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.Dd August 4, 1998 26.Dt PTHREAD_RWLOCK_RDLOCK 3 27.Os 28.Sh NAME 29.Nm pthread_rwlock_rdlock , 30.Nm pthread_rwlock_tryrdlock 31.Nd acquire a read/write lock for reading 32.Sh LIBRARY 33.Lb libpthread 34.Sh SYNOPSIS 35.In pthread.h 36.Ft int 37.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock" 38.Ft int 39.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock" 40.Sh DESCRIPTION 41The 42.Fn pthread_rwlock_rdlock 43function acquires a read lock on 44.Fa lock 45provided that 46.Fa lock 47is not presently held for writing and no writer threads are 48presently blocked on the lock. 49If the read lock cannot be 50immediately acquired, the calling thread blocks until it can 51acquire the lock. 52.Pp 53The 54.Fn pthread_rwlock_tryrdlock 55function performs the same action, but does not block if the lock 56cannot be immediately obtained (i.e., the lock is held for writing 57or there are waiting writers). 58.Pp 59A thread may hold multiple concurrent read locks. 60If so, 61.Fn pthread_rwlock_unlock 62must be called once for each lock obtained. 63.Pp 64The results of acquiring a read lock while the calling thread holds 65a write lock are undefined. 66.Sh IMPLEMENTATION NOTES 67To prevent writer starvation, writers are favored over readers. 68.Sh RETURN VALUES 69If successful, the 70.Fn pthread_rwlock_rdlock 71and 72.Fn pthread_rwlock_tryrdlock 73functions will return zero. 74Otherwise an error number will be returned 75to indicate the error. 76.Sh ERRORS 77The 78.Fn pthread_rwlock_tryrdlock 79function will fail if: 80.Bl -tag -width Er 81.It Bq Er EBUSY 82The lock could not be acquired because a writer holds the lock or 83was blocked on it. 84.El 85.Pp 86The 87.Fn pthread_rwlock_rdlock 88and 89.Fn pthread_rwlock_tryrdlock 90functions may fail if: 91.Bl -tag -width Er 92.It Bq Er EAGAIN 93The lock could not be acquired because the maximum number of read locks 94against 95.Fa lock 96has been exceeded. 97.It Bq Er EDEADLK 98The current thread already owns 99.Fa lock 100for writing. 101.It Bq Er EINVAL 102The value specified by 103.Fa lock 104is invalid. 105.It Bq Er ENOMEM 106Insufficient memory exists to initialize the lock (applies to 107statically initialized locks only). 108.El 109.Sh SEE ALSO 110.Xr pthread_rwlock_init 3 , 111.Xr pthread_rwlock_trywrlock 3 , 112.Xr pthread_rwlock_unlock 3 , 113.Xr pthread_rwlock_wrlock 3 114.Sh STANDARDS 115The 116.Fn pthread_rwlock_rdlock 117and 118.Fn pthread_rwlock_tryrdlock 119functions are expected to conform to 120.St -susv2 . 121.Sh HISTORY 122The 123.Fn pthread_rwlock_rdlock 124function first appeared in 125.Fx 3.0 . 126