PTHREAD_COND_BROADCAST(P)				     POSIX Programmer's Manual					 PTHREAD_COND_BROADCAST(P)

NAME

       pthread_cond_broadcast, pthread_cond_signal - broadcast or signal a condition

SYNOPSIS

       #include <pthread.h>

       int pthread_cond_broadcast(pthread_cond_t *cond);
       int pthread_cond_signal(pthread_cond_t *cond);

DESCRIPTION

       These functions shall unblock threads blocked on a condition variable.

       The pthread_cond_broadcast() function shall unblock all threads currently blocked on the specified condition variable cond.

       The  pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if
       any threads are blocked on cond).

       If more than one thread is blocked on a condition variable, the scheduling policy shall determine the order in which threads are unblocked.
       When  each thread unblocked as a result of a pthread_cond_broadcast() or pthread_cond_signal() returns from its call to pthread_cond_wait()
       or pthread_cond_timedwait(), the thread shall own the mutex with which  it  called  pthread_cond_wait()	or  pthread_cond_timedwait().  The
       thread(s)  that	are  unblocked	shall  contend for the mutex according to the scheduling policy (if applicable), and as if each had called
       pthread_mutex_lock().

       The pthread_cond_broadcast() or pthread_cond_signal() functions may be called by a thread whether or not it currently owns the  mutex  that
       threads calling pthread_cond_wait() or pthread_cond_timedwait() have associated with the condition variable during their waits; however, if
       predictable scheduling behavior is  required,  then  that  mutex  shall	be  locked  by	the  thread  calling  pthread_cond_broadcast()	or
       pthread_cond_signal().

       The pthread_cond_broadcast() and pthread_cond_signal() functions shall have no effect if there are no threads currently blocked on cond.

RETURN VALUE

       If  successful,	the  pthread_cond_broadcast()  and  pthread_cond_signal() functions shall return zero; otherwise, an error number shall be
       returned to indicate the error.

ERRORS

       The pthread_cond_broadcast() and pthread_cond_signal() function may fail if:

       EINVAL The value cond does not refer to an initialized condition variable.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The pthread_cond_broadcast() function is used whenever the shared-variable state has been changed in a way that more than  one  thread  can
       proceed with its task. Consider a single producer/multiple consumer problem, where the producer can insert multiple items on a list that is
       accessed one item at a time by the consumers.  By calling the pthread_cond_broadcast() function, the producer would  notify  all  consumers
       that  might  be	waiting,  and thereby the application would receive more throughput on a multi-processor. In addition, pthread_cond_broad-
       cast() makes it easier to implement a read-write lock. The pthread_cond_broadcast() function is needed in order	to  wake  up  all  waiting
       readers	when a writer releases its lock.  Finally, the two-phase commit algorithm can use this broadcast function to notify all clients of
       an impending transaction commit.

       It is not safe to use the pthread_cond_signal() function in a signal handler that is invoked asynchronously. Even if it	were  safe,  there
       would still be a race between the test of the Boolean pthread_cond_wait() that could not be efficiently eliminated.

       Mutexes and condition variables are thus not suitable for releasing a waiting thread by signaling from code running in a signal handler.

RATIONALE

   Multiple Awakenings by Condition Signal
       On  a  multi-processor, it may be impossible for an implementation of pthread_cond_signal() to avoid the unblocking of more than one thread
       blocked on a condition variable. For example, consider the following partial implementation of  pthread_cond_wait()  and  pthread_cond_sig-
       nal(),  executed by two threads in the order given. One thread is trying to wait on the condition variable, another is concurrently execut-
       ing pthread_cond_signal(), while a third thread is already waiting.

	      pthread_cond_wait(mutex, cond):
		  value = cond->value; /* 1 */
		  pthread_mutex_unlock(mutex); /* 2 */
		  pthread_mutex_lock(cond->mutex); /* 10 */
		  if (value == cond->value) { /* 11 */
		      me->next_cond = cond->waiter;
		      cond->waiter = me;
		      pthread_mutex_unlock(cond->mutex);
		      unable_to_run(me);
		  } else
		      pthread_mutex_unlock(cond->mutex); /* 12 */
		  pthread_mutex_lock(mutex); /* 13 */

	      pthread_cond_signal(cond):
		  pthread_mutex_lock(cond->mutex); /* 3 */
		  cond->value++; /* 4 */
		  if (cond->waiter) { /* 5 */
		      sleeper = cond->waiter; /* 6 */
		      cond->waiter = sleeper->next_cond; /* 7 */
		      able_to_run(sleeper); /* 8 */
		  }
		  pthread_mutex_unlock(cond->mutex); /* 9 */

       The effect is that more than one thread can return from its call to pthread_cond_wait() or pthread_cond_timedwait() as a result of one call
       to  pthread_cond_signal().  This  effect  is  called  "spurious	wakeup".  Note that the situation is self-correcting in that the number of
       threads that are so awakened is finite; for example, the next thread to call pthread_cond_wait() after the sequence of events above blocks.

       While this problem could be resolved, the loss of efficiency for a fringe condition that occurs only  rarely  is  unacceptable,	especially
       given  that  one has to check the predicate associated with a condition variable anyway. Correcting this problem would unnecessarily reduce
       the degree of concurrency in this basic building block for all higher-level synchronization operations.

       An added benefit of allowing spurious wakeups is that applications are forced to code a predicate-testing-loop around the  condition  wait.
       This  also  makes  the application tolerate superfluous condition broadcasts or signals on the same condition variable that may be coded in
       some other part of the application. The resulting applications are thus more robust. Therefore, IEEE Std 1003.1-2001  explicitly  documents
       that spurious wakeups may occur.

FUTURE DIRECTIONS

       None.

SEE ALSO

       pthread_cond_destroy() , pthread_cond_timedwait() , the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- 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 .

IEEE
/The Open Group						       2003						 PTHREAD_COND_BROADCAST(P)