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.Lb libthr 37.Sh SYNOPSIS 38.In pthread.h 39.Ft int 40.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock" 41.Ft int 42.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock" 43.Sh DESCRIPTION 44The 45.Fn pthread_rwlock_rdlock 46function acquires a read lock on 47.Fa lock 48provided that 49.Fa lock 50is not presently held for writing and no writer threads are 51presently blocked on the lock. 52If the read lock cannot be 53immediately acquired, the calling thread blocks until it can 54acquire the lock. 55.Pp 56The 57.Fn pthread_rwlock_tryrdlock 58function performs the same action, but does not block if the lock 59cannot be immediately obtained (i.e., the lock is held for writing 60or there are waiting writers). 61.Pp 62A thread may hold multiple concurrent read locks. 63If so, 64.Fn pthread_rwlock_unlock 65must be called once for each lock obtained. 66.Pp 67The results of acquiring a read lock while the calling thread holds 68a write lock are undefined. 69.Sh IMPLEMENTATION NOTES 70To prevent writer starvation, writers are favored over readers. 71.Sh RETURN VALUES 72If successful, the 73.Fn pthread_rwlock_rdlock 74and 75.Fn pthread_rwlock_tryrdlock 76functions will return zero. 77Otherwise an error number will be returned 78to indicate the error. 79.Sh ERRORS 80The 81.Fn pthread_rwlock_tryrdlock 82function will fail if: 83.Bl -tag -width Er 84.It Bq Er EBUSY 85The lock could not be acquired because a writer holds the lock or 86was blocked on it. 87.El 88.Pp 89The 90.Fn pthread_rwlock_rdlock 91and 92.Fn pthread_rwlock_tryrdlock 93functions may fail if: 94.Bl -tag -width Er 95.It Bq Er EAGAIN 96The lock could not be acquired because the maximum number of read locks 97against 98.Fa lock 99has been exceeded. 100.It Bq Er EDEADLK 101The current thread already owns 102.Fa lock 103for writing. 104.It Bq Er EINVAL 105The value specified by 106.Fa lock 107is invalid. 108.It Bq Er ENOMEM 109Insufficient memory exists to initialize the lock (applies to 110statically initialized locks only). 111.El 112.Sh SEE ALSO 113.Xr pthread_rwlock_init 3 , 114.Xr pthread_rwlock_trywrlock 3 , 115.Xr pthread_rwlock_unlock 3 , 116.Xr pthread_rwlock_wrlock 3 117.Sh STANDARDS 118The 119.Fn pthread_rwlock_rdlock 120and 121.Fn pthread_rwlock_tryrdlock 122functions are expected to conform to 123.St -susv2 . 124.Sh HISTORY 125The 126.Fn pthread_rwlock_rdlock 127function first appeared in 128.Fx 3.0 . 129