PTHREAD_COND_BROADCAST(3POSIX)      POSIX Programmer's Manual      PTHREAD_COND_BROADCAST(3POSIX)

PROLOG
       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

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.

       The  behavior  is  undefined  if  the  value   specified   by   the   cond   argument   to
       pthread_cond_broadcast()  or pthread_cond_signal() does not refer to an initialized condi-
       tion variable.

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
       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_broad-
       cast()  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_broadcast()   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
       If   an  implementation  detects  that  the  value  specified  by  the  cond  argument  to
       pthread_cond_broadcast() or pthread_cond_signal() does not refer to an initialized  condi-
       tion  variable,  it  is  recommended  that the function should fail and report an [EINVAL]
       error.

   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 exam-
       ple,  consider  the  following   partial   implementation   of   pthread_cond_wait()   and
       pthread_cond_signal(), executed by two threads in the order given. One thread is trying to
       wait on the condition variable, another is concurrently  executing  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  syn-
       chronization 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  toler-
       ate 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, POSIX.1-2008 explicitly documents that spurious wakeups may occur.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_cond_destroy(), pthread_cond_timedwait()

       The  Base  Definitions  volume  of  POSIX.1-2008,  Section  4.11,  Memory Synchronization,
       <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2013  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013  by  the
       Institute  of  Electrical  and  Electronics  Engineers,  Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any  discrep-
       ancy  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.unix.org/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                            2013                PTHREAD_COND_BROADCAST(3POSIX)