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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="atomic.interface"></a><a class="link" href="interface.html" title="Programming interfaces">Programming interfaces</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="interface.html#atomic.interface.configuration">Configuration and building</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.interface_memory_order">Memory order</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.interface_atomic_object">Atomic objects</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.interface_fences">Fences</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.feature_macros">Feature testing macros</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.interface.configuration"></a><a class="link" href="interface.html#atomic.interface.configuration" title="Configuration and building">Configuration and building</a>
</h3></div></div></div>
<p>
The library contains header-only and compiled parts. The library is header-only
for lock-free cases but requires a separate binary to implement the lock-based
emulation. Users are able to detect whether linking to the compiled part
is required by checking the <a class="link" href="interface.html#atomic.interface.feature_macros" title="Feature testing macros">feature
macros</a>.
</p>
<p>
The following macros affect library behavior:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Macro
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_NO_CMPXCHG8B</span></code>
</p>
</td>
<td>
<p>
Affects 32-bit x86 Oracle Studio builds. When defined, the library
assumes the target CPU does not support <code class="computeroutput"><span class="identifier">cmpxchg8b</span></code>
instruction used to support 64-bit atomic operations. This is the
case with very old CPUs (pre-Pentium). The library does not perform
runtime detection of this instruction, so running the code that
uses 64-bit atomics on such CPUs will result in crashes, unless
this macro is defined. Note that the macro does not affect MSVC,
GCC and compatible compilers because the library infers this information
from the compiler-defined macros.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_NO_CMPXCHG16B</span></code>
</p>
</td>
<td>
<p>
Affects 64-bit x86 MSVC and Oracle Studio builds. When defined,
the library assumes the target CPU does not support <code class="computeroutput"><span class="identifier">cmpxchg16b</span></code> instruction used to
support 128-bit atomic operations. This is the case with some early
64-bit AMD CPUs, all Intel CPUs and current AMD CPUs support this
instruction. The library does not perform runtime detection of
this instruction, so running the code that uses 128-bit atomics
on such CPUs will result in crashes, unless this macro is defined.
Note that the macro does not affect GCC and compatible compilers
because the library infers this information from the compiler-defined
macros.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_NO_MFENCE</span></code>
</p>
</td>
<td>
<p>
Affects 32-bit x86 Oracle Studio builds. When defined, the library
assumes the target CPU does not support <code class="computeroutput"><span class="identifier">mfence</span></code>
instruction used to implement thread fences. This instruction was
added with SSE2 instruction set extension, which was available
in CPUs since Intel Pentium 4. The library does not perform runtime
detection of this instruction, so running the library code on older
CPUs will result in crashes, unless this macro is defined. Note
that the macro does not affect MSVC, GCC and compatible compilers
because the library infers this information from the compiler-defined
macros.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_FORCE_FALLBACK</span></code>
</p>
</td>
<td>
<p>
When defined, all operations are implemented with locks. This is
mostly used for testing and should not be used in real world projects.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_DYN_LINK</span></code>
and <code class="computeroutput"><span class="identifier">BOOST_ALL_DYN_LINK</span></code>
</p>
</td>
<td>
<p>
Control library linking. If defined, the library assumes dynamic
linking, otherwise static. The latter macro affects all Boost libraries,
not just <span class="bold"><strong>Boost.Atomic</strong></span>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_NO_LIB</span></code>
and <code class="computeroutput"><span class="identifier">BOOST_ALL_NO_LIB</span></code>
</p>
</td>
<td>
<p>
Control library auto-linking on Windows. When defined, disables
auto-linking. The latter macro affects all Boost libraries, not
just <span class="bold"><strong>Boost.Atomic</strong></span>.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
Besides macros, it is important to specify the correct compiler options for
the target CPU. With GCC and compatible compilers this affects whether particular
atomic operations are lock-free or not.
