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study/Ref-docs/POSIX/susv3/functions/pthread_mutex_lock.html
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study/Ref-docs/POSIX/susv3/functions/pthread_mutex_lock.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 -->
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>pthread_mutex_lock</title>
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</head>
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<body bgcolor="white">
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<script type="text/javascript" language="JavaScript" src="../jscript/codes.js">
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</script>
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<basefont size="3"> <a name="pthread_mutex_lock"></a> <a name="tag_03_540"></a><!-- pthread_mutex_lock -->
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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_540_01"></a>NAME</h4>
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<blockquote>pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock and unlock a mutex</blockquote>
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<h4><a name="tag_03_540_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_mutex_lock(pthread_mutex_t *</tt><i>mutex</i><tt>);<br>
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int pthread_mutex_trylock(pthread_mutex_t *</tt><i>mutex</i><tt>);<br>
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int pthread_mutex_unlock(pthread_mutex_t *</tt><i>mutex</i><tt>); <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_540_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The mutex object referenced by <i>mutex</i> shall be locked by calling <i>pthread_mutex_lock</i>(). If the mutex is already
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locked, the calling thread shall block until the mutex becomes available. This operation shall return with the mutex object
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referenced by <i>mutex</i> in the locked state with the calling thread as its owner.</p>
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<p><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not be provided. Attempting to relock the mutex causes
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deadlock. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior
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results.</p>
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<p>If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking shall be provided. If a thread attempts to relock a mutex
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that it has already locked, an error shall be returned. If a thread attempts to unlock a mutex that it has not locked or a mutex
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which is unlocked, an error shall be returned.</p>
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<p>If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex shall maintain the concept of a lock count. When a thread
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successfully acquires a mutex for the first time, the lock count shall be set to one. Every time a thread relocks this mutex, the
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lock count shall be incremented by one. Each time the thread unlocks the mutex, the lock count shall be decremented by one. When
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the lock count reaches zero, the mutex shall become available for other threads to acquire. If a thread attempts to unlock a mutex
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that it has not locked or a mutex which is unlocked, an error shall be returned.</p>
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<p>If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively lock the mutex results in undefined behavior. Attempting
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to unlock the mutex if it was not locked by the calling thread results in undefined behavior. Attempting to unlock the mutex if it
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is not locked results in undefined behavior. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p>The <i>pthread_mutex_trylock</i>() function shall be equivalent to <i>pthread_mutex_lock</i>(), except that if the mutex object
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referenced by <i>mutex</i> is currently locked (by any thread, including the current thread), the call shall return immediately. If
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the mutex type is PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling thread, the mutex lock count shall be
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incremented by one and the <i>pthread_mutex_trylock</i>() function shall immediately return success.</p>
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<p>The <i>pthread_mutex_unlock</i>() function shall release the mutex object referenced by <i>mutex</i>. <sup>[<a href=
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"javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> The manner
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in which a mutex is released is dependent upon the mutex's type attribute. <img src="../images/opt-end.gif" alt="[Option End]"
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border="0"> If there are threads blocked on the mutex object referenced by <i>mutex</i> when <i>pthread_mutex_unlock</i>() is
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called, resulting in the mutex becoming available, the scheduling policy shall determine which thread shall acquire the mutex.</p>
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<p><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become available when the count reaches zero and the calling
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thread no longer has any locks on this mutex.) <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p>If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread shall resume waiting
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for the mutex as if it was not interrupted.</p>
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</blockquote>
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<h4><a name="tag_03_540_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>If successful, the <i>pthread_mutex_lock</i>() and <i>pthread_mutex_unlock</i>() functions shall return zero; otherwise, an
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error number shall be returned to indicate the error.</p>
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<p>The <i>pthread_mutex_trylock</i>() function shall return zero if a lock on the mutex object referenced by <i>mutex</i> is
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acquired. Otherwise, an error number is returned to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_540_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>pthread_mutex_lock</i>() and <i>pthread_mutex_trylock</i>() functions shall fail if:</p>
