.\" $OpenBSD: rwlock.9,v 1.3 2006/08/16 10:19:00 dlg Exp $
.\"
.\" Copyright (c) 2006 Pedro Martelletto <pedro@openbsd.org>
.\" All rights reserved.
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd April 19, 2006
.Dt RWLOCK 9
.Os
.Sh NAME
.Nm rwlock ,
.Nm rw_init ,
.Nm rw_enter_read ,
.Nm rw_enter_write ,
.Nm rw_exit_read ,
.Nm rw_exit_write
.Nd interface to read/write locks
.Sh SYNOPSIS
.Fd #include <sys/rwlock.h>
.Ft void
.Fn rw_init "struct rwlock *rwl" "const char *name"
.Ft void
.Fn rw_enter_read "struct rwlock *rwl"
.Ft void
.Fn rw_enter_write "struct rwlock *rwl"
.Ft void
.Fn rw_exit_read "struct rwlock *rwl"
.Ft void
.Fn rw_exit_write "struct rwlock *rwl"
.Sh DESCRIPTION
The
.Nm
set of functions provides a multiple-reader, single-writer locking mechanism to
ensure mutual exclusion between different processes.
.Pp
The
.Fn rw_init
function is used to initiate the lock pointed to by
.Fa rwl .
The
.Fa name
argument specifies the name of the lock, which is used as the wait message
if the process needs to sleep.
.Pp
The
.Fn rw_enter_read
function acquires a read lock, sleeping if necessary.
.Pp
The
.Fn rw_enter_write
function acquires a write lock, sleeping if necessary.
.Pp
The
.Fn rw_exit_read
function releases a read lock.
.Pp
The
.Fn rw_exit_write
function releases a write lock.
.Pp
Read locks can be acquired while the write lock is not held, and may coexist in
distinct processes at any time.
A write lock, however, can only be acquired when there are no read locks held,
granting exclusive access to a single process.
.Sh SEE ALSO
.Xr lockmgr 9 ,
.Xr spl 9
.Sh HISTORY
The
.Nm
functions first appeared in
.Ox 3.5 .
.Sh AUTHORS
The
.Nm
functions were written by
.An Artur Grabowski
.Aq art@openbsd.org .