The pthread_rwlock_unlock() function shall release a lock held on the
read-write lock object referenced by rwlock. Results are undefined if
the read-write lock rwlock is not held by the calling thread.
If this function is called to release a read lock from the read-write lock
object and there are other read locks currently held on this read-write lock
object, the read-write lock object remains in the read locked state. If this
function releases the last read lock for this read-write lock object, the
read-write lock object shall be put in the unlocked state with no owners.
If this function is called to release a write lock for this read-write lock
object, the read-write lock object shall be put in the unlocked state.
If there are threads blocked on the lock when it becomes available, the
scheduling policy shall determine which thread(s) shall acquire the lock.
If the Thread Execution Scheduling option is supported, when threads
executing with the scheduling policies SCHED_FIFO, SCHED_RR, or SCHED_SPORADIC
are waiting on the lock, they shall acquire the lock in priority order when
the lock becomes available. For equal priority threads, write locks shall take
precedence over read locks. If the Thread Execution Scheduling option is not
supported, it is implementation-defined whether write locks take precedence
over read locks.
Results are undefined if any of these functions are called with an uninitialized
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html