trusty (3) pthread_cond_signal.3posix.gz
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.
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
The pthread_cond_broadcast() and pthread_cond_signal() function may fail if:
EINVAL The value cond does not refer to an initialized condition variable.
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
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, IEEE Std 1003.1-2001 explicitly
documents that spurious wakeups may occur.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_cond_destroy() , pthread_cond_timedwait() , the Base Definitions volume of IEEE Std 1003.1-2001,
<pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .