"><tt>exclusive_trylock_function(...)</tt></a></li>
<li><a href="#ts_stf"><tt>shared_trylock_function(...)</tt></a></li>
<li><a href="#ts_uf"><tt>unlock_function(...)</tt></a></li>
<li><a href="#ts_lr"><tt>lock_returned(l)</tt></a></li>
<li><a href="#ts_le"><tt>locks_excluded(...)</tt></a></li>
<li><a href="#ts_elr"><tt>exclusive_locks_required(...)</tt></a></li>
<li><a href="#ts_slr"><tt>shared_locks_required(...)</tt></a></li>
</ul>
</li>
</ul>
<!-- ======================================================================= -->
<h2 id="intro">Introduction</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
href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
more information on these extensions.</p>
<!-- ======================================================================= -->
<h2 id="feature_check">Feature Checking Macros</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>
<!-- ======================================================================= -->
<h3><a name="__has_builtin">__has_builtin</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>
<blockquote>
<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>
</blockquote>
<!-- ======================================================================= -->
<h3><a name="__has_feature_extension"> __has_feature and __has_extension</a></h3>
<!-- ======================================================================= -->
<p>These function-like macros take a single identifier argument that is the
name of a feature. <code>__has_feature</code> 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 href="#has_feature_back_compat">below</a>), while
<code>__has_extension</code> 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>
<blockquote>
<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>
</blockquote>
<p id="has_feature_back_compat">For backwards compatibility reasons,
<code>__has_feature</code> can also be used to test for support for
non-standardized features, i.e. features not prefixed <code>c_</code>,
<code>cxx_</code> or <code>objc_</code>.</p>
<p id="has_feature_for_non_language_features">
Another use of <code>__has_feature</code> is to check for compiler features
not related to the language standard, such as e.g.
<a href="AddressSanitizer.html">AddressSanitizer</a>.
<p>If the <code>-pedantic-errors</code> option is given,
<code>__has_extension</code> is equivalent to <code>__has_feature</code>.</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 <code>__</code> (double underscore) to avoid interference from a macro
with the same name. For instance, <code>__cxx_rvalue_references__</code> can be
used instead of <code>cxx_rvalue_references</code>.</p>
<!-- ======================================================================= -->
<h3><a name="__has_attribute">__has_attribute</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>
<blockquote>
<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>
</blockquote>
<p>The attribute name can also be specified with a preceding and
following <code>__</code> (double underscore) to avoid interference from a macro
with the same name. For instance, <code>__always_inline__</code> can be used
instead of <code>always_inline</code>.</p>
<!-- ======================================================================= -->
<h2 id="has_include">Include File Checking Macros</h2>
<!-- ======================================================================= -->
<p>Not all developments systems have the same include files.
The <a href="#__has_include">__has_include</a> and
<a href="#__has_include_next">__has_include_next</a> macros allow you to
check for the existence of an include file before doing
a possibly failing #include directive.</p>
<!-- ======================================================================= -->
<h3><a name="__has_include">__has_include</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>
<blockquote>
<pre>
// Note the two possible file name string formats.
#if __has_include("myinclude.h") && __has_include(<stdint.h>)
# include "myinclude.h"
#endif
// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include) && __has_include("myinclude.h")
# include "myinclude.h"
#endif
</pre>
</blockquote>
<p>To test for this feature, use #if defined(__has_include).</p>
<!-- ======================================================================= -->
<h3><a name="__has_include_next">__has_include_next</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 __has_include 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>
<blockquote>
<pre>
// Note the two possible file name string formats.
#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
# include_next "myinclude.h"
#endif
// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include_next) && __has_include_next("myinclude.h")
# include_next "myinclude.h"
#endif
</pre>
</blockquote>
<p>Note that __has_include_next, like the GNU extension
#include_next 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>
<!-- ======================================================================= -->
<h3><a name="__has_warning">__has_warning</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>
<blockquote>
<pre>
#if __has_warning("-Wformat")
...
#endif
</pre>
</blockquote>
<!-- ======================================================================= -->
<h2 id="builtinmacros">Builtin Macros</h2>
<!-- ======================================================================= -->
<dl>
<dt><code>__BASE_FILE__</code></dt>
<dd>Defined to a string that contains the name of the main input
file passed to Clang.</dd>
<dt><code>__COUNTER__</code></dt>
<dd>Defined to an integer value that starts at zero and is
incremented each time the <code>__COUNTER__</code> macro is
expanded.</dd>
<dt><code>__INCLUDE_LEVEL__</code></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><code>__TIMESTAMP__</code></dt>
<dd>Defined to the date and time of the last modification of the
current source file.</dd>
<dt><code>__clang__</code></dt>
<dd>Defined when compiling with Clang</dd>
<dt><code>__clang_major__</code></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 href="#feature_check">feature checking
macros</a>.</dd>
<dt><code>__clang_minor__</code></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 href="#feature_check">feature checking
macros</a>.</dd>
<dt><code>__clang_patchlevel__</code></dt>
<dd>Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).</dd>
<dt><code>__clang_version__</code></dt>
<dd>Defined to a string that captures the Clang marketing version, including
the Subversion tag or revision number, e.g., "1.5 (trunk 102332)".</dd>
</dl>
<!-- ======================================================================= -->
<h2 id="vectors">Vectors and Extended Vectors</h2>
<!-- ======================================================================= -->
<p>Supports the GCC, OpenCL, AltiVec and NEON vector extensions.</p>
<p>OpenCL vector types are created using <tt>ext_vector_type</tt> attribute. It
support for <tt>V.xyzw</tt> syntax and other tidbits as seen in OpenCL. An
example is:</p>
<blockquote>
<pre>
typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
float4 foo(float2 a, float2 b) {
float4 c;
c.xz = a;
c.yw = b;
return c;
}
</pre>
</blockquote>
<p>Query for this feature with
<tt>__has_extension(attribute_ext_vector_type)</tt>.</p>
<p>Giving <tt>-faltivec</tt> option to clang enables support for AltiVec vector
syntax and functions. For example:</p>
<blockquote>
<pre>
vector float foo(vector int a) {
vector int b;
b = vec_add(a, a) + a;
return (vector float)b;
}
</pre>
</blockquote>
<p>NEON vector types are created using <tt>neon_vector_type</tt> and
<tt>neon_polyvector_type</tt> attributes. For example:</p>
<blockquote>
<pre>
typedef <b>__attribute__((neon_vector_type(8)))</b> int8_t int8x8_t;
typedef <b>__attribute__((neon_polyvector_type(16)))</b> poly8_t poly8x16_t;
int8x8_t foo(int8x8_t a) {
int8x8_t v;
v = a;
return v;
}
</pre>
</blockquote>
<!-- ======================================================================= -->
<h3><a name="vector_literals">Vector Literals</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>
<blockquote>
<pre>
typedef int v4si __attribute__((__vector_size__(16)));
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));
v4si vsi = (v4si){1, 2, 3, 4};
float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
vector int vi1 = (vector int)(1); // vi1 will be (1, 1, 1, 1).
