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