1 .\" Copyright (c) 1998 Alex Nash
2 .\" All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
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.
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
25 .\" $FreeBSD: src/lib/libc_r/man/pthread_rwlock_rdlock.3,v 1.2.2.2 2001/12/17 10:08:26 ru Exp $
28 .Dt PTHREAD_RWLOCK_RDLOCK 3
31 .Nm pthread_rwlock_rdlock ,
32 .Nm pthread_rwlock_tryrdlock
33 .Nd acquire a read/write lock for reading
39 .Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
41 .Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
44 .Fn pthread_rwlock_rdlock
45 function acquires a read lock on
49 is not presently held for writing and no writer threads are
50 presently blocked on the lock. If the read lock cannot be
51 immediately acquired, the calling thread blocks until it can
55 .Fn pthread_rwlock_tryrdlock
56 function performs the same action, but does not block if the lock
57 cannot be immediately obtained (i.e. the lock is held for writing
58 or there are waiting writers).
60 A thread may hold multiple concurrent read locks. If so,
61 .Fn pthread_rwlock_unlock
62 must be called once for each lock obtained.
64 The results of acquiring a read lock while the calling thread holds
65 a write lock are undefined.
66 .Sh IMPLEMENTATION NOTES
67 To prevent writer starvation, writers are favored over readers.
70 .Fn pthread_rwlock_rdlock
72 .Fn pthread_rwlock_tryrdlock
73 functions will return zero. Otherwise an error number will be returned
74 to indicate the error.
76 .Xr pthread_rwlock_init 3 ,
77 .Xr pthread_rwlock_trywrlock 3 ,
78 .Xr pthread_rwlock_unlock 3 ,
79 .Xr pthread_rwlock_wrlock 3
82 .Fn pthread_rwlock_rdlock
84 .Fn pthread_rwlock_tryrdlock
85 functions are expected to conform to
89 .Fn pthread_rwlock_tryrdlock
90 function will fail if:
93 The lock could not be acquired because a writer holds the lock or
98 .Fn pthread_rwlock_rdlock
100 .Fn pthread_rwlock_tryrdlock
101 functions may fail if:
104 The lock could not be acquired because the maximum number of read locks
109 The current thread already owns
113 The value specified by
117 Insufficient memory exists to initialize the lock (applies to
118 statically initialized locks only).
122 .Fn pthread_rwlock_rdlock
123 function first appeared in