REFCOUNT(9)                                                BSD Kernel Developer's Manual                                               REFCOUNT(9)

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 call-
     er is also responsible for managing the life cycle of any containing objects including explicitly releasing any resources when the last ref-
     erence 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.

BSD                                                              January 20, 2009                                                              BSD