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<title>pthread_once</title>
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<basefont size="3"> <a name="pthread_once"></a> <a name="tag_03_554"></a><!-- pthread_once -->
<!--header start-->
<center><font size="2">The Open Group Base Specifications Issue 6<br>
IEEE Std 1003.1-2001<br>
Copyright &copy; 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
<!--header end-->
<hr size="2" noshade>
<h4><a name="tag_03_554_01"></a>NAME</h4>
<blockquote>pthread_once - dynamic package initialization</blockquote>
<h4><a name="tag_03_554_02"></a>SYNOPSIS</h4>
<blockquote class="synopsis">
<div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt=
"[Option Start]" border="0"> #include &lt;<a href="../basedefs/pthread.h.html">pthread.h</a>&gt;<br>
<br>
int pthread_once(pthread_once_t *</tt><i>once_control</i><tt>,<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; void (*</tt><i>init_routine</i><tt>)(void));<br>
pthread_once_t</tt> <i>once_control</i> <tt>= PTHREAD_ONCE_INIT; <img src="../images/opt-end.gif" alt="[Option End]" border=
"0"></tt></code></div>
<tt><br>
</tt></blockquote>
<h4><a name="tag_03_554_03"></a>DESCRIPTION</h4>
<blockquote>
<p>The first call to <i>pthread_once</i>() by any thread in a process, with a given <i>once_control</i>, shall call the
<i>init_routine</i> with no arguments. Subsequent calls of <i>pthread_once</i>() with the same <i>once_control</i> shall not call
the <i>init_routine</i>. On return from <i>pthread_once</i>(), <i>init_routine</i> shall have completed. The <i>once_control</i>
parameter shall determine whether the associated initialization routine has been called.</p>
<p>The <i>pthread_once</i>() function is not a cancelation point. However, if <i>init_routine</i> is a cancelation point and is
canceled, the effect on <i>once_control</i> shall be as if <i>pthread_once</i>() was never called.</p>
<p>The constant PTHREAD_ONCE_INIT is defined in the <a href="../basedefs/pthread.h.html"><i>&lt;pthread.h&gt;</i></a> header.</p>
<p>The behavior of <i>pthread_once</i>() is undefined if <i>once_control</i> has automatic storage duration or is not initialized
by PTHREAD_ONCE_INIT.</p>
</blockquote>
<h4><a name="tag_03_554_04"></a>RETURN VALUE</h4>
<blockquote>
<p>Upon successful completion, <i>pthread_once</i>() shall return zero; otherwise, an error number shall be returned to indicate
the error.</p>
</blockquote>
<h4><a name="tag_03_554_05"></a>ERRORS</h4>
<blockquote>
<p>The <i>pthread_once</i>() function may fail if:</p>
<dl compact>
<dt>[EINVAL]</dt>
<dd>If either <i>once_control</i> or <i>init_routine</i> is invalid.</dd>
</dl>
<p>The <i>pthread_once</i>() function shall not return an error code of [EINTR].</p>
</blockquote>
<hr>
<div class="box"><em>The following sections are informative.</em></div>
<h4><a name="tag_03_554_06"></a>EXAMPLES</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_554_07"></a>APPLICATION USAGE</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_554_08"></a>RATIONALE</h4>
<blockquote>
<p>Some C libraries are designed for dynamic initialization. That is, the global initialization for the library is performed when
the first procedure in the library is called. In a single-threaded program, this is normally implemented using a static variable
whose value is checked on entry to a routine, as follows:</p>
<pre>
<tt>static int random_is_initialized = 0;
extern int initialize_random();
<br>
int random_function()
{
if (random_is_initialized == 0) {
initialize_random();
random_is_initialized = 1;
}
... /* Operations performed after initialization. */
}
</tt>
</pre>
<p>To keep the same structure in a multi-threaded program, a new primitive is needed. Otherwise, library initialization has to be
accomplished by an explicit call to a library-exported initialization function prior to any use of the library.</p>
<p>For dynamic library initialization in a multi-threaded process, a simple initialization flag is not sufficient; the flag needs
to be protected against modification by multiple threads simultaneously calling into the library. Protecting the flag requires the
use of a mutex; however, mutexes have to be initialized before they are used. Ensuring that the mutex is only initialized once
requires a recursive solution to this problem.</p>
<p>The use of <i>pthread_once</i>() not only supplies an implementation-guaranteed means of dynamic initialization, it provides an
aid to the reliable construction of multi-threaded and realtime systems. The preceding example then becomes:</p>
<pre>
<tt>#include &lt;pthread.h&gt;
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern int initialize_random();
<br>
int random_function()
{
(void) pthread_once(&amp;random_is_initialized, initialize_random);
... /* Operations performed after initialization. */
}
</tt>
</pre>
<p>Note that a <b>pthread_once_t</b> cannot be an array because some compilers do not accept the construct
<b>&amp;&lt;array_name&gt;</b>.</p>
</blockquote>
<h4><a name="tag_03_554_09"></a>FUTURE DIRECTIONS</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_554_10"></a>SEE ALSO</h4>
<blockquote>
<p>The Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/pthread.h.html"><i>&lt;pthread.h&gt;</i></a></p>
</blockquote>
<h4><a name="tag_03_554_11"></a>CHANGE HISTORY</h4>
<blockquote>
<p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p>
</blockquote>
<h4><a name="tag_03_554_12"></a>Issue 6</h4>
<blockquote>
<p>The <i>pthread_once</i>() function is marked as part of the Threads option.</p>
<p>The [EINVAL] error is added as a may fail case for if either argument is invalid.</p>
</blockquote>
<div class="box"><em>End of informative text.</em></div>
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