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<title>Using the GNU Compiler Collection (GCC): __atomic Builtins</title>
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<a name="g_t_005f_005fatomic-Builtins"></a>
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<p>
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Next: <a href="Integer-Overflow-Builtins.html#Integer-Overflow-Builtins" accesskey="n" rel="next">Integer Overflow Builtins</a>, Previous: <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" accesskey="p" rel="prev">__sync Builtins</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Built_002din-Functions-for-Memory-Model-Aware-Atomic-Operations"></a>
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<h3 class="section">6.52 Built-in Functions for Memory Model Aware Atomic Operations</h3>
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<p>The following built-in functions approximately match the requirements
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for the C++11 memory model. They are all
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identified by being prefixed with ‘<samp>__atomic</samp>’ and most are
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overloaded so that they work with multiple types.
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</p>
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<p>These functions are intended to replace the legacy ‘<samp>__sync</samp>’
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builtins. The main difference is that the memory order that is requested
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is a parameter to the functions. New code should always use the
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‘<samp>__atomic</samp>’ builtins rather than the ‘<samp>__sync</samp>’ builtins.
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</p>
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<p>Note that the ‘<samp>__atomic</samp>’ builtins assume that programs will
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conform to the C++11 memory model. In particular, they assume
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that programs are free of data races. See the C++11 standard for
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detailed requirements.
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</p>
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<p>The ‘<samp>__atomic</samp>’ builtins can be used with any integral scalar or
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pointer type that is 1, 2, 4, or 8 bytes in length. 16-byte integral
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types are also allowed if ‘<samp>__int128</samp>’ (see <a href="_005f_005fint128.html#g_t_005f_005fint128">__int128</a>) is
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supported by the architecture.
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</p>
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<p>The four non-arithmetic functions (load, store, exchange, and
|
compare_exchange) all have a generic version as well. This generic
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version works on any data type. It uses the lock-free built-in function
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if the specific data type size makes that possible; otherwise, an
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external call is left to be resolved at run time. This external call is
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the same format with the addition of a ‘<samp>size_t</samp>’ parameter inserted
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as the first parameter indicating the size of the object being pointed to.
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All objects must be the same size.
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</p>
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<p>There are 6 different memory orders that can be specified. These map
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to the C++11 memory orders with the same names, see the C++11 standard
|
or the <a href="http://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync">GCC wiki
|
on atomic synchronization</a> for detailed definitions. Individual
|
targets may also support additional memory orders for use on specific
|
architectures. Refer to the target documentation for details of
|
these.
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</p>
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<p>An atomic operation can both constrain code motion and
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be mapped to hardware instructions for synchronization between threads
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(e.g., a fence). To which extent this happens is controlled by the
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memory orders, which are listed here in approximately ascending order of
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strength. The description of each memory order is only meant to roughly
|
illustrate the effects and is not a specification; see the C++11
|
memory model for precise semantics.
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</p>
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<dl compact="compact">
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<dt><code>__ATOMIC_RELAXED</code></dt>
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<dd><p>Implies no inter-thread ordering constraints.
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</p></dd>
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<dt><code>__ATOMIC_CONSUME</code></dt>
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<dd><p>This is currently implemented using the stronger <code>__ATOMIC_ACQUIRE</code>
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memory order because of a deficiency in C++11’s semantics for
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<code>memory_order_consume</code>.
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</p></dd>
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<dt><code>__ATOMIC_ACQUIRE</code></dt>
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<dd><p>Creates an inter-thread happens-before constraint from the release (or
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stronger) semantic store to this acquire load. Can prevent hoisting
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of code to before the operation.
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</p></dd>
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<dt><code>__ATOMIC_RELEASE</code></dt>
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<dd><p>Creates an inter-thread happens-before constraint to acquire (or stronger)
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semantic loads that read from this release store. Can prevent sinking
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of code to after the operation.
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</p></dd>
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<dt><code>__ATOMIC_ACQ_REL</code></dt>
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<dd><p>Combines the effects of both <code>__ATOMIC_ACQUIRE</code> and
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<code>__ATOMIC_RELEASE</code>.
