NAME¶
refcount
,
refcount_init
,
refcount_acquire
,
refcount_release
—
manage a simple reference counter
SYNOPSIS¶
#include
<sys/param.h>
#include
<sys/refcount.h>
void
refcount_init
(
volatile
u_int *count, u_int value);
void
refcount_acquire
(
volatile
u_int *count);
int
refcount_release
(
volatile
u_int *count);
DESCRIPTION¶
The
refcount
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
count. Usually the counter is used to manage
the lifetime of an object and is stored as a member of the object.
The
refcount_init
() function is used to set
the initial value of the counter to
value. It
is normally used when creating a reference-counted object.
The
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.
The
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.
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.
RETURN VALUES¶
The
refcount_release
function returns
non-zero when releasing the last reference and zero when releasing any other
reference.
HISTORY¶
These functions were introduced in
FreeBSD 6.0.