vector int vi2 = (vector int){1}; // vi2 will be (1, 0, 0, 0).
vector int vi3 = (vector int)(1, 2); // error
vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
vector int vi5 = (vector int)(1, 2, 3, 4);
float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
</pre>
</blockquote>
<!-- ======================================================================= -->
<h3><a name="vector_operations">Vector Operations</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 width="500" border="1" cellspacing="0">
<tr>
<th>Operator</th>
<th>OpenCL</th>
<th>AltiVec</th>
<th>GCC</th>
<th>NEON</th>
</tr>
<tr>
<td>[]</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
</tr>
<tr>
<td>unary operators +, -</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
</tr>
<tr>
<td>++, --</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td>+, -, *, /, %</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
</tr>
<tr>
<td>bitwise operators &, |, ^, ~</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
</tr>
<tr>
<td>>>, <<</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
</tr>
<tr>
<td>!, &&,||</td>
<td align="center">no</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td>==,!=, >, <, >=, <=</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td>=</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
</tr>
<tr>
<td>:?</td>
<td align="center">yes</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td>sizeof</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
<td align="center">yes</td>
</tr>
</table>
<p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
<!-- ======================================================================= -->
<h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
<!-- ======================================================================= -->
<p>An optional string message can be added to the <tt>deprecated</tt>
and <tt>unavailable</tt> attributes. For example:</p>
<blockquote>
<pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
</blockquote>
<p>If the deprecated or unavailable declaration is used, the message
will be incorporated into the appropriate diagnostic:</p>
<blockquote>
<pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
[-Wdeprecated-declarations]
explode();
^</pre>
</blockquote>
<p>Query for this feature
with <tt>__has_extension(attribute_deprecated_with_message)</tt>
and <tt>__has_extension(attribute_unavailable_with_message)</tt>.</p>
<!-- ======================================================================= -->
<h2 id="attributes-on-enumerators">Attributes on Enumerators</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>
<blockquote>
<pre>enum OperationMode {
OM_Invalid,
OM_Normal,
OM_Terrified __attribute__((deprecated)),
OM_AbortOnError __attribute__((deprecated)) = 4
};</pre>
</blockquote>
<p>Attributes on the <tt>enum</tt> declaration do not apply to
individual enumerators.</p>
<p>Query for this feature with <tt>__has_extension(enumerator_attributes)</tt>.</p>
<!-- ======================================================================= -->
<h2 id="user_specified_system_framework">'User-Specified' System Frameworks</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
'.system_framework' file at the top-level of their framework. That is, the
framework should have contents like:</p>
<pre>
.../TestFramework.framework
.../TestFramework.framework/.system_framework
.../TestFramework.framework/Headers
.../TestFramework.framework/Headers/TestFramework.h
...
</pre>
<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>
<!-- ======================================================================= -->
<h2 id="availability">Availability attribute</h2
<!-- ======================================================================= -->
<p>Clang introduces the <code>availability</code> 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 <code>f</code>:</p>
<pre>
void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));
</pre>
<p>The availability attribute states that <code>f</code> 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 <code>f</code>: for example, if Clang is instructed to compile code for Mac OS X 10.5, a call to <code>f()</code> 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 <code>f()</code> is no longer available.</p>
<p>The availablility 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>
<dt>introduced=<i>version</i></dt>
<dd>The first version in which this declaration was introduced.</dd>
<dt>deprecated=<i>version</i></dt>
<dd>The first version in which this declaration was deprecated, meaning that users should migrate away from this API.</dd>
<dt>obsoleted=<i>version</i></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=<i>string-literal</i></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>
<dt>ios</dt>
<dd>Apple's iOS operating system. The minimum deployment target is specified by the <code>-mios-version-min=<i>version</i></code> or <code>-miphoneos-version-min=<i>version</i></code> command-line arguments.</dd>
<dt>macosx</dt>
<dd>Apple's Mac OS X operating system. The minimum deployment target is specified by the <code>-mmacosx-version-min=<i>version</i></code> 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