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</p></dd>
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<dt><code>__ATOMIC_SEQ_CST</code></dt>
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<dd><p>Enforces total ordering with all other <code>__ATOMIC_SEQ_CST</code> operations.
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</p></dd>
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</dl>
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<p>Note that in the C++11 memory model, <em>fences</em> (e.g.,
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‘<samp>__atomic_thread_fence</samp>’) take effect in combination with other
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atomic operations on specific memory locations (e.g., atomic loads);
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operations on specific memory locations do not necessarily affect other
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operations in the same way.
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</p>
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<p>Target architectures are encouraged to provide their own patterns for
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each of the atomic built-in functions. If no target is provided, the original
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non-memory model set of ‘<samp>__sync</samp>’ atomic built-in functions are
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used, along with any required synchronization fences surrounding it in
|
order to achieve the proper behavior. Execution in this case is subject
|
to the same restrictions as those built-in functions.
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</p>
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<p>If there is no pattern or mechanism to provide a lock-free instruction
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sequence, a call is made to an external routine with the same parameters
|
to be resolved at run time.
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</p>
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<p>When implementing patterns for these built-in functions, the memory order
|
parameter can be ignored as long as the pattern implements the most
|
restrictive <code>__ATOMIC_SEQ_CST</code> memory order. Any of the other memory
|
orders execute correctly with this memory order but they may not execute as
|
efficiently as they could with a more appropriate implementation of the
|
relaxed requirements.
|
</p>
|
<p>Note that the C++11 standard allows for the memory order parameter to be
|
determined at run time rather than at compile time. These built-in
|
functions map any run-time value to <code>__ATOMIC_SEQ_CST</code> rather
|
than invoke a runtime library call or inline a switch statement. This is
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standard compliant, safe, and the simplest approach for now.
|
</p>
|
<p>The memory order parameter is a signed int, but only the lower 16 bits are
|
reserved for the memory order. The remainder of the signed int is reserved
|
for target use and should be 0. Use of the predefined atomic values
|
ensures proper usage.
|
</p>
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<dl>
|
<dt><a name="index-_005f_005fatomic_005fload_005fn"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_load_n</strong> <em>(<var>type</var> *ptr, int memorder)</em></dt>
|
<dd><p>This built-in function implements an atomic load operation. It returns the
|
contents of <code>*<var>ptr</var></code>.
|
</p>
|
<p>The valid memory order variants are
|
<code>__ATOMIC_RELAXED</code>, <code>__ATOMIC_SEQ_CST</code>, <code>__ATOMIC_ACQUIRE</code>,
|
and <code>__ATOMIC_CONSUME</code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fload"></a>Built-in Function: <em>void</em> <strong>__atomic_load</strong> <em>(<var>type</var> *ptr, <var>type</var> *ret, int memorder)</em></dt>
|
<dd><p>This is the generic version of an atomic load. It returns the
|
contents of <code>*<var>ptr</var></code> in <code>*<var>ret</var></code>.
|
</p>
|
</dd></dl>
|
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<dl>
|
<dt><a name="index-_005f_005fatomic_005fstore_005fn"></a>Built-in Function: <em>void</em> <strong>__atomic_store_n</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dd><p>This built-in function implements an atomic store operation. It writes
|
<code><var>val</var></code> into <code>*<var>ptr</var></code>.
|
</p>
|
<p>The valid memory order variants are
|
<code>__ATOMIC_RELAXED</code>, <code>__ATOMIC_SEQ_CST</code>, and <code>__ATOMIC_RELEASE</code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fstore"></a>Built-in Function: <em>void</em> <strong>__atomic_store</strong> <em>(<var>type</var> *ptr, <var>type</var> *val, int memorder)</em></dt>
|
<dd><p>This is the generic version of an atomic store. It stores the value
|
of <code>*<var>val</var></code> into <code>*<var>ptr</var></code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fexchange_005fn"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_exchange_n</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dd><p>This built-in function implements an atomic exchange operation. It writes
|
<var>val</var> into <code>*<var>ptr</var></code>, and returns the previous contents of
|
<code>*<var>ptr</var></code>.
