97 lines
3.5 KiB
Groff
97 lines
3.5 KiB
Groff
|
.\"
|
||
|
.\" Copyright (c) 2009 Advanced Computing Technologies LLC
|
||
|
.\" Written by: John H. Baldwin <jhb@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, 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 AUTHOR 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 AUTHOR 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.
|
||
|
.\"
|
||
|
.\" $FreeBSD$
|
||
|
.\"
|
||
|
.Dd January 20, 2009
|
||
|
.Dt REFCOUNT 9
|
||
|
.Os
|
||
|
.Sh NAME
|
||
|
.Nm refcount ,
|
||
|
.Nm refcount_init ,
|
||
|
.Nm refcount_acquire ,
|
||
|
.Nm refcount_release
|
||
|
.Nd manage a simple reference counter
|
||
|
.Sh SYNOPSIS
|
||
|
.In sys/param.h
|
||
|
.In sys/refcount.h
|
||
|
.Ft void
|
||
|
.Fn refcount_init "volatile u_int *count, u_int value"
|
||
|
.Ft void
|
||
|
.Fn refcount_acquire "volatile u_int *count"
|
||
|
.Ft int
|
||
|
.Fn refcount_release "volatile u_int *count"
|
||
|
.Sh DESCRIPTION
|
||
|
The
|
||
|
.Nm
|
||
|
functions provide an API to manage a simple reference counter.
|
||
|
The caller provides the storage for the counter in an unsigned integer.
|
||
|
A pointer to this integer is passed via
|
||
|
.Fa count .
|
||
|
Usually the counter is used to manage the lifetime of an object and is
|
||
|
stored as a member of the object.
|
||
|
.Pp
|
||
|
The
|
||
|
.Fn refcount_init
|
||
|
function is used to set the initial value of the counter to
|
||
|
.Fa value .
|
||
|
It is normally used when creating a reference-counted object.
|
||
|
.Pp
|
||
|
The
|
||
|
.Fn refcount_acquire
|
||
|
function is used to acquire a new reference.
|
||
|
The caller is responsible for ensuring that it holds a valid reference
|
||
|
while obtaining a new reference.
|
||
|
For example,
|
||
|
if an object is stored on a list and the list holds a reference on the
|
||
|
object, then holding a lock that protects the list provides sufficient
|
||
|
protection for acquiring a new reference.
|
||
|
.Pp
|
||
|
The
|
||
|
.Fn refcount_release
|
||
|
function is used to release an existing reference.
|
||
|
The function returns a non-zero value if the reference being released was
|
||
|
the last reference;
|
||
|
otherwise, it returns zero.
|
||
|
.Pp
|
||
|
Note that these routines do not provide any inter-CPU synchronization,
|
||
|
data protection,
|
||
|
or memory ordering guarantees except for managing the counter.
|
||
|
The caller is responsible for any additional synchronization needed by
|
||
|
consumers of any containing objects.
|
||
|
In addition,
|
||
|
the caller is also responsible for managing the life cycle of any containing
|
||
|
objects including explicitly releasing any resources when the last reference
|
||
|
is released.
|
||
|
.Sh RETURN VALUES
|
||
|
The
|
||
|
.Nm refcount_release
|
||
|
function returns non-zero when releasing the last reference and zero when
|
||
|
releasing any other reference.
|
||
|
.Sh HISTORY
|
||
|
These functions were introduced in
|
||
|
.Fx 6.0 .
|