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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.