.\" Copyright (c) 2014 The FreeBSD Foundation
.\" All rights reserved.
.\"
.\" Part of this documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
.\"
.\" 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 AUTHORS 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 AUTHORS 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.
.\"
.\" $FreeBSD: head/share/man/man9/casuword.9 273783 2014-10-28 15:22:13Z kib $
.\"
.Dd October 21, 2014
.Dt CASU 9
.Os
.Sh NAME
.Nm casueword ,
.Nm casueword32 ,
.Nm casuword ,
.Nm casuword32
.Nd fetch, compare and store data from user-space
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft int
.Fn casueword "volatile u_long *base" "u_long oldval" "u_long *oldvalp" "u_long newval"
.Ft int
.Fn casueword32 "volatile uint32_t *base" "uint32_t oldval" "uint32_t *oldvalp" "uint32_t newval"
.Ft u_long
.Fn casuword "volatile u_long *base" "u_long oldval" "u_long newval"
.Ft uint32_t
.Fn casuword32 "volatile uint32_t *base" "uint32_t oldval" "uint32_t newval"
.Sh DESCRIPTION
The
.Nm
functions are designed to perform atomic compare-and-swap operation on
the value in the usermode memory of the current process.
.Pp
The
.Nm
routines reads the value from user memory with address
.Pa base ,
and compare the value read with
.Pa oldval .
If the values are equal,
.Pa newval
is written to the
.Pa *base .
In case of
.Fn casueword32
and
.Fn casueword ,
old value is stored into the (kernel-mode) variable pointed by
.Pa *oldvalp .
The userspace value must be naturally aligned.
.Pp
The callers of
.Fn casuword
and
.Fn casuword32
functions cannot distinguish between -1 read from
userspace and function failure.
.Sh RETURN VALUES
The
.Fn casuword
and
.Fn casuword32
functions return the data fetched or -1 on failure.
The
.Fn casueword
and
.Fn casueword32
functions return 0 on success and -1 on failure.
.Sh SEE ALSO
.Xr atomic 9 ,
.Xr fetch 9 ,
.Xr store 9