href="https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html">weakly
linked</a>, as if the <code>weak_import</code> 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>
<!-- ======================================================================= -->
<h2 id="checking_language_features">Checks for Standard Language Features</h2>
<!-- ======================================================================= -->
<p>The <tt>__has_feature</tt> macro can be used to query if certain standard
language features are enabled. The <tt>__has_extension</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>
<h3 id="cxx98">C++98</h3>
<p>The features listed below are part of the C++98 standard. These features are
enabled by default when compiling C++ code.</p>
<h4 id="cxx_exceptions">C++ exceptions</h4>
<p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
example, compiling code with <tt>-fno-exceptions</tt> disables C++ exceptions.</p>
<h4 id="cxx_rtti">C++ RTTI</h4>
<p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
<h3 id="cxx11">C++11</h3>
<p>The features listed below are part of the C++11 standard. As a result, all
these features are enabled with the <tt>-std=c++11</tt> or <tt>-std=gnu++11</tt>
option when compiling C++ code.</p>
<h4 id="cxx_access_control_sfinae">C++11 SFINAE includes access control</h4>
<p>Use <tt>__has_feature(cxx_access_control_sfinae)</tt> or <tt>__has_extension(cxx_access_control_sfinae)</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 href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170">C++ DR1170</a>.</p>
<h4 id="cxx_alias_templates">C++11 alias templates</h4>
<p>Use <tt>__has_feature(cxx_alias_templates)</tt> or
<tt>__has_extension(cxx_alias_templates)</tt> to determine if support for
C++11's alias declarations and alias templates is enabled.</p>
<h4 id="cxx_alignas">C++11 alignment specifiers</h4>
<p>Use <tt>__has_feature(cxx_alignas)</tt> or
<tt>__has_extension(cxx_alignas)</tt> to determine if support for alignment
specifiers using <tt>alignas</tt> is enabled.</p>
<h4 id="cxx_attributes">C++11 attributes</h4>
<p>Use <tt>__has_feature(cxx_attributes)</tt> or
<tt>__has_extension(cxx_attributes)</tt> to determine if support for attribute
parsing with C++11's square bracket notation is enabled.</p>
<h4 id="cxx_constexpr">C++11 generalized constant expressions</h4>
<p>Use <tt>__has_feature(cxx_constexpr)</tt> to determine if support
for generalized constant expressions (e.g., <tt>constexpr</tt>) is
enabled.</p>
<h4 id="cxx_decltype">C++11 <tt>decltype()</tt></h4>
<p>Use <tt>__has_feature(cxx_decltype)</tt> or
<tt>__has_extension(cxx_decltype)</tt> to determine if support for the
<tt>decltype()</tt> specifier is enabled. C++11's <tt>decltype</tt>
does not require type-completeness of a function call expression.
Use <tt>__has_feature(cxx_decltype_incomplete_return_types)</tt>
or <tt>__has_extension(cxx_decltype_incomplete_return_types)</tt>
to determine if support for this feature is enabled.</p>
<h4 id="cxx_default_function_template_args">C++11 default template arguments in function templates</h4>
<p>Use <tt>__has_feature(cxx_default_function_template_args)</tt> or
<tt>__has_extension(cxx_default_function_template_args)</tt> to determine
if support for default template arguments in function templates is enabled.</p>
<h4 id="cxx_defaulted_functions">C++11 <tt>default</tt>ed functions</h4>
<p>Use <tt>__has_feature(cxx_defaulted_functions)</tt> or
<tt>__has_extension(cxx_defaulted_functions)</tt> to determine if support for
defaulted function definitions (with <tt>= default</tt>) is enabled.</p>
<h4 id="cxx_delegating_constructors">C++11 delegating constructors</h4>
<p>Use <tt>__has_feature(cxx_delegating_constructors)</tt> to determine if
support for delegating constructors is enabled.</p>
<h4 id="cxx_deleted_functions">C++11 <tt>delete</tt>d functions</h4>
<p>Use <tt>__has_feature(cxx_deleted_functions)</tt> or
<tt>__has_extension(cxx_deleted_functions)</tt> to determine if support for
deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
<h4 id="cxx_explicit_conversions">C++11 explicit conversion functions</h4>
<p>Use <tt>__has_feature(cxx_explicit_conversions)</tt> to determine if support for <tt>explicit</tt> conversion functions is enabled.</p>
<h4 id="cxx_generalized_initializers">C++11 generalized initializers</h4>
<p>Use <tt>__has_feature(cxx_generalized_initializers)</tt> to determine if
support for generalized initializers (using braced lists and
<tt>std::initializer_list</tt>) is enabled.</p>
<h4 id="cxx_implicit_moves">C++11 implicit move constructors/assignment operators</h4>
<p>Use <tt>__has_feature(cxx_implicit_moves)</tt> to determine if Clang will
implicitly generate move constructors and move assignment operators where needed.</p>
<h4 id="cxx_inheriting_constructors">C++11 inheriting constructors</h4>
<p>Use <tt>__has_feature(cxx_inheriting_constructors)</tt> to determine if support for inheriting constructors is enabled. Clang does not currently implement this feature.</p>
<h4 id="cxx_inline_namespaces">C++11 inline namespaces</h4>
