#include <pthread.h> int pthread_cond_signal(pthread_cond_t *cond);Service Program Name: QP0WPTHR
The pthread_cond_signal() function wakes up at least one thread that is currently waiting on the condition variable specified by cond. If no threads are currently blocked on the condition variable, this call has no effect.
When the thread that was the target of the signal wakes up, it contends for the mutex that it has associated with the condition variable on the call to pthread_cond_timedwait() or pthread_cond_wait().
The signal and broadcast functions can be called by a thread whether or not it currently owns the mutex associated with the condition variable. If predictable scheduling behavior is required from the applications viewpoint, however, the mutex should be locked by the thread that calls pthread_cond_signal() or pthread_cond_broadcast().
Note: For dependable use of condition variables, and to ensure that you do not lose wake-up operations on condition variables, your application should always use a Boolean predicate and a mutex with the condition variable.
None.
If pthread_cond_signal() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.
The condition specified is not valid.
See Code disclaimer information for information pertaining to code examples.
#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include "check.h"
/* For safe condition variable usage, must use a boolean predicate and */
/* a mutex with the condition. */
int workToDo = 0;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
#define NTHREADS 2
void *threadfunc(void *parm)
{
int rc;
while (1) {
/* Usually worker threads will loop on these operations */
rc = pthread_mutex_lock(&mutex);
checkResults("pthread_mutex_lock()\n", rc);
while (!workToDo) {
printf("Thread blocked\n");
rc = pthread_cond_wait(&cond, &mutex);
checkResults("pthread_cond_wait()\n", rc);
}
printf("Thread awake, finish work!\n");
/* Under protection of the lock, complete or remove the work */
/* from whatever worker queue we have. Here it is simply a flag */
workToDo = 0;
rc = pthread_mutex_unlock(&mutex);
checkResults("pthread_mutex_lock()\n", rc);
}
return NULL;
}
int main(int argc, char **argv)
{
int rc=0;
int i;
pthread_t threadid[NTHREADS];
printf("Enter Testcase - %s\n", argv[0]);
printf("Create %d threads\n", NTHREADS);
for(i=0; i<NTHREADS; ++i) {
rc = pthread_create(&threadid[i], NULL, threadfunc, NULL);
checkResults("pthread_create()\n", rc);
}
sleep(5); /* Sleep is not a very robust way to serialize threads */
for(i=0; i<5; ++i) {
printf("Wake up a worker, work to do...\n");
rc = pthread_mutex_lock(&mutex);
checkResults("pthread_mutex_lock()\n", rc);
/* In the real world, all the threads might be busy, and */
/* we would add work to a queue instead of simply using a flag */
/* In that case the boolean predicate might be some boolean */
/* statement like: if (the-queue-contains-work) */
if (workToDo) {
printf("Work already present, likely threads are busy\n");
}
workToDo = 1;
rc = pthread_cond_signal(&cond);
checkResults("pthread_cond_broadcast()\n", rc);
rc = pthread_mutex_unlock(&mutex);
checkResults("pthread_mutex_unlock()\n", rc);
sleep(5); /* Sleep is not a very robust way to serialize threads */
}
printf("Main completed\n");
exit(0);
return 0;
}
Output:
Enter Testcase - QP0WTEST/TPCOS0 Create 2 threads Thread blocked Thread blocked Wake up a worker, work to do... Thread awake, finish work! Thread blocked Wake up a worker, work to do... Thread awake, finish work! Thread blocked Wake up a worker, work to do... Thread awake, finish work! Thread blocked Wake up a worker, work to do... Thread awake, finish work! Thread blocked Wake up a worker, work to do... Thread awake, finish work! Thread blocked Main completed
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