source: downloads/boost_1_33_1/libs/thread/doc/once-ref.xml @ 12

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  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd" [
  <!ENTITY % threads.entities SYSTEM "entities.xml">
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<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(&amp;init, once);
}</programlisting>
                        </para></description>
                        
                        <parameter name="func">
                                <paramtype>void (*func)()</paramtype>
                        </parameter>
                        
                        <parameter name="flag">
                                <paramtype>once_flag&amp;</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>