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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
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_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
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;
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 spurious wakeups may occur.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_cond_destroy(), pthread_cond_timedwait()
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Synchronization,
<pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 .