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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>pthread_rwlock_wrlock</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_007_1630"> </a>NAME</h4><blockquote>pthread_rwlock_wrlock, pthread_rwlock_trywrlock- lock a read-write lock object for writing</blockquote><h4><a name = "tag_000_007_1631"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="pthread.h.html">pthread.h</a>>int pthread_rwlock_wrlock(pthread_rwlock_t <i>*rwlock</i>);int pthread_rwlock_trywrlock(pthread_rwlock_t <i>*rwlock</i>);</code></pre></blockquote><h4><a name = "tag_000_007_1632"> </a>DESCRIPTION</h4><blockquote>The <i>pthread_rwlock_wrlock()</i>function applies a write lock tothe read-write lock referenced by <i>rwlock</i>. Thecalling thread acquires the write lock if no other thread(reader or writer) holds the read-write lock <i>rwlock</i>.Otherwise, the thread blocks (that is, does not return from the<i>pthread_rwlock_wrlock()</i>call) until it canacquire the lock. Results are undefined if the calling threadholds the read-write lock (whether a read or writelock) at the time the call is made.<p>Implementations are allowed to favour writers overreaders to avoid writer starvation.<p>The function <i>pthread_rwlock_trywrlock()</i>applies a write locklike the <i>pthread_rwlock_wrlock()</i>function, with the exception that the function fails if anythread currently holds <i>rwlock</i> (for reading or writing).<p>Results are undefined if any of these functions are called withan uninitialised read-write lock.<p>If a signal is delivered to a thread waiting for a read-writelock for writing, upon return from the signal handlerthe thread resumes waiting for the read-write lock forwriting as if it was not interrupted.</blockquote><h4><a name = "tag_000_007_1633"> </a>RETURN VALUE</h4><blockquote>If successful, the <i>pthread_rwlock_wrlock()</i>function returns zero. Otherwise, an error number is returnedto indicate the error.<p>The function <i>pthread_rwlock_trywrlock()</i>returns zero if thelock for writing on the read-write lock objectreferenced by <i>rwlock</i> is acquired. Otherwise an error number isreturned to indicate the error.</blockquote><h4><a name = "tag_000_007_1634"> </a>ERRORS</h4><blockquote>The<i>pthread_rwlock_trywrlock()</i>function will fail if:<dl compact><dt>[EBUSY]<dd>The read-write lock could not be acquired for writing because itwas already locked for reading or writing.</dl><p>The<i>pthread_rwlock_wrlock()</i>and<i>pthread_rwlock_trywrlock()</i>functions may fail if:<dl compact><dt>[EINVAL]<dd>The value specified by <i>rwlock</i> does not refer to an initialisedread-write lock object.<dt>[EDEADLK]<dd>The current thread already owns the read-write lock for writing orreading.</dl></blockquote><h4><a name = "tag_000_007_1635"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1636"> </a>APPLICATION USAGE</h4><blockquote>Similar functions are being developed by IEEE PASC.In keeping with its objective of ensuring that CAESpecifications are fully aligned with formal standards, The Open Groupintends to add any new interfaces adopted by an official IEEE standardin this area.<p>Realtime applications may encounter priority inversion when usingread-write locks.The problem occurs when a high priority thread "locks" a read-writelock that is about to be "unlocked" by a low priority thread, butthe low priority thread is preempted by a medium priority thread.This scenario leads to priority inversion; a high priority thread isblocked by lower priority threads for an unlimited period of time.During system design, realtime programmers must take into account thepossibility of this kind of priority inversion.They can deal with it in a number of ways, such as by having criticalsections that are guarded by read-write locks execute at a highpriority, so that a thread cannot be preempted while executing in itscritical section.</blockquote><h4><a name = "tag_000_007_1637"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1638"> </a>SEE ALSO</h4><blockquote><i><a href="pthread.h.html"><pthread.h></a></i>,<i><a href="pthread_rwlock_init.html">pthread_rwlock_init()</a></i>,<i><a href="pthread_rwlock_unlock.html">pthread_rwlock_unlock()</a></i>,<i><a href="pthread_rwlockattr_init.html">pthread_rwlockattr_init()</a></i>,<i><a href="pthread_rwlock_rdlock.html">pthread_rwlock_rdlock()</a></i>.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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