.\" $OpenBSD: rwlock.9,v 1.8 2007/12/09 20:54:01 jmc Exp $
.\"
.\" Copyright (c) 2006 Pedro Martelletto <pedro@ambientworks.net>
.\" All rights reserved.
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
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.Dd $Mdocdate: December 9 2007 $
.Dt RWLOCK 9
.Os
.Sh NAME
.Nm rwlock ,
.Nm rw_init ,
.Nm rw_enter ,
.Nm rw_exit ,
.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 int
.Fn rw_enter "struct rwlock *rwl" "int flags"
.Ft void
.Fn rw_exit "struct rwlock *rwl"
.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
function acquires a lock.
The
.Fa flags
argument specifies what kind of lock should be obtained and also
modifies the operation.
The possible flags are:
.Pp
.Bl -tag -offset indent -width RW_DOWNGRADEXXX -compact
.It Dv RW_READ
Acquire a shared lock.
.It Dv RW_WRITE
Acquire an exclusive lock.
.It Dv RW_DOWNGRADE
Safely release an exclusive lock and acquire a shared lock without
letting other exclusive locks in between.
.It Dv RW_INTR
When waiting for a lock, allow signals to interrupt the sleep.
.It Dv RW_NOSLEEP
Do not wait for busy locks, fail with
.Dv EBUSY
instead.
.It Dv RW_SLEEPFAIL
Wait for busy locks, but do not obtain them, fail with
.Dv EAGAIN
instead.
.El
.Pp
The
.Fn rw_exit
function is used to release a held lock.
.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 mutex 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 .
.Sh CAVEATS
While it is safe to sleep with an rwlock held, they cannot
be used in an interrupt handler as an rwlock is bound to a process.