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97 lines
3.5 KiB
Groff
97 lines
3.5 KiB
Groff
.\"
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.\" Copyright (c) 2009 Advanced Computing Technologies LLC
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.\" Written by: John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 20, 2009
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.Dt REFCOUNT 9
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.Os
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.Sh NAME
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.Nm refcount ,
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.Nm refcount_init ,
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.Nm refcount_acquire ,
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.Nm refcount_release
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.Nd manage a simple reference counter
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/refcount.h
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.Ft void
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.Fn refcount_init "volatile u_int *count, u_int value"
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.Ft void
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.Fn refcount_acquire "volatile u_int *count"
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.Ft int
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.Fn refcount_release "volatile u_int *count"
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.Sh DESCRIPTION
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The
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.Nm
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functions provide an API to manage a simple reference counter.
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The caller provides the storage for the counter in an unsigned integer.
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A pointer to this integer is passed via
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.Fa count .
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Usually the counter is used to manage the lifetime of an object and is
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stored as a member of the object.
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.Pp
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The
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.Fn refcount_init
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function is used to set the initial value of the counter to
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.Fa value .
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It is normally used when creating a reference-counted object.
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.Pp
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The
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.Fn refcount_acquire
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function is used to acquire a new reference.
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The caller is responsible for ensuring that it holds a valid reference
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while obtaining a new reference.
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For example,
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if an object is stored on a list and the list holds a reference on the
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object, then holding a lock that protects the list provides sufficient
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protection for acquiring a new reference.
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.Pp
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The
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.Fn refcount_release
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function is used to release an existing reference.
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The function returns a non-zero value if the reference being released was
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the last reference;
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otherwise, it returns zero.
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.Pp
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Note that these routines do not provide any inter-CPU synchronization,
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data protection,
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or memory ordering guarantees except for managing the counter.
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The caller is responsible for any additional synchronization needed by
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consumers of any containing objects.
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In addition,
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the caller is also responsible for managing the life cycle of any containing
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objects including explicitly releasing any resources when the last reference
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is released.
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.Sh RETURN VALUES
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The
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.Nm refcount_release
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function returns non-zero when releasing the last reference and zero when
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releasing any other reference.
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.Sh HISTORY
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These functions were introduced in
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.Fx 6.0 .
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