</p>
<p>
Boost building process is described in the <a href="http://www.boost.org/doc/libs/release/more/getting_started/" target="_top">Getting
Started guide</a>. For example, you can build <span class="bold"><strong>Boost.Atomic</strong></span>
with the following command line:
</p>
<pre class="programlisting">bjam --with-atomic variant=release instruction-set=core2 stage
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.interface.interface_memory_order"></a><a class="link" href="interface.html#atomic.interface.interface_memory_order" title="Memory order">Memory order</a>
</h3></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">memory_order</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
The enumeration <code class="literal">boost::memory_order</code> defines the following
values to represent memory ordering constraints:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Constant
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_relaxed</span></code>
</p>
</td>
<td>
<p>
No ordering constraint. Informally speaking, following operations
may be reordered before, preceding operations may be reordered
after the atomic operation. This constraint is suitable only when
either a) further operations do not depend on the outcome of the
atomic operation or b) ordering is enforced through stand-alone
<code class="computeroutput"><span class="identifier">atomic_thread_fence</span></code>
operations. The operation on the atomic value itself is still atomic
though.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_release</span></code>
</p>
</td>
<td>
<p>
Perform <code class="computeroutput"><span class="identifier">release</span></code>
operation. Informally speaking, prevents all preceding memory operations
to be reordered past this point.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_acquire</span></code>
</p>
</td>
<td>
<p>
Perform <code class="computeroutput"><span class="identifier">acquire</span></code>
operation. Informally speaking, prevents succeeding memory operations
to be reordered before this point.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_consume</span></code>
</p>
</td>
<td>
<p>
Perform <code class="computeroutput"><span class="identifier">consume</span></code>
operation. More relaxed (and on some architectures more efficient)
than <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code>
as it only affects succeeding operations that are computationally-dependent
on the value retrieved from an atomic variable.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_acq_rel</span></code>
</p>
</td>
<td>
<p>
Perform both <code class="computeroutput"><span class="identifier">release</span></code>
and <code class="computeroutput"><span class="identifier">acquire</span></code> operation
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">memory_order_seq_cst</span></code>
</p>
</td>
<td>
<p>
Enforce sequential consistency. Implies <code class="computeroutput"><span class="identifier">memory_order_acq_rel</span></code>,
but additionally enforces total order for all operations such qualified.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
See section <a class="link" href="thread_coordination.html" title="Thread coordination using Boost.Atomic"><span class="emphasis"><em>happens-before</em></span></a>
for explanation of the various ordering constraints.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.interface.interface_atomic_object"></a><a class="link" href="interface.html#atomic.interface.interface_atomic_object" title="Atomic objects">Atomic objects</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_generic"><code class="literal">boost::atomic<<span class="emphasis"><em>T</em></span>></code>
template class</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_integral"><code class="literal">boost::atomic<<span class="emphasis"><em>integral</em></span>></code>
template class</a></span></dt>
<dt><span class="section"><a href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_pointer"><code class="literal">boost::atomic<<span class="emphasis"><em>pointer</em></span>></code>
template class</a></span></dt>
</dl></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
<code class="literal">boost::atomic<<span class="emphasis"><em>T</em></span>></code> provides methods
for atomically accessing variables of a suitable type <code class="literal"><span class="emphasis"><em>T</em></span></code>.
The type is suitable if it is <span class="emphasis"><em>trivially copyable</em></span> (3.9/9
[basic.types]). Following are examples of the types compatible with this
requirement:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
a scalar type (e.g. integer, boolean, enum or pointer type)
</li>
<li class="listitem">
a <code class="literal">class</code> or <code class="literal">struct</code> that has no non-trivial
copy or move constructors or assignment operators, has a trivial destructor,
and that is comparable via <code class="literal">memcmp</code>.