<p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> or
<tt>__has_extension(cxx_inline_namespaces)</tt> to determine if support for
inline namespaces is enabled.</p>
<h4 id="cxx_lambdas">C++11 lambdas</h4>
<p>Use <tt>__has_feature(cxx_lambdas)</tt> or
<tt>__has_extension(cxx_lambdas)</tt> to determine if support for lambdas
is enabled. </p>
<h4 id="cxx_local_type_template_args">C++11 local and unnamed types as template arguments</h4>
<p>Use <tt>__has_feature(cxx_local_type_template_args)</tt> or
<tt>__has_extension(cxx_local_type_template_args)</tt> to determine if
support for local and unnamed types as template arguments is enabled.</p>
<h4 id="cxx_noexcept">C++11 noexcept</h4>
<p>Use <tt>__has_feature(cxx_noexcept)</tt> or
<tt>__has_extension(cxx_noexcept)</tt> to determine if support for noexcept
exception specifications is enabled.</p>
<h4 id="cxx_nonstatic_member_init">C++11 in-class non-static data member initialization</h4>
<p>Use <tt>__has_feature(cxx_nonstatic_member_init)</tt> to determine whether in-class initialization of non-static data members is enabled.</p>
<h4 id="cxx_nullptr">C++11 <tt>nullptr</tt></h4>
<p>Use <tt>__has_feature(cxx_nullptr)</tt> or
<tt>__has_extension(cxx_nullptr)</tt> to determine if support for
<tt>nullptr</tt> is enabled.</p>
<h4 id="cxx_override_control">C++11 <tt>override control</tt></h4>
<p>Use <tt>__has_feature(cxx_override_control)</tt> or
<tt>__has_extension(cxx_override_control)</tt> to determine if support for
the override control keywords is enabled.</p>
<h4 id="cxx_reference_qualified_functions">C++11 reference-qualified functions</h4>
<p>Use <tt>__has_feature(cxx_reference_qualified_functions)</tt> or
<tt>__has_extension(cxx_reference_qualified_functions)</tt> to determine
if support for reference-qualified functions (e.g., member functions with
<code>&</code> or <code>&&</code> applied to <code>*this</code>)
is enabled.</p>
<h4 id="cxx_range_for">C++11 range-based <tt>for</tt> loop</h4>
<p>Use <tt>__has_feature(cxx_range_for)</tt> or
<tt>__has_extension(cxx_range_for)</tt> to determine if support for the
range-based for loop is enabled. </p>
<h4 id="cxx_raw_string_literals">C++11 raw string literals</h4>
<p>Use <tt>__has_feature(cxx_raw_string_literals)</tt> to determine if support
for raw string literals (e.g., <tt>R"x(foo\bar)x"</tt>) is enabled.</p>
<h4 id="cxx_rvalue_references">C++11 rvalue references</h4>
<p>Use <tt>__has_feature(cxx_rvalue_references)</tt> or
<tt>__has_extension(cxx_rvalue_references)</tt> to determine if support for
rvalue references is enabled. </p>
<h4 id="cxx_static_assert">C++11 <tt>static_assert()</tt></h4>
<p>Use <tt>__has_feature(cxx_static_assert)</tt> or
<tt>__has_extension(cxx_static_assert)</tt> to determine if support for
compile-time assertions using <tt>static_assert</tt> is enabled.</p>
<h4 id="cxx_auto_type">C++11 type inference</h4>
<p>Use <tt>__has_feature(cxx_auto_type)</tt> or
<tt>__has_extension(cxx_auto_type)</tt> to determine C++11 type inference is
supported using the <tt>auto</tt> specifier. If this is disabled, <tt>auto</tt>
will instead be a storage class specifier, as in C or C++98.</p>
<h4 id="cxx_strong_enums">C++11 strongly typed enumerations</h4>
<p>Use <tt>__has_feature(cxx_strong_enums)</tt> or
<tt>__has_extension(cxx_strong_enums)</tt> to determine if support for
strongly typed, scoped enumerations is enabled.</p>
<h4 id="cxx_trailing_return">C++11 trailing return type</h4>
<p>Use <tt>__has_feature(cxx_trailing_return)</tt> or
<tt>__has_extension(cxx_trailing_return)</tt> to determine if support for the
alternate function declaration syntax with trailing return type is enabled.</p>
<h4 id="cxx_unicode_literals">C++11 Unicode string literals</h4>
<p>Use <tt>__has_feature(cxx_unicode_literals)</tt> to determine if
support for Unicode string literals is enabled.</p>
<h4 id="cxx_unrestricted_unions">C++11 unrestricted unions</h4>
<p>Use <tt>__has_feature(cxx_unrestricted_unions)</tt> to determine if support for unrestricted unions is enabled.</p>
<h4 id="cxx_user_literals">C++11 user-defined literals</h4>
<p>Use <tt>__has_feature(cxx_user_literals)</tt> to determine if support for user-defined literals is enabled.</p>
<h4 id="cxx_variadic_templates">C++11 variadic templates</h4>
<p>Use <tt>__has_feature(cxx_variadic_templates)</tt> or
<tt>__has_extension(cxx_variadic_templates)</tt> to determine if support
for variadic templates is enabled.</p>
<h3 id="c11">C11</h3>
<p>The features listed below are part of the C11 standard. As a result, all
these features are enabled with the <tt>-std=c11</tt> or <tt>-std=gnu11</tt>
option when compiling C code. Additionally, because these features are all
backward-compatible, they are available as extensions in all language modes.</p>
<h4 id="c_alignas">C11 alignment specifiers</h4>
<p>Use <tt>__has_feature(c_alignas)</tt> or <tt>__has_extension(c_alignas)</tt>
to determine if support for alignment specifiers using <tt>_Alignas</tt>
is enabled.</p>
<h4 id="c_atomic">C11 atomic operations</h4>
<p>Use <tt>__has_feature(c_atomic)</tt> or <tt>__has_extension(c_atomic)</tt>
to determine if support for atomic types using <tt>_Atomic</tt> is enabled.
