.\" $OpenBSD: pthread_rwlock_init.3,v 1.12 2025/07/08 02:23:49 jsg Exp $
.\" Copyright (c) 1998 Alex Nash
.\" All rights reserved.
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.\" $FreeBSD: pthread_rwlock_init.3,v 1.2 1999/08/28 00:03:09 peter Exp $
.\" $FreeBSD: pthread_rwlock_destroy.3,v 1.3 1999/08/28 00:03:09 peter Exp $
.\" $FreeBSD: pthread_rwlock_rdlock.3,v 1.2 1999/08/28 00:03:09 peter Exp $
.\" $FreeBSD: pthread_rwlock_wrlock.3,v 1.2 1999/08/28 00:03:10 peter Exp $
.\" $FreeBSD: pthread_rwlock_unlock.3,v 1.2 1999/08/28 00:03:10 peter Exp $
.\"
.Dd $Mdocdate: July 8 2025 $
.Dt PTHREAD_RWLOCK_INIT 3
.Os
.Sh NAME
.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 operations on read/write locks
.Sh SYNOPSIS
.Lb libpthread
.In pthread.h
.Ft int
.Fo pthread_rwlock_init
.Fa "pthread_rwlock_t *lock"
.Fa "const pthread_rwlockattr_t *attr"
.Fc
.Ft int
.Fo pthread_rwlock_destroy
.Fa "pthread_rwlock_t *lock"
.Fc
.Ft int
.Fo pthread_rwlock_rdlock
.Fa "pthread_rwlock_t *lock"
.Fc
.Ft int
.Fo pthread_rwlock_timedrdlock
.Fa "pthread_rwlock_t *lock"
.Fa "const struct timespec *abstime"
.Fc
.Ft int
.Fo pthread_rwlock_tryrdlock
.Fa "pthread_rwlock_t *lock"
.Fc
.Ft int
.Fo pthread_rwlock_wrlock
.Fa "pthread_rwlock_t *lock"
.Fc
.Ft int
.Fo pthread_rwlock_timedwrlock
.Fa "pthread_rwlock_t *lock"
.Fa "const struct timespec *abstime"
.Fc
.Ft int
.Fo pthread_rwlock_trywrlock
.Fa "pthread_rwlock_t *lock"
.Fc
.Ft int
.Fo pthread_rwlock_unlock
.Fa "pthread_rwlock_t *lock"
.Fc
.Sh DESCRIPTION
The
.Fn pthread_rwlock_init
function initializes a read/write lock, with attributes
specified by
.Fa attr .
If
.Fa attr
is NULL, the default read/write lock attributes are used.
The results of calling
.Fn pthread_rwlock_init
with an already initialized lock are undefined.
.Pp
The
.Fn pthread_rwlock_destroy
function destroys a read/write lock previously created with
.Fn pthread_rwlock_init .
.Pp
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_wrlock
function blocks until a write lock can be acquired against
.Fa lock .
.Pp
The
.Fn pthread_rwlock_timedrdlock
and
.Fn pthread_rwlock_timedwrlock
functions perform the same action,
but will not wait beyond
.Fa abstime
to obtain the lock before returning.
.Pp
The
.Fn pthread_rwlock_tryrdlock
and
.Fn pthread_rwlock_trywrlock
functions perform the same action
but do not block if the lock cannot be immediately obtained.
.Pp
The
.Fn pthread_rwlock_unlock
function releases the read/write lock previously obtained.
.Pp
A thread may hold multiple concurrent read locks.
If so,
.Fn pthread_rwlock_unlock
must be called once for each lock obtained.
.Sh RETURN VALUES
If successful, these functions return zero.
Otherwise an error number will be returned to indicate the error.
.Sh ERRORS
.Fn pthread_rwlock_init
fails if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The system lacked the necessary resources (other than memory) to
initialize the lock.
.It Bq Er ENOMEM
Insufficient memory exists to initialize the lock.
.It Bq Er EPERM
The caller does not have sufficient privilege to perform the
operation.
.It 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.
.It Bq Er EINVAL
The value specified by
.Fa attr
is invalid.
.El
.Pp
Other functions fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.It Bq Er ENOMEM
Insufficient memory exists to initialize the lock (applies to
statically initialized locks only).
.El
.Pp
.Fn pthread_rwlock_destroy
fails if:
.Bl -tag -width Er
.It Bq Er EBUSY
The system has detected an attempt to destroy the object referenced by
.Fa lock
while it is locked.
.El
.Pp
.Fn pthread_rwlock_tryrdlock
and
.Fn pthread_rwlock_trywrlock
fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The lock could not be acquired without blocking.
.El
.Pp
.Fn pthread_rwlock_timedrdlock
and
.Fn pthread_rwlock_timedwrlock
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 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.
.El
.Pp
The
.Fn pthread_rwlock_wrlock ,
.Fn pthread_rwlock_timedwrlock ,
and
.Fn pthread_rwlock_trywrlock
functions fail if:
.Bl -tag -width Er
.It Bq Er EDEADLK
The calling thread already owns the read/write lock (for reading
or writing).
.El
.Pp
The
.Fn pthread_rwlock_unlock
fails if:
.Bl -tag -width Er
.It Bq Er EPERM
The current thread does not own the read/write lock.
.El
.Sh SEE ALSO
.Xr pthread_rwlockattr_init 3 ,
.Xr pthreads 3
.Sh STANDARDS
These functions are expected to conform to
.St -susv2 .
.Sh HISTORY
.Fn pthread_rwlock_init ,
.Fn pthread_rwlock_destroy ,
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_tryrdlock ,
.Fn pthread_rwlock_wrlock ,
.Fn pthread_rwlock_trywrlock ,
and
.Fn pthread_rwlock_unlock
have been available since
.Ox 2.5 ,
and
.Fn pthread_rwlock_timedrdlock
and
.Fn pthread_rwlock_timedwrlock
since
.Ox 4.8 .
.Sh BUGS
The PTHREAD_PROCESS_SHARED attribute is not supported.