fe02148d3f
UNIX Specification. As with our standard mutexes, process shared locks are not supported at this time.
123 lines
3.6 KiB
Groff
123 lines
3.6 KiB
Groff
.\" Copyright (c) 1998 Alex Nash
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $Id$
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.\"
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.Dd August 4, 1998
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.Dt PTHREAD_RWLOCK_RDLOCK 3
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.Os
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.Sh NAME
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.Nm pthread_rwlock_rdlock ,
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.Nm pthread_rwlock_tryrdlock
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.Nd acquire a read/write lock for reading
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.Sh SYNOPSIS
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.Fd #include <pthread.h>
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.Ft int
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.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
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.Ft int
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.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
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.Sh DESCRIPTION
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The
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.Fn pthread_rwlock_rdlock
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function acquires a read lock on
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.Fa lock
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provided that
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.Fa lock
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is not presently held for writing and no writer threads are
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presently blocked on the lock. If the read lock cannot be
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immediately acquired, the calling thread blocks until it can
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acquire the lock.
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.Pp
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The
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.Fn pthread_rwlock_tryrdlock
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function performs the same action, but does not block if the lock
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cannot be immediately obtained (i.e. the lock is held for writing
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or there are waiting writers).
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.Pp
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A thread may hold multiple concurrent read locks. If so,
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.Fn pthread_rwlock_unlock
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must be called once for each lock obtained.
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.Pp
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The results of acquiring a read lock while the calling thread holds
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a write lock are undefined.
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.Sh IMPLEMENTATION NOTES
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To prevent writer starvation, writers are favored over readers.
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.Sh RETURN VALUES
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If successful, the
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.Fn pthread_rwlock_rdlock
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and
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.Fn pthread_rwlock_tryrdlock
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functions will return zero. Otherwise an error number will be returned
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to indicate the error.
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.Sh SEE ALSO
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.Xr pthread_rwlock_init 3 ,
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.Xr pthread_rwlock_trywrlock 3 ,
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.Xr pthread_rwlock_unlock 3 ,
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.Xr pthread_rwlock_wrlock 3
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.Sh STANDARDS
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The
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.Fn pthread_rwlock_rdlock
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and
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.Fn pthread_rwlock_tryrdlock
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functions are expected to conform to
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.St -susv2 .
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.Sh ERRORS
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The
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.Fn pthread_rwlock_tryrdlock
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function will fail if:
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.Bl -tag -width Er
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.It Bq Er EBUSY
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The lock could not be acquired because a writer holds the lock or
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was blocked on it.
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.El
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.Pp
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The
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.Fn pthread_rwlock_rdlock
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and
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.Fn pthread_rwlock_tryrdlock
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functions may fail if:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The lock could not be acquired because the maximum number of read locks
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against
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.Fa lock
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has been exceeded.
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.It Bq Er EDEADLK
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The current thread already owns
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.Fa lock
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for writing.
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.It Bq Er EINVAL
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The value specified by
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.Fa lock
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is invalid.
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.It Bq Er ENOMEM
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Insufficient memory exists to initialize the lock (applies to
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statically initialized locks only).
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.El
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.Sh HISTORY
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The
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.Fn pthread_rwlock_rdlock
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function first appeared in
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.Fx 3.0 .
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