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128 lines
3.7 KiB
Groff
128 lines
3.7 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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.\" $FreeBSD$
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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 LIBRARY
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.Lb libpthread
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.Sh SYNOPSIS
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.In 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.
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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.
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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.
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Otherwise an error number will be returned
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to indicate the error.
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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 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 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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