</li>
</ul></div>
<p>
Note that classes with virtual functions or virtual base classes do not satisfy
the requirements. Also be warned that structures with "padding"
between data members may compare non-equal via <code class="literal">memcmp</code>
even though all members are equal.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="atomic.interface.interface_atomic_object.interface_atomic_generic"></a><a class="link" href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_generic" title="boost::atomic<T> template class"><code class="literal">boost::atomic<<span class="emphasis"><em>T</em></span>></code>
template class</a>
</h4></div></div></div>
<p>
All atomic objects supports the following operations:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Syntax
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">atomic</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
Initialize to an unspecified value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">atomic</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">initial_value</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Initialize to <code class="literal">initial_value</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">is_lock_free</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
Checks if the atomic object is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">load</span><span class="special">(</span><span class="identifier">memory_order</span>
<span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Return current value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">store</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">value</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Write new value to atomic variable
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">exchange</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">new_value</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Exchange current value with <code class="computeroutput"><span class="identifier">new_value</span></code>,
returning current value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">compare_exchange_weak</span><span class="special">(</span><span class="identifier">T</span>
<span class="special">&</span> <span class="identifier">expected</span><span class="special">,</span> <span class="identifier">T</span>
<span class="identifier">desired</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Compare current value with <code class="computeroutput"><span class="identifier">expected</span></code>,
change it to <code class="computeroutput"><span class="identifier">desired</span></code>
if matches. Returns <code class="computeroutput"><span class="keyword">true</span></code>
if an exchange has been performed, and always writes the previous
value back in <code class="computeroutput"><span class="identifier">expected</span></code>.
May fail spuriously, so must generally be retried in a loop.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">compare_exchange_weak</span><span class="special">(</span><span class="identifier">T</span>
<span class="special">&</span> <span class="identifier">expected</span><span class="special">,</span> <span class="identifier">T</span>
<span class="identifier">desired</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">success_order</span><span class="special">,</span> <span class="identifier">memory_order</span>
<span class="identifier">failure_order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Compare current value with <code class="computeroutput"><span class="identifier">expected</span></code>,
change it to <code class="computeroutput"><span class="identifier">desired</span></code>
if matches. Returns <code class="computeroutput"><span class="keyword">true</span></code>
if an exchange has been performed, and always writes the previous
value back in <code class="computeroutput"><span class="identifier">expected</span></code>.
May fail spuriously, so must generally be retried in a loop.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">compare_exchange_strong</span><span class="special">(</span><span class="identifier">T</span>
<span class="special">&</span> <span class="identifier">expected</span><span class="special">,</span> <span class="identifier">T</span>
<span class="identifier">desired</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Compare current value with <code class="computeroutput"><span class="identifier">expected</span></code>,
change it to <code class="computeroutput"><span class="identifier">desired</span></code>
if matches. Returns <code class="computeroutput"><span class="keyword">true</span></code>
if an exchange has been performed, and always writes the previous
value back in <code class="computeroutput"><span class="identifier">expected</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">compare_exchange_strong</span><span class="special">(</span><span class="identifier">T</span>
<span class="special">&</span> <span class="identifier">expected</span><span class="special">,</span> <span class="identifier">T</span>
<span class="identifier">desired</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">success_order</span><span class="special">,</span> <span class="identifier">memory_order</span>
<span class="identifier">failure_order</span><span class="special">))</span></code>
</p>
</td>
<td>
<p>
Compare current value with <code class="computeroutput"><span class="identifier">expected</span></code>,
change it to <code class="computeroutput"><span class="identifier">desired</span></code>
if matches. Returns <code class="computeroutput"><span class="keyword">true</span></code>
if an exchange has been performed, and always writes the previous
value back in <code class="computeroutput"><span class="identifier">expected</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
<code class="computeroutput"><span class="identifier">order</span></code> always has <code class="computeroutput"><span class="identifier">memory_order_seq_cst</span></code> as default parameter.
</p>
<p>
The <code class="computeroutput"><span class="identifier">compare_exchange_weak</span></code>/<code class="computeroutput"><span class="identifier">compare_exchange_strong</span></code> variants taking
four parameters differ from the three parameter variants in that they allow
a different memory ordering constraint to be specified in case the operation
fails.
