Copyright (c) 2002, 2010 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
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.
THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
Copyright (c) 1998 Alex Nash
All rights reserved.
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.
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
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
----------------------------------------------------------------------------
.Dd July 8, 2010 .Dt PTHREAD_RWLOCK 3 .Os .Sh NAME .Nm pthread_rwlock , .Nm pthread_rwlock_init , .Nm pthread_rwlock_destroy , .Nm pthread_rwlock_rdlock , .Nm pthread_rwlock_timedrdlock , .Nm pthread_rwlock_tryrdlock , .Nm pthread_rwlock_wrlock , .Nm pthread_rwlock_timedwrlock , .Nm pthread_rwlock_trywrlock , .Nm pthread_rwlock_unlock .Nd read/write lock interface .Sh LIBRARY .Lb libpthread ----------------------------------------------------------------------------
.Sh SYNOPSIS n pthread.h .Ft int .Fn pthread_rwlock_init "pthread_rwlock_t * restrict lock" \ "const pthread_rwlockattr_t * restrict attr" .Vt pthread_rwlock_t lock No = Dv PTHREAD_RWLOCK_INITIALIZER ; .Ft int .Fn pthread_rwlock_destroy "pthread_rwlock_t *lock" .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" .Ft int .Fn pthread_rwlock_wrlock "pthread_rwlock_t *lock" .Ft int .Fn pthread_rwlock_timedwrlock "pthread_rwlock_t * restrict lock" \ "const struct timespec * restrict abstime" .Ft int .Fn pthread_rwlock_trywrlock "pthread_rwlock_t *lock" .Ft int .Fn pthread_rwlock_unlock "pthread_rwlock_t *lock" ----------------------------------------------------------------------------
.Sh DESCRIPTION The .Fn pthread_rwlock_init function is used to initialize a read/write lock, with attributes specified by .Fa attr . If .Fa attr is .Dv NULL , the default read/write lock attributes are used.
p The results of calling .Fn pthread_rwlock_init with an already initialized lock are undefined.
p The macro .Dv PTHREAD_RWLOCK_INITIALIZER can be used to initialize a read/write lock when the allocation can be done statically, no error checking is required, and the default attributes are appropriate. The behavior is similar to calling .Fn pthread_rwlock_init with .Fa attr specified as .Dv NULL .
p
-----
The
.Fn pthread_rwlock_destroy
function is used to destroy a read/write lock previously created with
.Fn pthread_rwlock_init .
p
-----
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.
p The .Fn pthread_rwlock_timedrdlock performs the same action, but will not wait beyond .Fa abstime to obtain the lock before returning.
p 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).
p A thread may hold multiple concurrent read locks. If so, .Fn pthread_rwlock_unlock must be called once for each lock obtained.
p The results of acquiring a read lock while the calling thread holds a write lock are undefined.
p
-----
The
.Fn pthread_rwlock_wrlock
function blocks until a write lock can be acquired against
.Fa lock .
p The .Fn pthread_rwlock_timedwrlock performs the same action, but will not wait beyond .Fa abstime to obtain the lock before returning.
p The .Fn pthread_rwlock_trywrlock function performs the same action as .Fn pthread_rwlock_wrlock , but does not block if the lock cannot be immediately obtained.
p The results are undefined if the calling thread already holds the lock at the time the call is made.
p
-----
The
.Fn pthread_rwlock_unlock
function is used to release the read/write lock previously obtained by
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_wrlock ,
.Fn pthread_rwlock_tryrdlock ,
or
.Fn pthread_rwlock_trywrlock .
----------------------------------------------------------------------------
.Sh RETURN VALUES
If successful, the
.Fn pthread_rwlock_init
function will return zero.
Otherwise an error number will be returned to indicate the error.
p If successful, the .Fn pthread_rwlock_destroy , .Fn pthread_rwlock_rdlock , .Fn pthread_rwlock_timedrdlock , .Fn pthread_rwlock_tryrdlock , .Fn pthread_rwlock_wrlock , .Fn pthread_rwlock_timedwrlock , .Fn pthread_rwlock_trywrlock , and .Fn pthread_rwlock_unlock will return zero. Otherwise an error number will be returned to indicate the error.
p
The results are undefined if
.Fa lock
is not held by the calling thread.
----------------------------------------------------------------------------
.Sh ERRORS
The
.Fn pthread_rwlock_init
function may fail if:
l -tag -width Er t Bq Er EAGAIN The system lacks the resources to initialize another read-write lock.
t Bq Er EBUSY The system has detected an attempt to re-initialize the object
referenced by
.Fa lock ,
a previously initialized but not yet destroyed read/write lock.
t Bq Er EINVAL The value specified by
.Fa attr
is invalid.
t Bq Er ENOMEM Insufficient memory exists to initialize the read-write lock.
.El
p
-----
The
.Fn pthread_rwlock_destroy
function may fail if:
l -tag -width Er t Bq Er EBUSY The system has detected an attempt to destroy the object referenced by
.Fa lock
while it is locked.
t Bq Er EINVAL The value specified by
.Fa lock
is invalid.
.El
p
-----
The
.Fn pthread_rwlock_tryrdlock
function shall fail if:
l -tag -width Er t Bq Er EBUSY The lock could not be acquired because a writer holds the lock or
was blocked on it.
.El
p The .Fn pthread_rwlock_timedrdlock function shall fail if: l -tag -width Er t Bq Er ETIMEDOUT The time specified by .Fa abstime was reached before the lock could be obtained. .El
p The .Fn pthread_rwlock_rdlock , .Fn pthread_rwlock_timedrdlock , and .Fn pthread_rwlock_tryrdlock functions may fail if: l -tag -width Er t Bq Er EAGAIN The lock could not be acquired because the maximum number of read locks against .Fa lock has been exceeded. t Bq Er EDEADLK The current thread already owns .Fa lock for writing. t Bq Er EINVAL The value specified by .Fa lock is invalid. .El
p
-----
The
.Fn pthread_rwlock_trywrlock
function shall fail if:
l -tag -width Er t Bq Er EBUSY The calling thread is not able to acquire the lock without blocking.
.El
p The .Fn pthread_rwlock_timedrdlock function shall fail if: l -tag -width Er t Bq Er ETIMEDOUT The time specified by .Fa abstime was reached before the lock could be obtained. .El
p The .Fn pthread_rwlock_wrlock , .Fn pthread_rwlock_timedwrlock , and .Fn pthread_rwlock_trywrlock functions may fail if: l -tag -width Er t Bq Er EDEADLK The calling thread already owns the read/write lock (for reading or writing). t Bq Er EINVAL The value specified by .Fa lock is invalid. .El
p
-----
The
.Fn pthread_rwlock_unlock
function may fail if:
l -tag -width Er t Bq Er EINVAL The value specified by
.Fa lock
is invalid.
t Bq Er EPERM The current thread does not own the read/write lock.
.El
----------------------------------------------------------------------------
.Sh SEE ALSO
.Xr pthread 3 ,
.Xr pthread_barrier 3 ,
.Xr pthread_cond 3 ,
.Xr pthread_mutex 3 ,
.Xr pthread_rwlockattr 3 ,
.Xr pthread_spin 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2001 .
----------------------------------------------------------------------------
.Sh BUGS
The
.Dv PTHREAD_PROCESS_SHARED
attribute is not supported.
Indexes created Sat Sep 20 22:09:52 GMT 2025