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 libc_r 36.Lb libpthread 37.Lb libthr 38.Sh SYNOPSIS 39.In pthread.h 40.Ft int 41.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock" 42.Ft int 43.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock" 44.Sh DESCRIPTION 45The 46.Fn pthread_rwlock_rdlock 47function acquires a read lock on 48.Fa lock 49provided that 50.Fa lock 51is not presently held for writing and no writer threads are 52presently blocked on the lock. If 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. If 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. Otherwise an error number will be returned 76to indicate the error. 77.Sh SEE ALSO 78.Xr pthread_rwlock_init 3 , 79.Xr pthread_rwlock_trywrlock 3 , 80.Xr pthread_rwlock_unlock 3 , 81.Xr pthread_rwlock_wrlock 3 82.Sh STANDARDS 83The 84.Fn pthread_rwlock_rdlock 85and 86.Fn pthread_rwlock_tryrdlock 87functions are expected to conform to 88.St -susv2 . 89.Sh ERRORS 90The 91.Fn pthread_rwlock_tryrdlock 92function will fail if: 93.Bl -tag -width Er 94.It Bq Er EBUSY 95The lock could not be acquired because a writer holds the lock or 96was blocked on it. 97.El 98.Pp 99The 100.Fn pthread_rwlock_rdlock 101and 102.Fn pthread_rwlock_tryrdlock 103functions may fail if: 104.Bl -tag -width Er 105.It Bq Er EAGAIN 106The lock could not be acquired because the maximum number of read locks 107against 108.Fa lock 109has been exceeded. 110.It Bq Er EDEADLK 111The current thread already owns 112.Fa lock 113for writing. 114.It Bq Er EINVAL 115The value specified by 116.Fa lock 117is invalid. 118.It Bq Er ENOMEM 119Insufficient memory exists to initialize the lock (applies to 120statically initialized locks only). 121.El 122.Sh HISTORY 123The 124.Fn pthread_rwlock_rdlock 125function first appeared in 126.Fx 3.0 . 127