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All Rights Reserved. .\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved. .\" .TH PTHREAD_RWLOCK_RDLOCK 3C "Mar 23, 2005" .SH NAME pthread_rwlock_rdlock, pthread_rwlock_tryrdlock \- lock or attempt to lock read-write lock object for reading .SH SYNOPSIS .LP .nf cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ] #include \fBint\fR \fBpthread_rwlock_rdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR); .fi .LP .nf \fBint\fR \fBpthread_rwlock_tryrdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR); .fi .SH DESCRIPTION .sp .LP The \fBpthread_rwlock_rdlock()\fR function applies a read lock to the read-write lock referenced by \fIrwlock\fR. The calling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock. .sp .LP The calling thread does not acquire the lock if a writer holds the lock or if writers of higher or equal priority are blocked on the lock; otherwise, the calling thread acquires the lock. If the read lock is not acquired, the calling thread blocks until it can acquire the lock. .sp .LP A thread can hold multiple concurrent read locks on \fIrwlock\fR (that is, successfully call the \fBpthread_rwlock_rdlock()\fR function \fIn\fR times). If so, the thread must perform matching unlocks (that is, it must call the \fBpthread_rwlock_unlock()\fR function \fIn\fR times). .sp .LP The maximum number of concurrent read locks that a thread can hold on one read-write lock is currently set at 100,000, though this number could change in a future release. There is no imposed limit on the number of different threads that can apply a read lock to one read-write lock. .sp .LP The \fBpthread_rwlock_tryrdlock()\fR function applies a read lock like the \fBpthread_rwlock_rdlock()\fR function, with the exception that the function fails if the equivalent \fBpthread_rwlock_rdlock()\fR call would have blocked the calling thread. In no case will the \fBpthread_rwlock_tryrdlock()\fR function ever bloc. It always either acquires the lock or fails and returns immediately. .sp .LP Results are undefined if any of these functions are called with an uninitialized read-write lock. .sp .LP If a signal is delivered to a thread waiting for a read-write lock for reading, upon return from the signal handler the thread resumes waiting for the read-write lock for reading as if it was not interrupted. .SH RETURN VALUES .sp .LP If successful, the \fBpthread_rwlock_rdlock()\fR function returns \fB0\fR. Otherwise, an error number is returned to indicate the error. .sp .LP The \fBpthread_rwlock_tryrdlock()\fR function returns \fB0\fR if the lock for reading on the read-write lock object referenced by \fIrwlock\fR is acquired. Otherwise an error number is returned to indicate the error. .SH ERRORS .sp .LP The \fBpthread_rwlock_rdlock()\fR and \fBpthread_rwlock_tryrdlock()\fR functions will fail if: .sp .ne 2 .na \fB\fBEAGAIN\fR\fR .ad .RS 10n The read lock could not be acquired because the maximum number of read locks by the current thread for \fIrwlock\fR has been exceeded. .RE .sp .LP The \fBpthread_rwlock_rdlock()\fR function will fail if: .sp .ne 2 .na \fB\fBEDEADLK\fR\fR .ad .RS 11n The current thread already owns the read-write lock for writing. .RE .sp .LP The \fBpthread_rwlock_tryrdlock()\fR function will fail if: .sp .ne 2 .na \fB\fBEBUSY\fR\fR .ad .RS 9n The read-write lock could not be acquired for reading because a writer holds the lock or a writer with the appropriate priority was blocked on it. .RE .SH ATTRIBUTES .sp .LP See \fBattributes\fR(5) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ Interface Stability Standard _ MT-Level MT-Safe .TE .SH SEE ALSO .sp .LP \fBpthread_rwlock_init\fR(3C), \fBpthread_rwlock_wrlock\fR(3C), \fBpthread_rwlockattr_init\fR(3C), \fBpthread_rwlock_unlock\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)