Clang also provides <a href="#__c11_atomic">a set of builtins</a> which can be
used to implement the <tt><stdatomic.h></tt> operations on _Atomic
types.</p>
<h4 id="c_generic_selections">C11 generic selections</h4>
<p>Use <tt>__has_feature(c_generic_selections)</tt> or
<tt>__has_extension(c_generic_selections)</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>
<h4 id="c_static_assert">C11 <tt>_Static_assert()</tt></h4>
<p>Use <tt>__has_feature(c_static_assert)</tt> or
<tt>__has_extension(c_static_assert)</tt> to determine if support for
compile-time assertions using <tt>_Static_assert</tt> is enabled.</p>
<!-- ======================================================================= -->
<h2 id="checking_type_traits">Checks for Type Traits</h2>
<!-- ======================================================================= -->
<p>Clang supports the <a href="http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html">GNU C++ type traits</a> and a subset of the <a href="http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx">Microsoft Visual C++ Type traits</a>. For each supported type trait <code>__X</code>, <code>__has_extension(X)</code> indicates the presence of the type trait. For example:
<blockquote>
<pre>
#if __has_extension(is_convertible_to)
template<typename From, typename To>
struct is_convertible_to {
static const bool value = __is_convertible_to(From, To);
};
#else
// Emulate type trait
#endif
</pre>
</blockquote>
<p>The following type traits are supported by Clang:</p>
<ul>
<li><code>__has_nothrow_assign</code> (GNU, Microsoft)</li>
<li><code>__has_nothrow_copy</code> (GNU, Microsoft)</li>
<li><code>__has_nothrow_constructor</code> (GNU, Microsoft)</li>
<li><code>__has_trivial_assign</code> (GNU, Microsoft)</li>
<li><code>__has_trivial_copy</code> (GNU, Microsoft)</li>
<li><code>__has_trivial_constructor</code> (GNU, Microsoft)</li>
<li><code>__has_trivial_destructor</code> (GNU, Microsoft)</li>
<li><code>__has_virtual_destructor</code> (GNU, Microsoft)</li>
<li><code>__is_abstract</code> (GNU, Microsoft)</li>
<li><code>__is_base_of</code> (GNU, Microsoft)</li>
<li><code>__is_class</code> (GNU, Microsoft)</li>
<li><code>__is_convertible_to</code> (Microsoft)</li>
<li><code>__is_empty</code> (GNU, Microsoft)</li>
<li><code>__is_enum</code> (GNU, Microsoft)</li>
<li><code>__is_pod</code> (GNU, Microsoft)</li>
<li><code>__is_polymorphic</code> (GNU, Microsoft)</li>
<li><code>__is_union</code> (GNU, Microsoft)</li>
<li><code>__is_literal(type)</code>: Determines whether the given type is a literal type</li>
<li><code>__is_final</code>: Determines whether the given type is declared with a <code>final</code> class-virt-specifier.</li>
<li><code>__underlying_type(type)</code>: Retrieves the underlying type for a given <code>enum</code> type. This trait is required to implement the C++11 standard library.</li>
<li><code>__is_trivially_assignable(totype, fromtype)</code>: Determines whether a value of type <tt>totype</tt> can be assigned to from a value of type <tt>fromtype</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><code>__is_trivially_constructible(type, argtypes...)</code>: Determines whether a value of type <tt>type</tt> can be direct-initialized with arguments of types <tt>argtypes...</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>
<!-- ======================================================================= -->
<h2 id="blocks">Blocks</h2>
<!-- ======================================================================= -->
<p>The syntax and high level language feature description is in <a
href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
details for the clang implementation are in <a
href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
<p>Query for this feature with __has_extension(blocks).</p>
<!-- ======================================================================= -->
<h2 id="objc_features">Objective-C Features</h2>
<!-- ======================================================================= -->
<h3 id="objc_instancetype">Related result types</h3>
<p>According to Cocoa conventions, Objective-C methods with certain names ("init", "alloc", 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>
<blockquote>
<pre>
@interface NSObject
+ (id)alloc;
- (id)init;
@end
@interface NSArray : NSObject
@end
</pre>
</blockquote>
<p>and this common initialization pattern</p>
<blockquote>
<pre>
NSArray *array = [[NSArray alloc] init];
</pre>
</blockquote>
<p>the type of the expression <code>[NSArray alloc]</code> is
<code>NSArray*</code> because <code>alloc</code> implicitly has a
related result type. Similarly, the type of the expression
<code>[[NSArray alloc] init]</code> is <code>NSArray*</code>, since
<code>init</code> has a related result type and its receiver is known
to have the type <code>NSArray *</code>. If neither <code>alloc</code> nor <code>init</code> had a related result type, the expressions would have had type <code>id</code>, as declared in the method signature.</p>
<p>A method with a related result type can be declared by using the
type <tt>instancetype</tt> as its result type. <tt>instancetype</tt>
is a contextual keyword that is only permitted in the result type of
an Objective-C method, e.g.</p>
<pre>
@interface A
+ (<b>instancetype</b>)constructAnA;
@end
</pre>
<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., "init" in "initWithObjects") 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>
<li>the first word is "alloc" or "new", and the method is a class
method, or</li>
<li>the first word is "autorelease", "init", "retain", or "self",
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>
<blockquote>
<pre>
@interface NSString : NSObject
- (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
@end
</pre>
</blockquote>
<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>id</tt>.</p>
<p>Use <tt>__has_feature(objc_instancetype)</tt> to determine whether
the <tt>instancetype</tt> contextual keyword is available.</p>
<!-- ======================================================================= -->
<h2 id="objc_arc">Automatic reference counting </h2>
<!-- ======================================================================= -->
<p>Clang provides support for <a href="AutomaticReferenceCounting.html">automated reference counting</a> in Objective-C, which eliminates the need for manual retain/release/autorelease message sends. There are two feature macros associated with automatic reference counting: <code>__has_feature(objc_arc)</code> indicates the availability of automated reference counting in general, while <code>__has_feature(objc_arc_weak)</code> indicates that automated reference counting also includes support for <code>__weak</code> pointers to Objective-C objects.</p>
<!-- ======================================================================= -->
<h2 id="objc_fixed_enum">Enumerations with a fixed underlying type</h2>
<!-- ======================================================================= -->
<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>
<pre>
typedef enum : unsigned char { Red, Green, Blue } Color;
</pre>
<p>This specifies that the underlying type, which is used to store the
enumeration value, is <tt>unsigned char</tt>.</p>
<p>Use <tt>__has_feature(objc_fixed_enum)</tt> to determine whether
support for fixed underlying types is available in Objective-C.</p>
<!-- ======================================================================= -->
<h2 id="objc_lambdas">Interoperability with C++11 lambdas</h2>
<!-- ======================================================================= -->
<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 <code>NSArray</code>'s array-sorting
method:</p>
<pre> - (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr; </pre>
<p><code>NSComparator</code> is simply a typedef for the block pointer
<code>NSComparisonResult (^)(id, id)</code>, 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
<code>id</code> and returning an <code>NSComparisonResult</code>):</p>
<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:<b>[=](id s1, id s2) -> NSComparisonResult {
NSRange string1Range = NSMakeRange(0, [s1 length]);
return [s1 compare:s2 options:comparisonOptions
range:string1Range locale:currentLocale];
