1.\" 2.\" Copyright (c) 2009 Advanced Computing Technologies LLC 3.\" Written by: John H. Baldwin <jhb@FreeBSD.org> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 16.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 17.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 18.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 19.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 20.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 21.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 22.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 25.\" SUCH DAMAGE. 26.\" 27.\" $FreeBSD$ 28.\" 29.Dd January 20, 2009 30.Dt REFCOUNT 9 31.Os 32.Sh NAME 33.Nm refcount , 34.Nm refcount_init , 35.Nm refcount_acquire , 36.Nm refcount_release 37.Nd manage a simple reference counter 38.Sh SYNOPSIS 39.In sys/param.h 40.In sys/refcount.h 41.Ft void 42.Fn refcount_init "volatile u_int *count, u_int value" 43.Ft void 44.Fn refcount_acquire "volatile u_int *count" 45.Ft int 46.Fn refcount_release "volatile u_int *count" 47.Sh DESCRIPTION 48The 49.Nm 50functions provide an API to manage a simple reference counter. 51The caller provides the storage for the counter in an unsigned integer. 52A pointer to this integer is passed via 53.Fa count . 54Usually the counter is used to manage the lifetime of an object and is 55stored as a member of the object. 56.Pp 57The 58.Fn refcount_init 59function is used to set the initial value of the counter to 60.Fa value . 61It is normally used when creating a reference-counted object. 62.Pp 63The 64.Fn refcount_acquire 65function is used to acquire a new reference. 66The caller is responsible for ensuring that it holds a valid reference 67while obtaining a new reference. 68For example, 69if an object is stored on a list and the list holds a reference on the 70object, then holding a lock that protects the list provides sufficient 71protection for acquiring a new reference. 72.Pp 73The 74.Fn refcount_release 75function is used to release an existing reference. 76The function returns a non-zero value if the reference being released was 77the last reference; 78otherwise, it returns zero. 79.Pp 80Note that these routines do not provide any inter-CPU synchronization, 81data protection, 82or memory ordering guarantees except for managing the counter. 83The caller is responsible for any additional synchronization needed by 84consumers of any containing objects. 85In addition, 86the caller is also responsible for managing the life cycle of any containing 87objects including explicitly releasing any resources when the last reference 88is released. 89.Sh RETURN VALUES 90The 91.Nm refcount_release 92function returns non-zero when releasing the last reference and zero when 93releasing any other reference. 94.Sh HISTORY 95These functions were introduced in 96.Fx 6.0 . 97