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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PTHREAD_...ROADCAST(3P) POSIX Programmer's Manual PTHREAD_...ROADCAST(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.1‐2017, Section 4.12, Memory
Synchronization, pthread.h(0p)
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 .
IEEE/The Open Group 2017 PTHREAD_...ROADCAST(3P)
Pages that refer to this page: pthread.h(0p), pthread_cond_destroy(3p), pthread_cond_signal(3p), pthread_cond_timedwait(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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