|
</p>
|
<p>The valid memory order variants are
|
<code>__ATOMIC_RELAXED</code>, <code>__ATOMIC_SEQ_CST</code>, <code>__ATOMIC_ACQUIRE</code>,
|
<code>__ATOMIC_RELEASE</code>, and <code>__ATOMIC_ACQ_REL</code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fexchange"></a>Built-in Function: <em>void</em> <strong>__atomic_exchange</strong> <em>(<var>type</var> *ptr, <var>type</var> *val, <var>type</var> *ret, int memorder)</em></dt>
|
<dd><p>This is the generic version of an atomic exchange. It stores the
|
contents of <code>*<var>val</var></code> into <code>*<var>ptr</var></code>. The original value
|
of <code>*<var>ptr</var></code> is copied into <code>*<var>ret</var></code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fcompare_005fexchange_005fn"></a>Built-in Function: <em>bool</em> <strong>__atomic_compare_exchange_n</strong> <em>(<var>type</var> *ptr, <var>type</var> *expected, <var>type</var> desired, bool weak, int success_memorder, int failure_memorder)</em></dt>
|
<dd><p>This built-in function implements an atomic compare and exchange operation.
|
This compares the contents of <code>*<var>ptr</var></code> with the contents of
|
<code>*<var>expected</var></code>. If equal, the operation is a <em>read-modify-write</em>
|
operation that writes <var>desired</var> into <code>*<var>ptr</var></code>. If they are not
|
equal, the operation is a <em>read</em> and the current contents of
|
<code>*<var>ptr</var></code> are written into <code>*<var>expected</var></code>. <var>weak</var> is true
|
for weak compare_exchange, which may fail spuriously, and false for
|
the strong variation, which never fails spuriously. Many targets
|
only offer the strong variation and ignore the parameter. When in doubt, use
|
the strong variation.
|
</p>
|
<p>If <var>desired</var> is written into <code>*<var>ptr</var></code> then true is returned
|
and memory is affected according to the
|
memory order specified by <var>success_memorder</var>. There are no
|
restrictions on what memory order can be used here.
|
</p>
|
<p>Otherwise, false is returned and memory is affected according
|
to <var>failure_memorder</var>. This memory order cannot be
|
<code>__ATOMIC_RELEASE</code> nor <code>__ATOMIC_ACQ_REL</code>. It also cannot be a
|
stronger order than that specified by <var>success_memorder</var>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fcompare_005fexchange"></a>Built-in Function: <em>bool</em> <strong>__atomic_compare_exchange</strong> <em>(<var>type</var> *ptr, <var>type</var> *expected, <var>type</var> *desired, bool weak, int success_memorder, int failure_memorder)</em></dt>
|
<dd><p>This built-in function implements the generic version of
|
<code>__atomic_compare_exchange</code>. The function is virtually identical to
|
<code>__atomic_compare_exchange_n</code>, except the desired value is also a
|
pointer.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fadd_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_add_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005fsub_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_sub_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005fand_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_and_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005fxor_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_xor_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005for_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_or_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005fnand_005ffetch"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_nand_fetch</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dd><p>These built-in functions perform the operation suggested by the name, and
|
return the result of the operation. Operations on pointer arguments are
|
performed as if the operands were of the <code>uintptr_t</code> type. That is,
|
they are not scaled by the size of the type to which the pointer points.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">{ *ptr <var>op</var>= val; return *ptr; }
|
</pre></div>
|
|
<p>The object pointed to by the first argument must be of integer or pointer
|
type. It must not be a Boolean type. All memory orders are valid.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005fadd"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_add</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005fsub"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_sub</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005fand"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_and</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005fxor"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_xor</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005for"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_or</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dt><a name="index-_005f_005fatomic_005ffetch_005fnand"></a>Built-in Function: <em><var>type</var></em> <strong>__atomic_fetch_nand</strong> <em>(<var>type</var> *ptr, <var>type</var> val, int memorder)</em></dt>
|
<dd><p>These built-in functions perform the operation suggested by the name, and
|
return the value that had previously been in <code>*<var>ptr</var></code>. Operations
|
on pointer arguments are performed as if the operands were of
|
the <code>uintptr_t</code> type. That is, they are not scaled by the size of
|
the type to which the pointer points.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">{ tmp = *ptr; *ptr <var>op</var>= val; return tmp; }
|
</pre></div>
|
|
<p>The same constraints on arguments apply as for the corresponding
|
<code>__atomic_op_fetch</code> built-in functions. All memory orders are valid.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005ftest_005fand_005fset"></a>Built-in Function: <em>bool</em> <strong>__atomic_test_and_set</strong> <em>(void *ptr, int memorder)</em></dt>
|
<dd>
|
<p>This built-in function performs an atomic test-and-set operation on
|
the byte at <code>*<var>ptr</var></code>. The byte is set to some implementation
|
defined nonzero “set” value and the return value is <code>true</code> if and only
|
if the previous contents were “set”.
