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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>pthread_rwlock_rdlock</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="pthread_rwlock_rdlock"></a> <a name="tag_03_556"></a><!-- pthread_rwlock_rdlock --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_556_01"></a>NAME</h4><blockquote>pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write lock object for reading</blockquote><h4><a name="tag_03_556_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 <<a href="../basedefs/pthread.h.html">pthread.h</a>><br><br> int pthread_rwlock_rdlock(pthread_rwlock_t</tt> <i>*rwlock</i><tt>);<br> int pthread_rwlock_tryrdlock(pthread_rwlock_t</tt> <i>*rwlock</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]"border="0"></tt></code></div><tt><br></tt></blockquote><h4><a name="tag_03_556_03"></a>DESCRIPTION</h4><blockquote><p>The <i>pthread_rwlock_rdlock</i>() function shall apply a read lock to the read-write lock referenced by <i>rwlock</i>. Thecalling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock.</p><p><sup>[<a href="javascript:open_code('TPS')">TPS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">If the Thread Execution Scheduling option is supported, and the threads involved in the lock are executing with the schedulingpolicies SCHED_FIFO or SCHED_RR, the calling thread shall not acquire the lock if a writer holds the lock or if writers of higheror equal priority are blocked on the lock; otherwise, the calling thread shall acquire the lock. <img src="../images/opt-end.gif"alt="[Option End]" border="0"></p><p><sup>[<a href="javascript:open_code('TPS TSP')">TPS TSP</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"border="0"> If the Threads Execution Scheduling option is supported, and the threads involved in the lock are executing with theSCHED_SPORADIC scheduling policy, the calling thread shall not acquire the lock if a writer holds the lock or if writers of higheror equal priority are blocked on the lock; otherwise, the calling thread shall acquire the lock. <img src="../images/opt-end.gif"alt="[Option End]" border="0"></p><p>If the Thread Execution Scheduling option is not supported, it is implementation-defined whether the calling thread acquires thelock when a writer does not hold the lock and there are writers blocked on the lock. If a writer holds the lock, the calling threadshall not acquire the read lock. If the read lock is not acquired, the calling thread shall block until it can acquire the lock.The calling thread may deadlock if at the time the call is made it holds a write lock.</p><p>A thread may hold multiple concurrent read locks on <i>rwlock</i> (that is, successfully call the <i>pthread_rwlock_rdlock</i>()function <i>n</i> times). If so, the application shall ensure that the thread performs matching unlocks (that is, it calls the <ahref="../functions/pthread_rwlock_unlock.html"><i>pthread_rwlock_unlock</i>()</a> function <i>n</i> times).</p><p>The maximum number of simultaneous read locks that an implementation guarantees can be applied to a read-write lock shall beimplementation-defined. The <i>pthread_rwlock_rdlock</i>() function may fail if this maximum would be exceeded.</p><p>The <i>pthread_rwlock_tryrdlock</i>() function shall apply a read lock as in the <i>pthread_rwlock_rdlock</i>() function, withthe exception that the function shall fail if the equivalent <i>pthread_rwlock_rdlock</i>() call would have blocked the callingthread. In no case shall the <i>pthread_rwlock_tryrdlock</i>() function ever block; it always either acquires the lock or fails andreturns immediately.</p><p>Results are undefined if any of these functions are called with an uninitialized read-write lock.</p><p>If a signal is delivered to a thread waiting for a read-write lock for reading, upon return from the signal handler the threadresumes waiting for the read-write lock for reading as if it was not interrupted.</p></blockquote><h4><a name="tag_03_556_04"></a>RETURN VALUE</h4><blockquote><p>If successful, the <i>pthread_rwlock_rdlock</i>() function shall return zero; otherwise, an error number shall be returned toindicate the error.</p><p>The <i>pthread_rwlock_tryrdlock</i>() function shall return zero if the lock for reading on the read-write lock objectreferenced by <i>rwlock</i> is acquired. Otherwise, an error number shall be returned to indicate the error.</p></blockquote><h4><a name="tag_03_556_05"></a>ERRORS</h4><blockquote><p>The <i>pthread_rwlock_tryrdlock</i>() function shall fail if:</p><dl compact><dt>[EBUSY]</dt><dd>The read-write lock could not be acquired for reading because a writer holds the lock or a writer with the appropriate prioritywas blocked on it.</dd></dl><p>The <i>pthread_rwlock_rdlock</i>() and <i>pthread_rwlock_tryrdlock</i>() functions may fail if:</p><dl compact><dt>[EINVAL]</dt><dd>The value specified by <i>rwlock</i> does not refer to an initialized read-write lock object.</dd><dt>[EAGAIN]</dt><dd>The read lock could not be acquired because the maximum number of read locks for <i>rwlock</i> has been exceeded.</dd></dl><p>The <i>pthread_rwlock_rdlock</i>() function may fail if:</p><dl compact><dt>[EDEADLK]</dt><dd>The current thread already owns the read-write lock for writing.</dd></dl><p>These functions 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_556_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_556_07"></a>APPLICATION USAGE</h4><blockquote><p>Applications using these functions may be subject to priority inversion, as discussed in the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap03.html#tag_03_285">Section 3.285, Priority Inversion</a>.</p></blockquote><h4><a name="tag_03_556_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_556_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_556_10"></a>SEE ALSO</h4><blockquote><p><a href="pthread_rwlock_destroy.html"><i>pthread_rwlock_destroy</i>()</a> , <a href="pthread_rwlock_timedrdlock.html"><i>pthread_rwlock_timedrdlock</i>()</a> , <a href="pthread_rwlock_timedwrlock.html"><i>pthread_rwlock_timedwrlock</i>()</a> , <a href="pthread_rwlock_trywrlock.html"><i>pthread_rwlock_trywrlock</i>()</a> , <a href="pthread_rwlock_unlock.html"><i>pthread_rwlock_unlock</i>()</a> , <a href="pthread_rwlock_wrlock.html"><i>pthread_rwlock_wrlock</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <ahref="../basedefs/pthread.h.html"><i><pthread.h></i></a></p></blockquote><h4><a name="tag_03_556_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 5.</p></blockquote><h4><a name="tag_03_556_12"></a>Issue 6</h4><blockquote><p>The following changes are made for alignment with IEEE Std 1003.1j-2000:</p><ul><li><p>The margin code in the SYNOPSIS is changed to THR to indicate that the functionality is now part of the Threads option(previously it was part of the Read-Write Locks option in IEEE Std 1003.1j-2000 and also part of the XSI extension).</p></li><li><p>The DESCRIPTION is updated as follows:</p><ul><li><p>Conditions under which writers have precedence over readers are specified.</p></li><li><p>Failure of <i>pthread_rwlock_tryrdlock</i>() is clarified.</p></li><li><p>A paragraph on the maximum number of read locks is added.</p></li></ul></li><li><p>In the ERRORS sections, [EBUSY] is modified to take into account write priority, and [EDEADLK] is deleted as a<i>pthread_rwlock_tryrdlock</i>() error.</p></li><li><p>The SEE ALSO section is updated.</p></li></ul></blockquote><div class="box"><em>End of informative text.</em></div><hr><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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