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. 53If the read lock cannot be 54immediately acquired, the calling thread blocks until it can 55acquire the lock. 56.Pp 57The 58.Fn pthread_rwlock_tryrdlock 59function performs the same action, but does not block if the lock 60cannot be immediately obtained (i.e., the lock is held for writing 61or there are waiting writers). 62.Pp 63A thread may hold multiple concurrent read locks. 64If so, 65.Fn pthread_rwlock_unlock 66must be called once for each lock obtained. 67.Pp 68The results of acquiring a read lock while the calling thread holds 69a write lock are undefined. 70.Sh IMPLEMENTATION NOTES 71To prevent writer starvation, writers are favored over readers. 72.Sh RETURN VALUES 73If successful, the 74.Fn pthread_rwlock_rdlock 75and 76.Fn pthread_rwlock_tryrdlock 77functions will return zero. 78Otherwise an error number will be returned 79to indicate the error. 80.Sh ERRORS 81The 82.Fn pthread_rwlock_tryrdlock 83function will fail if: 84.Bl -tag -width Er 85.It Bq Er EBUSY 86The lock could not be acquired because a writer holds the lock or 87was blocked on it. 88.El 89.Pp 90The 91.Fn pthread_rwlock_rdlock 92and 93.Fn pthread_rwlock_tryrdlock 94functions may fail if: 95.Bl -tag -width Er 96.It Bq Er EAGAIN 97The lock could not be acquired because the maximum number of read locks 98against 99.Fa lock 100has been exceeded. 101.It Bq Er EDEADLK 102The current thread already owns 103.Fa lock 104for writing. 105.It Bq Er EINVAL 106The value specified by 107.Fa lock 108is invalid. 109.It Bq Er ENOMEM 110Insufficient memory exists to initialize the lock (applies to 111statically initialized locks only). 112.El 113.Sh SEE ALSO 114.Xr pthread_rwlock_init 3 , 115.Xr pthread_rwlock_trywrlock 3 , 116.Xr pthread_rwlock_unlock 3 , 117.Xr pthread_rwlock_wrlock 3 118.Sh STANDARDS 119The 120.Fn pthread_rwlock_rdlock 121and 122.Fn pthread_rwlock_tryrdlock 123functions are expected to conform to 124.St -susv2 . 125.Sh HISTORY 126The 127.Fn pthread_rwlock_rdlock 128function first appeared in 129.Fx 3.0 . 130