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1.\" Copyright (c) 1998 Alex Nash 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd August 4, 1998 28.Dt PTHREAD_RWLOCK_RDLOCK 3 29.Os 30.Sh NAME 31.Nm pthread_rwlock_rdlock , 32.Nm pthread_rwlock_tryrdlock 33.Nd acquire a read/write lock for reading 34.Sh LIBRARY 35.Lb libpthread 36.Sh SYNOPSIS 37.In pthread.h 38.Ft int 39.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock" 40.Ft int 41.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock" 42.Sh DESCRIPTION 43The 44.Fn pthread_rwlock_rdlock 45function acquires a read lock on 46.Fa lock 47provided that 48.Fa lock 49is not presently held for writing and no writer threads are 50presently blocked on the lock. 51If the read lock cannot be 52immediately acquired, the calling thread blocks until it can 53acquire the lock. 54.Pp 55The 56.Fn pthread_rwlock_tryrdlock 57function performs the same action, but does not block if the lock 58cannot be immediately obtained (i.e., the lock is held for writing 59or there are waiting writers). 60.Pp 61A thread may hold multiple concurrent read locks. 62If so, 63.Fn pthread_rwlock_unlock 64must be called once for each lock obtained. 65.Pp 66The results of acquiring a read lock while the calling thread holds 67a write lock are undefined. 68.Sh IMPLEMENTATION NOTES 69To prevent writer starvation, writers are favored over readers. 70.Sh RETURN VALUES 71If successful, the 72.Fn pthread_rwlock_rdlock 73and 74.Fn pthread_rwlock_tryrdlock 75functions will return zero. 76Otherwise an error number will be returned 77to indicate the error. 78.Sh ERRORS 79The 80.Fn pthread_rwlock_tryrdlock 81function will fail if: 82.Bl -tag -width Er 83.It Bq Er EBUSY 84The lock could not be acquired because a writer holds the lock or 85was blocked on it. 86.El 87.Pp 88The 89.Fn pthread_rwlock_rdlock 90and 91.Fn pthread_rwlock_tryrdlock 92functions may fail if: 93.Bl -tag -width Er 94.It Bq Er EAGAIN 95The lock could not be acquired because the maximum number of read locks 96against 97.Fa lock 98has been exceeded. 99.It Bq Er EDEADLK 100The current thread already owns 101.Fa lock 102for writing. 103.It Bq Er EINVAL 104The value specified by 105.Fa lock 106is invalid. 107.It Bq Er ENOMEM 108Insufficient memory exists to initialize the lock (applies to 109statically initialized locks only). 110.El 111.Sh SEE ALSO 112.Xr pthread_rwlock_init 3 , 113.Xr pthread_rwlock_trywrlock 3 , 114.Xr pthread_rwlock_unlock 3 , 115.Xr pthread_rwlock_wrlock 3 116.Sh STANDARDS 117The 118.Fn pthread_rwlock_rdlock 119and 120.Fn pthread_rwlock_tryrdlock 121functions are expected to conform to 122.St -susv2 . 123.Sh HISTORY 124The 125.Fn pthread_rwlock_rdlock 126function first appeared in 127.Fx 3.0 .