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.\"
.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. 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(s), this list of conditions and the following disclaimer as
.\" the first lines of this file unmodified other than the possible
.\" addition of one or more copyright notices.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice(s), 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 COPYRIGHT HOLDER(S) ``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 COPYRIGHT HOLDER(S) 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.
.\"
.\" $FreeBSD$
.\"
.Dd February 26, 2001
.Dt SX 9
.Os
.Sh NAME
.Nm sx ,
.Nm sx_init ,
.Nm sx_destroy ,
.Nm sx_slock ,
.Nm sx_xlock ,
.Nm sx_try_slock ,
.Nm sx_try_xlock ,
.Nm sx_sunlock ,
.Nm sx_xunlock
.Nm sx_try_upgrade
.Nm sx_downgrade
.Nd kernel shared/exclusive lock
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/lock.h>
.Fd #include <sys/mutex.h>
.Fd #include <sys/sx.h>
.Ft void
.Fn sx_init "struct sx *sx" "const char *description"
.Ft void
.Fn sx_destroy "struct sx *sx"
.Ft void
.Fn sx_slock "struct sx *sx"
.Ft void
.Fn sx_xlock "struct sx *sx"
.Ft int
.Fn sx_try_slock "struct sx *sx"
.Ft int
.Fn sx_try_xlock "struct sx *sx"
.Ft void
.Fn sx_sunlock "struct sx *sx"
.Ft void
.Fn sx_xunlock "struct sx *sx"
.Ft int
.Fn sx_try_upgrade "struct sx *sx"
.Ft void
.Fn sx_downgrade "struct sx *sx"
.Sh DESCRIPTION
Shared/exclusive locks are used to protect data that are read far more often
than they are written.
Mutexes are inherently more efficient than shared/exclusive locks, so
shared/exclusive locks should be used prudently.
.Pp
Shared/exclusive locks are created with
.Fn sx_init ,
where
.Fa sx
is a pointer to space for a
.Vt struct sx ,
and
.Fa description
is a pointer to a null-terminated character string that describes the
shared/exclusive lock.
Shared/exclusive locks are destroyed with
.Fn sx_destroy .
Threads acquire and release a shared lock by calling
.Fn sx_slock
or
.Fn sx_try_slock
and
.Fn sx_sunlock .
Threads acquire and release an exclusive lock by calling
.Fn sx_xlock
or
.Fn sx_try_xlock
and
.Fn sx_xunlock .
A thread can attempt to upgrade a currently owned shared lock to an exclusive
lock by calling
.Fn sx_try_upgrade .
A thread that owns an exclusive lock can downgrade it to a shared lock by
calling
.Fn sx_downgrade .
.Pp
.Fn sx_try_slock
and
.Fn sx_try_xlock
will return 0 if the shared/exclusive lock cannot be acquired immediately;
otherwise the shared/exclusive lock will be acquired and a non-zero value will
be returned.
.Pp
.Fn sx_try_upgrade
will return 0 if the shared lock cannot be upgraded to an exclusive lock
immediately; otherwise the exclusive lock will be acquired and a non-zero value
will be returned.
.Pp
A thread may not own a shared lock and an exclusive lock simultaneously;
attempting to do so will result in deadlock.
.Sh SEE ALSO
.Xr mutex 9
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