REFCOUNT(9)            FreeBSD 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);

     u_int
     refcount_load(volatile u_int *count);

     void
     refcount_acquire(volatile u_int *count);

     bool
     refcount_acquire_checked(volatile u_int *count);

     bool
     refcount_acquire_if_not_zero(volatile u_int *count);

     bool
     refcount_release(volatile u_int *count);

     bool
     refcount_release_if_last(volatile u_int *count);

     bool
     refcount_release_if_not_last(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.

     Currently all functions are implemented as static inline.

     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_load() function returns a snapshot of the counter value.
     This value may immediately become out-of-date in the absence of external
     synchronization.  refcount_load() should be used instead of relying on
     the properties of the volatile qualifier.

     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_acquire_checked() variant performs the same operation as
     refcount_acquire(), but additionally checks that the count value does not
     overflow as result of the operation.  It returns true if the reference
     was sucessfully obtained, and false if it was not, due to the overflow.

     The refcount_acquire_if_not_zero() function is yet another variant of
     refcount_acquire(), which only obtains the reference when some reference
     already exists.  In other words, *count must be already greater than zero
     for the function to succeed, in which case the return value is true,
     otherwise false is returned.

     The refcount_release() function is used to release an existing reference.
     The function returns true if the reference being released was the last
     reference; otherwise, it returns false.

     The refcount_release_if_last() and refcount_release_if_not_last()
     functions are variants of refcount_release() which only drop the
     reference when it is or is not the last reference, respectively.  In
     other words, refcount_release_if_last() returns true when *count is equal
     to one, in which case it is decremented to zero.  Otherwise, *count is
     not modified and the function returns false.  Similarly,
     refcount_release_if_not_last() returns true when *count is greater than
     one, in which case *count is decremented.  Otherwise, if *count is equal
     to one, the reference is not released and the function returns false.

     Note that these routines do not provide any inter-CPU synchronization or
     data protection 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() unconditionally executes a release fence (see
     atomic(9)) before releasing the reference, which synchronizes with an
     acquire fence executed right before returning the true value.  This
     ensures that the destructor, supposedly executed by the caller after the
     last reference was dropped, sees all updates done during the lifetime of
     the object.

RETURN VALUES
     The refcount_release function returns true when releasing the last
     reference and false when releasing any other reference.

HISTORY
     These functions were introduced in FreeBSD 6.0.

FreeBSD 13.1-RELEASE-p6        November 2, 2020        FreeBSD 13.1-RELEASE-p6