pthread_cond_broadcast(3p) — Linux manual page



PROLOG         top

       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         top

       pthread_cond_broadcast, pthread_cond_signal — broadcast or signal a

SYNOPSIS         top

       #include <pthread.h>

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

DESCRIPTION         top

       These functions shall unblock threads blocked on a condition

       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

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

       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 condition variable.

RETURN VALUE         top

       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         top

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

       The following sections are informative.

EXAMPLES         top



       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_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         top

       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 condition 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 example, 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;
               } else
                   pthread_mutex_unlock(cond->mutex); /* 12 */
               pthread_mutex_lock(mutex); /* 13 */

               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, POSIX.1‐2008 explicitly
       documents that spurious wakeups may occur.



SEE ALSO         top

       pthread_cond_destroy(3p), pthread_cond_timedwait(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
       Synchronization, pthread.h(0p)

COPYRIGHT         top

       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 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 .

       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 .

IEEE/The Open Group                 2013          PTHREAD_COND_BROADCAST(3P)

Pages that refer to this page: pthread.h(0p)pthread_cond_destroy(3p)pthread_cond_init(3p)pthread_cond_signal(3p)pthread_cond_timedwait(3p)pthread_cond_wait(3p)