Move pthread* manpages to lib/libpthread as they are not libc_r specific.
[dragonfly.git] / lib / libpthread / pthread_rwlock_rdlock.3
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1.\" Copyright (c) 1998 Alex Nash
2.\" All rights reserved.
3.\"
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5.\" modification, are permitted provided that the following conditions
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25.\" $FreeBSD: src/lib/libc_r/man/pthread_rwlock_rdlock.3,v 1.2.2.2 2001/12/17 10:08:26 ru Exp $
0b84df5c 26.\" $DragonFly: src/lib/libc_r/man/pthread_rwlock_rdlock.3,v 1.3 2006/02/17 19:35:07 swildner Exp $
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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 LIBRARY
36.Lb libc_r
37.Sh SYNOPSIS
38.In pthread.h
39.Ft int
40.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
41.Ft int
42.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
43.Sh DESCRIPTION
44The
45.Fn pthread_rwlock_rdlock
46function acquires a read lock on
47.Fa lock
48provided that
49.Fa lock
50is not presently held for writing and no writer threads are
51presently blocked on the lock. If the read lock cannot be
52immediately acquired, the calling thread blocks until it can
53acquire the lock.
54.Pp
55The
56.Fn pthread_rwlock_tryrdlock
57function performs the same action, but does not block if the lock
58cannot be immediately obtained (i.e. the lock is held for writing
59or there are waiting writers).
60.Pp
61A thread may hold multiple concurrent read locks. If so,
62.Fn pthread_rwlock_unlock
63must be called once for each lock obtained.
64.Pp
65The results of acquiring a read lock while the calling thread holds
66a write lock are undefined.
67.Sh IMPLEMENTATION NOTES
68To prevent writer starvation, writers are favored over readers.
69.Sh RETURN VALUES
70If successful, the
71.Fn pthread_rwlock_rdlock
72and
73.Fn pthread_rwlock_tryrdlock
74functions will return zero. Otherwise an error number will be returned
75to indicate the error.
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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
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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 .
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121.Sh HISTORY
122The
123.Fn pthread_rwlock_rdlock
124function first appeared in
125.Fx 3.0 .