Thread Condition Wait Coding Pattern

The reason why we need pthread_mutex_lock(&mutex) before pthread_cond_wait(&cond, &mutex) :
Need to protect the condition read/wirte of variables that changes the conditions.
Before we call pthread_cond_wait, we need to check if condition is met or not, if it is met, not call to pthread_cond_wait is needed.

Below sample code can be found at
http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=%2Fapis%2Fusers_78.htm

#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;
}