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<dl compact>
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<dt>[EINVAL]</dt>
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<dd>The <i>mutex</i> was created with the protocol attribute having the value PTHREAD_PRIO_PROTECT and the calling thread's
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priority is higher than the mutex's current priority ceiling.</dd>
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</dl>
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<p>The <i>pthread_mutex_trylock</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EBUSY]</dt>
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<dd>The <i>mutex</i> could not be acquired because it was already locked.</dd>
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</dl>
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<p>The <i>pthread_mutex_lock</i>(), <i>pthread_mutex_trylock</i>(), and <i>pthread_mutex_unlock</i>() functions may fail if:</p>
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<dl compact>
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<dt>[EINVAL]</dt>
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<dd>The value specified by <i>mutex</i> does not refer to an initialized mutex object.</dd>
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<dt>[EAGAIN]</dt>
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<dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The mutex could not be acquired because the maximum number of recursive locks for <i>mutex</i> has been exceeded. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></dd>
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</dl>
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<p>The <i>pthread_mutex_lock</i>() function may fail if:</p>
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<dl compact>
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<dt>[EDEADLK]</dt>
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<dd>The current thread already owns the mutex.</dd>
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</dl>
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<p>The <i>pthread_mutex_unlock</i>() function may fail if:</p>
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<dl compact>
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<dt>[EPERM]</dt>
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<dd>The current thread does not own the mutex.</dd>
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</dl>
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<p>These functions 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_540_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_540_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_540_08"></a>RATIONALE</h4>
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<blockquote>
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<p>Mutex objects are intended to serve as a low-level primitive from which other thread synchronization functions can be built. As
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such, the implementation of mutexes should be as efficient as possible, and this has ramifications on the features available at the
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interface.</p>
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<p>The mutex functions and the particular default settings of the mutex attributes have been motivated by the desire to not
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preclude fast, inlined implementations of mutex locking and unlocking.</p>
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<p>For example, deadlocking on a double-lock is explicitly allowed behavior in order to avoid requiring more overhead in the basic
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mechanism than is absolutely necessary. (More "friendly" mutexes that detect deadlock or that allow multiple locking by the same
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thread are easily constructed by the user via the other mechanisms provided. For example, <a href=
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"../functions/pthread_self.html"><i>pthread_self</i>()</a> can be used to record mutex ownership.) Implementations might also
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choose to provide such extended features as options via special mutex attributes.</p>
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<p>Since most attributes only need to be checked when a thread is going to be blocked, the use of attributes does not slow the
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(common) mutex-locking case.</p>
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<p>Likewise, while being able to extract the thread ID of the owner of a mutex might be desirable, it would require storing the
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current thread ID when each mutex is locked, and this could incur unacceptable levels of overhead. Similar arguments apply to a
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<i>mutex_tryunlock</i> operation.</p>
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</blockquote>
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<h4><a name="tag_03_540_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_540_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="pthread_mutex_destroy.html"><i>pthread_mutex_destroy</i>()</a> , <a href=
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"pthread_mutex_timedlock.html"><i>pthread_mutex_timedlock</i>()</a> , the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/pthread.h.html"><i><pthread.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_540_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_540_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>pthread_mutex_lock</i>(), <i>pthread_mutex_trylock</i>(), and <i>pthread_mutex_unlock</i>() functions are marked as part
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of the Threads option.</p>
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<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>The behavior when attempting to relock a mutex is defined.</p>
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</li>
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</ul>
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<p>The <a href="../functions/pthread_mutex_timedlock.html"><i>pthread_mutex_timedlock</i>()</a> function is added to the SEE ALSO
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section for alignment with IEEE Std 1003.1d-1999.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<hr size="2" noshade>
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<center><font size="2"><!--footer start-->
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POSIX ® is a registered Trademark of The IEEE.<br>
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