</p>
<p>
In addition to these explicit operations, each <code class="literal">atomic<<span class="emphasis"><em>T</em></span>></code>
object also supports implicit <code class="literal">store</code> and <code class="literal">load</code>
through the use of "assignment" and "conversion to <code class="literal">T</code>"
operators. Avoid using these operators, as they do not allow explicit specification
of a memory ordering constraint.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="atomic.interface.interface_atomic_object.interface_atomic_integral"></a><a class="link" href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_integral" title="boost::atomic<integral> template class"><code class="literal">boost::atomic<<span class="emphasis"><em>integral</em></span>></code>
template class</a>
</h4></div></div></div>
<p>
In addition to the operations listed in the previous section, <code class="literal">boost::atomic<<span class="emphasis"><em>I</em></span>></code>
for integral types <code class="literal"><span class="emphasis"><em>I</em></span></code> supports the
following operations:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Syntax
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_add</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Add <code class="computeroutput"><span class="identifier">v</span></code> to variable,
returning previous value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_sub</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Subtract <code class="computeroutput"><span class="identifier">v</span></code> from
variable, returning previous value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_and</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Apply bit-wise "and" with <code class="computeroutput"><span class="identifier">v</span></code>
to variable, returning previous value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_or</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Apply bit-wise "or" with <code class="computeroutput"><span class="identifier">v</span></code>
to variable, returning previous value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_xor</span><span class="special">(</span><span class="identifier">T</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Apply bit-wise "xor" with <code class="computeroutput"><span class="identifier">v</span></code>
to variable, returning previous value
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
<code class="computeroutput"><span class="identifier">order</span></code> always has <code class="computeroutput"><span class="identifier">memory_order_seq_cst</span></code> as default parameter.
</p>
<p>
In addition to these explicit operations, each <code class="literal">boost::atomic<<span class="emphasis"><em>I</em></span>></code>
object also supports implicit pre-/post- increment/decrement, as well as
the operators <code class="computeroutput"><span class="special">+=</span></code>, <code class="computeroutput"><span class="special">-=</span></code>, <code class="computeroutput"><span class="special">&=</span></code>,
<code class="computeroutput"><span class="special">|=</span></code> and <code class="computeroutput"><span class="special">^=</span></code>.
Avoid using these operators, as they do not allow explicit specification
of a memory ordering constraint.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="atomic.interface.interface_atomic_object.interface_atomic_pointer"></a><a class="link" href="interface.html#atomic.interface.interface_atomic_object.interface_atomic_pointer" title="boost::atomic<pointer> template class"><code class="literal">boost::atomic<<span class="emphasis"><em>pointer</em></span>></code>
template class</a>
</h4></div></div></div>
<p>
In addition to the operations applicable to all atomic object, <code class="literal">boost::atomic<<span class="emphasis"><em>P</em></span>></code>
for pointer types <code class="literal"><span class="emphasis"><em>P</em></span></code> (other than
<code class="literal">void</code> pointers) support the following operations:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Syntax
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_add</span><span class="special">(</span><span class="identifier">ptrdiff_t</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Add <code class="computeroutput"><span class="identifier">v</span></code> to variable,
returning previous value
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">fetch_sub</span><span class="special">(</span><span class="identifier">ptrdiff_t</span>
<span class="identifier">v</span><span class="special">,</span>
<span class="identifier">memory_order</span> <span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Subtract <code class="computeroutput"><span class="identifier">v</span></code> from
variable, returning previous value
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
<code class="computeroutput"><span class="identifier">order</span></code> always has <code class="computeroutput"><span class="identifier">memory_order_seq_cst</span></code> as default parameter.
