pthread_cond_wait()--Wait for Condition

 #include <pthread.h>
 int pthread_cond_wait(pthread_cond_t *cond,
                      pthread_mutex_t *mutex);   

The pthread_cond_wait() function blocks the calling thread, waiting for the condition specified by cond to be signaled or broadcast to.

When pthread_cond_wait() is called, the calling thread must have mutex locked. The pthread_cond_wait() function atomically unlocks mutex and performs the wait for the condition. In this case, atomically means with respect to the mutex and the condition variable and another threads access to those objects through the pthread condition variable interfaces.

If the wait is satisfied, or if the thread is canceled, before the thread is allowed to continue, the mutex is automatically acquired by the calling thread. If mutex is not currently locked, an EPERM error results. You should always associate only one mutex with a condition at a time. Using two different mutexes with the same condition at the same time could lead to unpredictable serialization issues in your application.

This function is a cancellation point.

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.

Authorities and Locks

For successful completion, the mutex lock associated with the condition variable is must be locked prior to calling pthread_cond_wait().

Parameters

cond

(Input) Address of the condition variable to wait on

mutex

(Input) Address of the mutex associated with the condition variable

Return Value

0

pthread_cond_wait() was successful.

value

pthread_cond_wait() was not successful. value is set to indicate the error condition.

Error Conditions

If pthread_cond_wait() 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.

[EINVAL]

The value specified for the argument is not correct.

[EPERM]

The mutex specified is not locked by the caller.

Related Information

• The <pthread.h> header file. See Header files for Pthread functions.
  
  
• pthread_cond_broadcast()--Broadcast Condition to All Waiting Threads
  
  
• pthread_cond_init()--Initialize Condition Variable
  
  
• pthread_cond_signal()--Signal Condition to One Waiting Thread
  
  
• pthread_cond_timedwait()--Timed Wait for Condition

Example

Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.

#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                 conditionMet = 0;
pthread_cond_t      cond  = PTHREAD_COND_INITIALIZER;
pthread_mutex_t     mutex = PTHREAD_MUTEX_INITIALIZER;

#define NTHREADS    5

void *threadfunc(void *parm)
{
  int           rc;

  rc = pthread_mutex_lock(&mutex);
  checkResults("pthread_mutex_lock()\n", rc);

  while (!conditionMet) {
    printf("Thread blocked\n");
    rc = pthread_cond_wait(&cond, &mutex);
    checkResults("pthread_cond_wait()\n", rc);
  }

  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 */
  rc = pthread_mutex_lock(&mutex);
  checkResults("pthread_mutex_lock()\n", rc);

  /* The condition has occured. Set the flag and wake up any waiting threads */
  conditionMet = 1;
  printf("Wake up all waiting threads...\n");
  rc = pthread_cond_broadcast(&cond);
  checkResults("pthread_cond_broadcast()\n", rc);

  rc = pthread_mutex_unlock(&mutex);
  checkResults("pthread_mutex_unlock()\n", rc);

  printf("Wait for threads and cleanup\n");
  for (i=0; i<NTHREADS; ++i) {
    rc = pthread_join(threadid[i], NULL);
    checkResults("pthread_join()\n", rc);
  }
  pthread_cond_destroy(&cond);
  pthread_mutex_destroy(&mutex);

  printf("Main completed\n");
  return 0;
}

Output:

Entering testcase
Create 5 threads
Thread blocked
Thread blocked
Thread blocked
Thread blocked
Thread blocked
Wake up all waiting threads...
Wait for threads and cleanup
Main completed

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