}</b>];
NSLog(@"sorted: %@", sorted);
</pre>
<p>This code relies on an implicit conversion from the type of the
lambda expression (an unnamed, local class type called the <i>closure
type</i>) 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>
<pre>
operator NSComparisonResult (^)(id, id)() const;
</pre>
<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>Block_copy</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>
<!-- ======================================================================= -->
<h2 id="object-literals-subscripting">Object Literals and Subscripting</h2>
<!-- ======================================================================= -->
<p>Clang provides support for <a href="ObjectiveCLiterals.html">Object Literals and Subscripting</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: <code>__has_feature(objc_array_literals)</code> tests the availability of array literals; <code>__has_feature(objc_dictionary_literals)</code> tests the availability of dictionary literals; <code>__has_feature(objc_subscripting)</code> tests the availability of object subscripting.</p>
<!-- ======================================================================= -->
<h2 id="overloading-in-c">Function Overloading in C</h2>
<!-- ======================================================================= -->
<p>Clang provides support for C++ function overloading in C. Function
overloading in C is introduced using the <tt>overloadable</tt> attribute. For
example, one might provide several overloaded versions of a <tt>tgsin</tt>
function that invokes the appropriate standard function computing the sine of a
value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
precision:</p>
<blockquote>
<pre>
#include <math.h>
float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); }
</pre>
</blockquote>
<p>Given these declarations, one can call <tt>tgsin</tt> with a
<tt>float</tt> value to receive a <tt>float</tt> result, with a
<tt>double</tt> to receive a <tt>double</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>
<li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long
double</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>T*</tt> to a pointer of type
<tt>U*</tt> is considered a pointer conversion (with conversion
rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li>
<li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt>
is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This
conversion is given "conversion" rank.</li>
</ul>
<p>The declaration of <tt>overloadable</tt> functions is restricted to
function declarations and definitions. Most importantly, if any
function with a given name is given the <tt>overloadable</tt>
attribute, then all function declarations and definitions with that
name (and in that scope) must have the <tt>overloadable</tt>
attribute. This rule even applies to redeclarations of functions whose original
declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
<blockquote>
<pre>
int f(int) __attribute__((overloadable));
float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
int g(int) __attribute__((overloadable));
int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
</pre>
</blockquote>
<p>Functions marked <tt>overloadable</tt> must have
prototypes. Therefore, the following code is ill-formed:</p>
<blockquote>
<pre>
int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
</pre>
</blockquote>
<p>However, <tt>overloadable</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>unavailable</tt> attribute:</p>
<blockquote>
<pre>
void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
</pre>
</blockquote>
<p>Functions declared with the <tt>overloadable</tt> attribute have
their names mangled according to the same rules as C++ function
names. For example, the three <tt>tgsin</tt> functions in our
motivating example get the mangled names <tt>_Z5tgsinf</tt>,
<tt>_Z5tgsind</tt>, and <tt>_Z5tgsine</tt>, respectively. There are two
caveats to this use of name mangling:</p>
<ul>
<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>static inline</tt> with <tt>overloadable</tt> functions.</li>
<li>The <tt>overloadable</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>overloadable</tt>
function occurs within an <tt>extern "C"</tt> linkage specification,
it's name <i>will</i> be mangled in the same way as it would in
C.</li>
</ul>
<p>Query for this feature with __has_extension(attribute_overloadable).</p>
<!-- ======================================================================= -->
<h2 id="complex-list-init">Initializer lists for complex numbers in C</h2>
<!-- ======================================================================= -->
<p>clang supports an extension which allows the following in C:</p>
<blockquote>
<pre>
#include <math.h>
#include <complex.h>
complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)
</pre>
</blockquote>
<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 <code>_Imaginary</code>.
(clang also supports the <code>__real__</code> and <code>__imag__</code>
extensions from gcc, which help in some cases, but are not usable in
static initializers.)
<p>Note that this extension does not allow eliding the braces; the
meaning of the following two lines is different:</p>
<blockquote>
<pre>
complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)
</pre>
</blockquote>
<p>This extension also works in C++ mode, as far as that goes, but does not
apply to the C++ <code>std::complex</code>. (In C++11, list
initialization allows the same syntax to be used with
<code>std::complex</code> with the same meaning.)
<!-- ======================================================================= -->
<h2 id="builtins">Builtin Functions</h2>
<!-- ======================================================================= -->
<p>Clang supports a number of builtin library functions with the same syntax as
GCC, including things like <tt>__builtin_nan</tt>,
<tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
<tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</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><xmmintrin.h></tt>, which
define portable wrappers for these. Many of the Clang versions of these
functions are implemented directly in terms of <a href="#vectors">extended
vector support</a> instead of builtins, in order to reduce the number of
builtins that we need to implement.</p>
<!-- ======================================================================= -->
<h3><a name="__builtin_shufflevector">__builtin_shufflevector</a></h3>
<!-- ======================================================================= -->
<p><tt>__builtin_shufflevector</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><xmmintrin.h></tt>.
</p>
<p><b>Syntax:</b></p>
<pre>
__builtin_shufflevector(vec1, vec2, index1, index2, ...)
</pre>
<p><b>Examples:</b></p>
<pre>
// Identity operation - return 4-element vector V1.
__builtin_shufflevector(V1, V1, 0, 1, 2, 3)
// "Splat" element 0 of V1 into a 4-element result.
__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
// Reverse 4-element vector V1.
__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
// Concatenate every other element of 4-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
// Concatenate every other element of 8-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
</pre>
<p><b>Description:</b></p>
<p>The first two arguments to __builtin_shufflevector 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
vec1 is a 4-element vector, index 5 would refer to the second element of vec2.
</p>
<p>The result of __builtin_shufflevector is a vector
with the same element type as vec1/vec2 but that has an element count equal to
the number of indices specified.
</p>
<p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>
<!-- ======================================================================= -->
<h3><a name="__builtin_unreachable">__builtin_unreachable</a></h3>
<!-- ======================================================================= -->
<p><tt>__builtin_unreachable</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>__builtin_unreachable</tt> in the example below,
the compiler assumes that the inline asm can fall through and prints a "function
declared 'noreturn' should not return" warning.
