NetBSD-5.0.2/lib/libpthread/pthread_rwlock_rdlock.3

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.\" $NetBSD: pthread_rwlock_rdlock.3,v 1.10 2008/05/04 19:43:05 martin Exp $
.\"
.\" Copyright (c) 2002 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" Copyright (c) 1998 Alex Nash
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.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" $FreeBSD: src/lib/libpthread/man/pthread_rwlock_rdlock.3,v 1.5 2002/09/16 19:29:29 mini Exp $
.\"
.Dd January 30, 2003
.Dt PTHREAD_RWLOCK_RDLOCK 3
.Os
.Sh NAME
.Nm pthread_rwlock_rdlock ,
.Nm pthread_rwlock_timedrdlock ,
.Nm pthread_rwlock_tryrdlock
.Nd acquire a read/write lock for reading
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_timedrdlock "pthread_rwlock_t * restrict lock" "const struct timespec * restrict abstime"
.Ft int
.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
.Sh DESCRIPTION
The
.Fn pthread_rwlock_rdlock
function acquires a read lock on
.Fa lock
provided that
.Fa lock
is not presently held for writing and no writer threads are
presently blocked on the lock.
If the read lock cannot be immediately acquired, the calling thread
blocks until it can acquire the lock.
.Pp
The
.Fn pthread_rwlock_timedrdlock
performs the same action, but will not wait beyond
.Fa abstime
to obtain the lock before returning.
.Pp
The
.Fn pthread_rwlock_tryrdlock
function performs the same action as
.Fn pthread_rwlock_rdlock ,
but does not block if the lock cannot be immediately obtained (i.e.,
the lock is held for writing or there are waiting writers).
.Pp
A thread may hold multiple concurrent read locks.
If so,
.Fn pthread_rwlock_unlock
must be called once for each lock obtained.
.Pp
The results of acquiring a read lock while the calling thread holds
a write lock are undefined.
.Sh RETURN VALUES
If successful, the
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_timedrdlock ,
and
.Fn pthread_rwlock_tryrdlock
functions will return zero.
Otherwise an error number will be returned to indicate the error.
.Sh ERRORS
The
.Fn pthread_rwlock_tryrdlock
function shall fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The lock could not be acquired because a writer holds the lock or
was blocked on it.
.El
.Pp
The
.Fn pthread_rwlock_timedrdlock
function shall fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The time specified by
.Fa abstime
was reached before the lock could be obtained.
.El
.Pp
The
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_timedrdlock ,
and
.Fn pthread_rwlock_tryrdlock
functions may fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The lock could not be acquired because the maximum number of read locks
against
.Fa lock
has been exceeded.
.It Bq Er EDEADLK
The current thread already owns
.Fa lock
for writing.
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.Sh SEE ALSO
.Xr pthread_rwlock_destroy 3 ,
.Xr pthread_rwlock_init 3 ,
.Xr pthread_rwlock_unlock 3 ,
.Xr pthread_rwlock_wrlock 3
.Sh STANDARDS
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_timedrdlock ,
and
.Fn pthread_rwlock_tryrdlock
conform to
.St -p1003.1-96 .