PTHREAD_COND_BROADCAST(3P) POSIX Programmer's Manual PTHREAD_COND_BROADCAST(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond-
ing 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 condition 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_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
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_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, POSIX.1-2008 explicitly documents that spu-
rious 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 Technol-
ogy -- 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 Stan-
dard 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(3P)