</p>
<p><b>Syntax:</b></p>
<pre>
__builtin_unreachable()
</pre>
<p><b>Example of Use:</b></p>
<pre>
void myabort(void) __attribute__((noreturn));
void myabort(void) {
asm("int3");
__builtin_unreachable();
}
</pre>
<p><b>Description:</b></p>
<p>The __builtin_unreachable() 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 __has_builtin(__builtin_unreachable).</p>
<!-- ======================================================================= -->
<h3><a name="__sync_swap">__sync_swap</a></h3>
<!-- ======================================================================= -->
<p><tt>__sync_swap</tt> is used to atomically swap integers or pointers in
memory.
</p>
<p><b>Syntax:</b></p>
<pre>
<i>type</i> __sync_swap(<i>type</i> *ptr, <i>type</i> value, ...)
</pre>
<p><b>Example of Use:</b></p>
<pre>
int old_value = __sync_swap(&value, new_value);
</pre>
<p><b>Description:</b></p>
<p>The __sync_swap() builtin extends the existing __sync_*() 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 __sync_bool_compare_and_swap() or
relying on the platform specific implementation details of
__sync_lock_test_and_set(). The __sync_swap() builtin is a full barrier.
</p>
<!-- ======================================================================= -->
<h3><a name="__c11_atomic">__c11_atomic builtins</a></h3>
<!-- ======================================================================= -->
<p>Clang provides a set of builtins which are intended to be used to implement
C11's <tt><stdatomic.h></tt> header. These builtins provide the semantics
of the <tt>_explicit</tt> form of the corresponding C11 operation, and are named
with a <tt>__c11_</tt> prefix. The supported operations are:</p>
<ul>
<li><tt>__c11_atomic_init</tt></li>
<li><tt>__c11_atomic_thread_fence</tt></li>
<li><tt>__c11_atomic_signal_fence</tt></li>
<li><tt>__c11_atomic_is_lock_free</tt></li>
<li><tt>__c11_atomic_store</tt></li>
<li><tt>__c11_atomic_load</tt></li>
<li><tt>__c11_atomic_exchange</tt></li>
<li><tt>__c11_atomic_compare_exchange_strong</tt></li>
<li><tt>__c11_atomic_compare_exchange_weak</tt></li>
<li><tt>__c11_atomic_fetch_add</tt></li>
<li><tt>__c11_atomic_fetch_sub</tt></li>
<li><tt>__c11_atomic_fetch_and</tt></li>
<li><tt>__c11_atomic_fetch_or</tt></li>
<li><tt>__c11_atomic_fetch_xor</tt></li>
</ul>
<!-- ======================================================================= -->
<h2 id="targetspecific">Target-Specific Extensions</h2>
<!-- ======================================================================= -->
<p>Clang supports some language features conditionally on some targets.</p>
<!-- ======================================================================= -->
<h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
<!-- ======================================================================= -->
<p>The X86 backend has these language extensions:</p>
<!-- ======================================================================= -->
<h4 id="x86-gs-segment">Memory references off the GS segment</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>
<pre>
#define GS_RELATIVE __attribute__((address_space(256)))
int foo(int GS_RELATIVE *P) {
return *P;
}
</pre>
<p>Which compiles to (on X86-32):</p>
<pre>
_foo:
movl 4(%esp), %eax
movl %gs:(%eax), %eax
ret
</pre>
<!-- ======================================================================= -->
<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
<!-- ======================================================================= -->
<p>Clang supports additional attributes that are useful for documenting program
invariants and rules for static analysis tools. The extensions documented here
are used by the <a
href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
engine</a> that is part of Clang's Analysis library.</p>
<h3 id="attr_analyzer_noreturn">The <tt>analyzer_noreturn</tt> attribute</h3>
<p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
attribute. This attribute, which is typically affixed to a function prototype,
indicates that a call to a given function never returns. Function prototypes for
common functions like <tt>exit</tt> are typically annotated with this attribute,
as well as a variety of common assertion handlers. Users can educate the static
analyzer about their own custom assertion handles (thus cutting down on false
positives due to false paths) by marking their own "panic" functions
with this attribute.</p>
<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
there are special functions that for all intents and purposes should be
considered panic functions (i.e., they are only called when an internal program
error occurs) but may actually return so that the program can fail gracefully.
The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
as being interpreted as "no return" functions by the analyzer (thus
pruning bogus paths) but will not affect compilation (as in the case of
<tt>noreturn</tt>).</p>
<p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
placed at the end of function prototypes:</p>
<pre>
void foo() <b>__attribute__((analyzer_noreturn))</b>;
</pre>
<p>Query for this feature with
<tt>__has_attribute(analyzer_noreturn)</tt>.</p>
<h3 id="attr_method_family">The <tt>objc_method_family</tt> attribute</h3>
<p>Many methods in Objective-C have conventional meanings determined
by their selectors. For the purposes of static analysis, 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 <q>method family</q> that a method belongs to.</p>
<p><b>Usage</b>: <tt>__attribute__((objc_method_family(X)))</tt>,
where <tt>X</tt> is one of <tt>none</tt>, <tt>alloc</tt>, <tt>copy</tt>,
<tt>init</tt>, <tt>mutableCopy</tt>, or <tt>new</tt>. This attribute
can only be placed at the end of a method declaration:</p>
<pre>
- (NSString*) initMyStringValue <b>__attribute__((objc_method_family(none)))</b>;
</pre>
<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 href="#attr_retain_release">retaining behavior attributes</a>
described below.</p>
<p>Query for this feature with
<tt>__has_attribute(objc_method_family)</tt>.</p>
<h3 id="attr_retain_release">Objective-C retaining behavior attributes</h3>
<p>In Objective-C, functions and methods are generally assumed to take
and return objects with +0 retain counts, with some exceptions for
special methods like <tt>+alloc</tt> and <tt>init</tt>. However,
there are exceptions, and so Clang provides attributes to allow these
exceptions to be documented, which helps the analyzer find leaks (and
ignore non-leaks). Some exceptions may be better described using
the <a href="#attr_method_family"><tt>objc_method_family</tt></a>
attribute instead.</p>
<p><b>Usage</b>: The <tt>ns_returns_retained</tt>, <tt>ns_returns_not_retained</tt>,
<tt>ns_returns_autoreleased</tt>, <tt>cf_returns_retained</tt>,
and <tt>cf_returns_not_retained</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>
<pre>
id foo() <b>__attribute__((ns_returns_retained))</b>;
- (NSString*) bar: (int) x <b>__attribute__((ns_returns_retained))</b>;
</pre>
<p>The <tt>*_returns_retained</tt> attributes specify that the
returned object has a +1 retain count.
