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     refcount, refcount_init, refcount_acquire, refcount_release — manage a simple reference


     #include <sys/param.h>
     #include <sys/refcount.h>

     refcount_init(volatile u_int *count, u_int value);

     refcount_acquire(volatile u_int *count);

     refcount_release(volatile u_int *count);


     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.


     The refcount_release function returns non-zero when releasing the last reference and zero
     when releasing any other reference.


     These functions were introduced in FreeBSD 6.0.