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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_rdlock</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_1603"> </a>NAME</h4><blockquote>pthread_rwlock_rdlock, pthread_rwlock_tryrdlock- lock a read-write lock object for reading</blockquote><h4><a name = "tag_000_007_1604"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="pthread.h.html">pthread.h</a>>int pthread_rwlock_rdlock(pthread_rwlock_t <i>*rwlock</i>);int pthread_rwlock_tryrdlock(pthread_rwlock_t <i>*rwlock</i>);</code></pre></blockquote><h4><a name = "tag_000_007_1605"> </a>DESCRIPTION</h4><blockquote>The <i>pthread_rwlock_rdlock()</i>function applies a read lock to the read-write lock referenced by<i>rwlock</i>. The calling thread acquires the read lock if a writer does nothold the lock and there are no writers blockedon the lock. It is unspecified whether the calling threadacquires the lock when a writer does not hold the lockand there are writers waiting for the lock. If a writer holdsthe lock, the calling thread will not acquire theread lock. If the read lock is not acquired, the callingthread blocks (that is, it does not return from the<i>pthread_rwlock_rdlock()</i>call) until it can acquire the lock.Results are undefined if the calling thread holdsa write lock on <i>rwlock</i>at the time the call is made.<p>Implementations are allowed to favour writers over readersto avoid writer starvation.<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 threadmust perform matching unlocks (that is, it mustcall the <i><a href="pthread_rwlock_unlock.html">pthread_rwlock_unlock()</a></i>function <i>n</i> times).<p>The function <i>pthread_rwlock_tryrdlock()</i>applies a read lockas in the <i>pthread_rwlock_rdlock()</i>functionwith the exception that the function fails if any thread holds awrite lock on <i>rwlock</i> or there are writers blockedon <i>rwlock</i>.<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 reading, upon return from the signal handlerthe thread resumes waiting for the read-write lock forreading as if it was not interrupted.</blockquote><h4><a name = "tag_000_007_1606"> </a>RETURN VALUE</h4><blockquote>If successful, the <i>pthread_rwlock_rdlock()</i>function returns zero. Otherwise, an error number is returnedto indicate the error.<p>The function <i>pthread_rwlock_tryrdlock()</i>returns zero if thelock for reading on the read-write lock objectreferenced by <i>rwlock</i> is acquired. Otherwise an error number is returned to indicate the error.</blockquote><h4><a name = "tag_000_007_1607"> </a>ERRORS</h4><blockquote>The<i>pthread_rwlock_tryrdlock()</i>function will fail if:<dl compact><dt>[EBUSY]<dd>The read-write lock could not be acquired for reading because awriter holds thelock or was blocked on it.</dl><p>The<i>pthread_rwlock_rdlock()</i>and<i>pthread_rwlock_tryrdlock()</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.<dt>[EAGAIN]<dd>The read lock could not be acquired because the maximum number of read locksfor <i>rwlock</i> has been exceeded.</dl></blockquote><h4><a name = "tag_000_007_1608"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1609"> </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_1610"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1611"> </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_wrlock.html">pthread_rwlock_wrlock()</a></i>,<i><a href="pthread_rwlockattr_init.html">pthread_rwlockattr_init()</a></i>,<i><a href="pthread_rwlock_unlock.html">pthread_rwlock_unlock()</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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