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PTHREAD_COND_BROADCAST(3P)POSIX Programmer's ManualTHREAD_COND_BROADCAST(3P)
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.
pthread_cond_broadcast, pthread_cond_signal — broadcast or signal a
condition
#include <pthread.h>
int pthread_cond_broadcast(pthread_cond_t *cond);
int pthread_cond_signal(pthread_cond_t *cond);
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.
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.
These functions shall not return an error code of [EINTR].
The following sections are informative.
None.
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.
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.
None.
pthread_cond_destroy(3p), pthread_cond_timedwait(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
Synchronization, pthread.h(0p)
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 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)
Pages that refer to this page: pthread.h(0p), pthread_cond_destroy(3p), pthread_cond_signal(3p), pthread_cond_timedwait(3p)