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.\" $FreeBSD: head/share/man/man9/refcount.9 281887 2015-04-23 14:22:20Z jhb $
.\"
.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 .