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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd" [
<!ENTITY % threads.entities SYSTEM "entities.xml">
%threads.entities;
]>
<header name="boost/thread/once.hpp"
last-revision="$Date: 2004/07/17 04:33:59 $">
<macro name="BOOST_ONCE_INIT">
<purpose>The <functionname>call_once</functionname> function and
<code>once_flag</code> type (statically initialized to
<macroname>BOOST_ONCE_INIT</macroname>) can be used to run a
routine exactly once. This can be used to initialize data in a
<link linkend="threads.glossary.thread-safe">thread-safe</link>
manner.</purpose>
<description>The implementation-defined macro
<macroname>BOOST_ONCE_INIT</macroname> is a constant value used to
initialize <code>once_flag</code> instances to indicate that the
logically associated routine has not been run yet. See
<functionname>call_once</functionname> for more details.</description>
</macro>
<namespace name="boost">
<typedef name="once_flag">
<purpose>The <functionname>call_once</functionname> function and
<code>once_flag</code> type (statically initialized to
<macroname>BOOST_ONCE_INIT</macroname>) can be used to run a
routine exactly once. This can be used to initialize data in a
<link linkend="threads.glossary.thread-safe">thread-safe</link>
manner.</purpose>
<description>The implementation-defined type <code>once_flag</code>
is used as a flag to insure a routine is called only once.
Instances of this type should be statically initialized to
<macroname>BOOST_ONCE_INIT</macroname>. See
<functionname>call_once</functionname> for more details.
</description>
<type><emphasis>implementation-defined</emphasis></type>
</typedef>
<function name="call_once">
<purpose>The <functionname>call_once</functionname> function and
<code>once_flag</code> type (statically initialized to
<macroname>BOOST_ONCE_INIT</macroname>) can be used to run a
routine exactly once. This can be used to initialize data in a
<link linkend="threads.glossary.thread-safe">thread-safe</link>
manner.</purpose>
<description>
<para>Example usage is as follows:</para>
<para>
<programlisting>//Example usage:
boost::once_flag once = BOOST_ONCE_INIT;
void init()
{
//...
}
void thread_proc()
{
boost::call_once(&init, once);
}</programlisting>
</para></description>
<parameter name="func">
<paramtype>void (*func)()</paramtype>
</parameter>
<parameter name="flag">
<paramtype>once_flag&</paramtype>
</parameter>
<requires>The function <code>func</code> shall not throw
exceptions.</requires>
<effects>As if (in an atomic fashion):
<code>if (flag == BOOST_ONCE_INIT) func();</code></effects>
<postconditions><code>flag != BOOST_ONCE_INIT</code>
</postconditions>
</function>
</namespace>
</header>
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