1.\" $OpenBSD: pthread_rwlock_rdlock.3,v 1.5 2000/04/12 21:48:02 aaron Exp $ 2.\" Copyright (c) 1998 Alex Nash 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" $FreeBSD: pthread_rwlock_rdlock.3,v 1.2 1999/08/28 00:03:09 peter Exp $ 27.\" 28.Dd August 4, 1998 29.Dt PTHREAD_RWLOCK_RDLOCK 3 30.Os 31.Sh NAME 32.Nm pthread_rwlock_rdlock , 33.Nm pthread_rwlock_tryrdlock 34.Nd acquire a read/write lock for reading 35.Sh SYNOPSIS 36.Fd #include <pthread.h> 37.Ft int 38.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock" 39.Ft int 40.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock" 41.Sh DESCRIPTION 42The 43.Fn pthread_rwlock_rdlock 44function acquires a read lock on 45.Fa lock 46provided that 47.Fa lock 48is not presently held for writing and no writer threads are 49presently blocked on the lock. If 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. If so, 60.Fn pthread_rwlock_unlock 61must be called once for each lock obtained. 62.Pp 63The results of acquiring a read lock while the calling thread holds 64a write lock are undefined. 65.Sh IMPLEMENTATION NOTES 66To prevent writer starvation, writers are favored over readers. 67.Sh RETURN VALUES 68If successful, the 69.Fn pthread_rwlock_rdlock 70and 71.Fn pthread_rwlock_tryrdlock 72functions will return zero. Otherwise an error number will be returned 73to indicate the error. 74.Sh SEE ALSO 75.Xr pthread_rwlock_init 3 , 76.Xr pthread_rwlock_trywrlock 3 , 77.Xr pthread_rwlock_unlock 3 , 78.Xr pthread_rwlock_wrlock 3 79.Sh STANDARDS 80The 81.Fn pthread_rwlock_rdlock 82and 83.Fn pthread_rwlock_tryrdlock 84functions are expected to conform to 85.St -susv2 . 86.Sh ERRORS 87The 88.Fn pthread_rwlock_tryrdlock 89function will fail if: 90.Bl -tag -width Er 91.It Bq Er EBUSY 92The lock could not be acquired because a writer holds the lock or 93was blocked on it. 94.El 95.Pp 96The 97.Fn pthread_rwlock_rdlock 98and 99.Fn pthread_rwlock_tryrdlock 100functions may fail if: 101.Bl -tag -width Er 102.It Bq Er EAGAIN 103The lock could not be acquired because the maximum number of read locks 104against 105.Fa lock 106has been exceeded. 107.It Bq Er EDEADLK 108The current thread already owns 109.Fa lock 110for writing. 111.It Bq Er EINVAL 112The value specified by 113.Fa lock 114is invalid. 115.It Bq Er ENOMEM 116Insufficient memory exists to initialize the lock (applies to 117statically initialized locks only). 118.El 119.Sh HISTORY 120The 121.Fn pthread_rwlock_rdlock 122function first appeared in 123.Fx 3.0 124and 125.Ox 2.5 . 126