The <tt>*_returns_not_retained</tt> attributes specify that the return
object has a +0 retain count, even if the normal convention for its
selector would be +1. <tt>ns_returns_autoreleased</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><b>Usage</b>: The <tt>ns_consumed</tt> and <tt>cf_consumed</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>ns_consumes_self</tt> attribute can only be placed on an
Objective-C method; it specifies that the method expects
its <tt>self</tt> parameter to have a +1 retain count, which it will
balance in some way.</p>
<pre>
void <b>foo(__attribute__((ns_consumed))</b> NSString *string);
- (void) bar <b>__attribute__((ns_consumes_self))</b>;
- (void) baz: (id) <b>__attribute__((ns_consumed))</b> x;
</pre>
<p>Query for these features with <tt>__has_attribute(ns_consumed)</tt>,
<tt>__has_attribute(ns_returns_retained)</tt>, etc.</p>
<!-- ======================================================================= -->
<h2 id="dynamicanalyzerspecific">Dynamic Analysis-Specific Extensions</h2>
<!-- ======================================================================= -->
<h3 id="address_sanitizer">AddressSanitizer</h3>
<p> Use <code>__has_feature(address_sanitizer)</code>
to check if the code is being built with <a
href="AddressSanitizer.html">AddressSanitizer</a>.
</p>
<p>Use <tt>__attribute__((no_address_safety_analysis))</tt> on a function
declaration to specify that address safety instrumentation (e.g.
AddressSanitizer) should not be applied to that function.
</p>
<!-- ======================================================================= -->
<h2 id="threadsafety">Thread-Safety Annotation Checking</h2>
<!-- ======================================================================= -->
<p>Clang supports additional attributes for checking basic locking policies in
multithreaded programs.
Clang currently parses the following list of attributes, although
<b>the implementation for these annotations is currently in development.</b>
For more details, see the
<a href="http://gcc.gnu.org/wiki/ThreadSafetyAnnotation">GCC implementation</a>.
</p>
<h4 id="ts_noanal">no_thread_safety_analysis</h4>
<p>Use <tt>__attribute__((no_thread_safety_analysis))</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>
<h4 id="ts_lockable">lockable</h4>
<p>Use <tt>__attribute__((lockable))</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>
<h4 id="ts_scopedlockable">scoped_lockable</h4>
<p>Use <tt>__attribute__((scoped_lockable))</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>
<h4 id="ts_guardedvar">guarded_var</h4>
<p>Use <tt>__attribute__((guarded_var))</tt> on a variable declaration to
specify that the variable must be accessed while holding some lock.</p>
<h4 id="ts_ptguardedvar">pt_guarded_var</h4>
<p>Use <tt>__attribute__((pt_guarded_var))</tt> on a pointer declaration to
specify that the pointer must be dereferenced while holding some lock.</p>
<h4 id="ts_guardedby">guarded_by(l)</h4>
<p>Use <tt>__attribute__((guarded_by(l)))</tt> on a variable declaration to
specify that the variable must be accessed while holding lock <tt>l</tt>.</p>
<h4 id="ts_ptguardedby">pt_guarded_by(l)</h4>
<p>Use <tt>__attribute__((pt_guarded_by(l)))</tt> on a pointer declaration to
specify that the pointer must be dereferenced while holding lock <tt>l</tt>.</p>
<h4 id="ts_acquiredbefore">acquired_before(...)</h4>
<p>Use <tt>__attribute__((acquired_before(...)))</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>
<h4 id="ts_acquiredafter">acquired_after(...)</h4>
<p>Use <tt>__attribute__((acquired_after(...)))</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>
<h4 id="ts_elf">exclusive_lock_function(...)</h4>
<p>Use <tt>__attribute__((exclusive_lock_function(...)))</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>this</tt> of the
enclosing object.</p>
<h4 id="ts_slf">shared_lock_function(...)</h4>
<p>Use <tt>__attribute__((shared_lock_function(...)))</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>this</tt> of the enclosing object.</p>
<h4 id="ts_etf">exclusive_trylock_function(...)</h4>
<p>Use <tt>__attribute__((exclusive_lock_function(...)))</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>this</tt> of the
enclosing object.</p>
<h4 id="ts_stf">shared_trylock_function(...)</h4>
<p>Use <tt>__attribute__((shared_lock_function(...)))</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>this</tt> of the enclosing object.</p>
<h4 id="ts_uf">unlock_function(...)</h4>
<p>Use <tt>__attribute__((unlock_function(...)))</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>this</tt> of the enclosing object.</p>
<h4 id="ts_lr">lock_returned(l)</h4>
<p>Use <tt>__attribute__((lock_returned(l)))</tt> on a function
declaration to specify that the function returns lock <tt>l</tt> (<tt>l</tt>
must be of lockable type). This annotation is used to aid in resolving lock
expressions.</p>
<h4 id="ts_le">locks_excluded(...)</h4>
<p>Use <tt>__attribute__((locks_excluded(...)))</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>
<h4 id="ts_elr">exclusive_locks_required(...)</h4>
<p>Use <tt>__attribute__((exclusive_locks_required(...)))</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>
<h4 id="ts_slr">shared_locks_required(...)</h4>
<p>Use <tt>__attribute__((shared_locks_required(...)))</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>
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