|
It should be only used for operands of type <code>bool</code> or <code>char</code>. For
|
other types only part of the value may be set.
|
</p>
|
<p>All memory orders are valid.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fclear"></a>Built-in Function: <em>void</em> <strong>__atomic_clear</strong> <em>(bool *ptr, int memorder)</em></dt>
|
<dd>
|
<p>This built-in function performs an atomic clear operation on
|
<code>*<var>ptr</var></code>. After the operation, <code>*<var>ptr</var></code> contains 0.
|
It should be only used for operands of type <code>bool</code> or <code>char</code> and
|
in conjunction with <code>__atomic_test_and_set</code>.
|
For other types it may only clear partially. If the type is not <code>bool</code>
|
prefer using <code>__atomic_store</code>.
|
</p>
|
<p>The valid memory order variants are
|
<code>__ATOMIC_RELAXED</code>, <code>__ATOMIC_SEQ_CST</code>, and
|
<code>__ATOMIC_RELEASE</code>.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fthread_005ffence"></a>Built-in Function: <em>void</em> <strong>__atomic_thread_fence</strong> <em>(int memorder)</em></dt>
|
<dd>
|
<p>This built-in function acts as a synchronization fence between threads
|
based on the specified memory order.
|
</p>
|
<p>All memory orders are valid.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005fsignal_005ffence"></a>Built-in Function: <em>void</em> <strong>__atomic_signal_fence</strong> <em>(int memorder)</em></dt>
|
<dd>
|
<p>This built-in function acts as a synchronization fence between a thread
|
and signal handlers based in the same thread.
|
</p>
|
<p>All memory orders are valid.
|
</p>
|
</dd></dl>
|
|
<dl>
|
<dt><a name="index-_005f_005fatomic_005falways_005flock_005ffree"></a>Built-in Function: <em>bool</em> <strong>__atomic_always_lock_free</strong> <em>(size_t size, void *ptr)</em></dt>
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<dd>
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<p>This built-in function returns true if objects of <var>size</var> bytes always
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generate lock-free atomic instructions for the target architecture.
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<var>size</var> must resolve to a compile-time constant and the result also
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resolves to a compile-time constant.
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</p>
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<p><var>ptr</var> is an optional pointer to the object that may be used to determine
|
alignment. A value of 0 indicates typical alignment should be used. The
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compiler may also ignore this parameter.
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</p>
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<div class="smallexample">
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<pre class="smallexample">if (__atomic_always_lock_free (sizeof (long long), 0))
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</pre></div>
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</dd></dl>
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<dl>
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<dt><a name="index-_005f_005fatomic_005fis_005flock_005ffree"></a>Built-in Function: <em>bool</em> <strong>__atomic_is_lock_free</strong> <em>(size_t size, void *ptr)</em></dt>
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<dd>
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<p>This built-in function returns true if objects of <var>size</var> bytes always
|
generate lock-free atomic instructions for the target architecture. If
|
the built-in function is not known to be lock-free, a call is made to a
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runtime routine named <code>__atomic_is_lock_free</code>.
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</p>
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<p><var>ptr</var> is an optional pointer to the object that may be used to determine
|
alignment. A value of 0 indicates typical alignment should be used. The
|
compiler may also ignore this parameter.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Integer-Overflow-Builtins.html#Integer-Overflow-Builtins" accesskey="n" rel="next">Integer Overflow Builtins</a>, Previous: <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" accesskey="p" rel="prev">__sync Builtins</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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