</p>
<p>
In addition to these explicit operations, each <code class="literal">boost::atomic<<span class="emphasis"><em>P</em></span>></code>
object also supports implicit pre-/post- increment/decrement, as well as
the operators <code class="computeroutput"><span class="special">+=</span></code>, <code class="computeroutput"><span class="special">-=</span></code>. Avoid using these operators, as they
do not allow explicit specification of a memory ordering constraint.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.interface.interface_fences"></a><a class="link" href="interface.html#atomic.interface.interface_fences" title="Fences">Fences</a>
</h3></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">/</span><span class="identifier">fences</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Syntax
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">atomic_thread_fence</span><span class="special">(</span><span class="identifier">memory_order</span>
<span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Issue fence for coordination with other threads.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">atomic_signal_fence</span><span class="special">(</span><span class="identifier">memory_order</span>
<span class="identifier">order</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Issue fence for coordination with signal handler (only in same
thread).
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.interface.feature_macros"></a><a class="link" href="interface.html#atomic.interface.feature_macros" title="Feature testing macros">Feature testing macros</a>
</h3></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">/</span><span class="identifier">capabilities</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
<span class="bold"><strong>Boost.Atomic</strong></span> defines a number of macros
to allow compile-time detection whether an atomic data type is implemented
using "true" atomic operations, or whether an internal "lock"
is used to provide atomicity. The following macros will be defined to <code class="computeroutput"><span class="number">0</span></code> if operations on the data type always require
a lock, to <code class="computeroutput"><span class="number">1</span></code> if operations on
the data type may sometimes require a lock, and to <code class="computeroutput"><span class="number">2</span></code>
if they are always lock-free:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Macro
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_FLAG_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic_flag</span></code>
is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_BOOL_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_CHAR_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_CHAR16_T_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">char16_t</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_CHAR32_T_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">char32_t</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_WCHAR_T_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">wchar_t</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_SHORT_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">short</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_LONG_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> (including signed/unsigned
variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_LLONG_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">long</span>
<span class="keyword">long</span><span class="special">></span></code>
(including signed/unsigned variants) is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_ADDRESS_LOCK_FREE</span></code>
or <code class="computeroutput"><span class="identifier">BOOST_ATOMIC_POINTER_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">T</span>
<span class="special">*></span></code> is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_THREAD_FENCE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic_thread_fence</span></code>
function is lock-free
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_SIGNAL_FENCE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic_signal_fence</span></code>
function is lock-free
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
In addition to these standard macros, <span class="bold"><strong>Boost.Atomic</strong></span>
also defines a number of extension macros, which can also be useful. Like
the standard ones, these macros are defined to values <code class="computeroutput"><span class="number">0</span></code>,
<code class="computeroutput"><span class="number">1</span></code> and <code class="computeroutput"><span class="number">2</span></code>
to indicate whether the corresponding operations are lock-free or not.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Macro
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT8_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">int8_type</span><span class="special">></span></code> is lock-free.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT16_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">int16_type</span><span class="special">></span></code> is lock-free.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT32_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">int32_type</span><span class="special">></span></code> is lock-free.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT64_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">int64_type</span><span class="special">></span></code> is lock-free.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_INT128_LOCK_FREE</span></code>
</p>
</td>
<td>
<p>
Indicate whether <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">int128_type</span><span class="special">></span></code> is lock-free.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">BOOST_ATOMIC_NO_ATOMIC_FLAG_INIT</span></code>
</p>
</td>
<td>
<p>
Defined after including <code class="computeroutput"><span class="identifier">atomic_flag</span><span class="special">.</span><span class="identifier">hpp</span></code>,
if the implementation does not support the <code class="computeroutput"><span class="identifier">BOOST_ATOMIC_FLAG_INIT</span></code>
macro for static initialization of <code class="computeroutput"><span class="identifier">atomic_flag</span></code>.
This macro is typically defined for pre-C++11 compilers.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
In the table above, <code class="computeroutput"><span class="identifier">intN_type</span></code>
is a type that fits storage of contiguous <code class="computeroutput"><span class="identifier">N</span></code>
bits, suitably aligned for atomic operations.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013 Andrey Semashev<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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