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.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. If the read lock cannot be 51immediately acquired, the calling thread blocks until it can 52acquire the lock. 53.Pp 54The 55.Fn pthread_rwlock_tryrdlock 56function performs the same action, but does not block if the lock 57cannot be immediately obtained (i.e. the lock is held for writing 58or there are waiting writers). 59.Pp 60A thread may hold multiple concurrent read locks. If 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. Otherwise an error number will be returned 74to indicate the error. 75.Sh SEE ALSO 76.Xr pthread_rwlock_init 3 , 77.Xr pthread_rwlock_trywrlock 3 , 78.Xr pthread_rwlock_unlock 3 , 79.Xr pthread_rwlock_wrlock 3 80.Sh STANDARDS 81The 82.Fn pthread_rwlock_rdlock 83and 84.Fn pthread_rwlock_tryrdlock 85functions are expected to conform to 86.St -susv2 . 87.Sh ERRORS 88The 89.Fn pthread_rwlock_tryrdlock 90function will fail if: 91.Bl -tag -width Er 92.It Bq Er EBUSY 93The lock could not be acquired because a writer holds the lock or 94was blocked on it. 95.El 96.Pp 97The 98.Fn pthread_rwlock_rdlock 99and 100.Fn pthread_rwlock_tryrdlock 101functions may fail if: 102.Bl -tag -width Er 103.It Bq Er EAGAIN 104The lock could not be acquired because the maximum number of read locks 105against 106.Fa lock 107has been exceeded. 108.It Bq Er EDEADLK 109The current thread already owns 110.Fa lock 111for writing. 112.It Bq Er EINVAL 113The value specified by 114.Fa lock 115is invalid. 116.It Bq Er ENOMEM 117Insufficient memory exists to initialize the lock (applies to 118statically initialized locks only). 119.El 120.Sh HISTORY 121The 122.Fn pthread_rwlock_rdlock 123function first appeared in 124.Fx 3.0 . 125