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Create a pthread then wait for a signal

Posted on 2004-09-17
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Last Modified: 2012-06-27
How do you go about creating a pthread which then runs through an infinite for loop waiting for a signal from a server?
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Question by:jewee
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jkr earned 500 total points
ID: 12088217
You'd do that like

#include <pthread.h>
#include <stdio.h>

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int     count = 0;
int     thread_ids[3] = {0,1,2};
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *idp)
{
  int j,i;
  double result=0.0;
  int *my_id = idp;

  for (i=0; i < TCOUNT; i++) {
    pthread_mutex_lock(&count_mutex);
    count++;

    /*
    Check the value of count and signal waiting thread when condition is
    reached.  Note that this occurs while mutex is locked.
    */
    if (count == COUNT_LIMIT) {
      pthread_cond_signal(&count_threshold_cv);
      printf("inc_count(): thread %d, count = %d  Threshold reached.\n",
             *my_id, count);
      }
    printf("inc_count(): thread %d, count = %d, unlocking mutex\n",
         *my_id, count);
    pthread_mutex_unlock(&count_mutex);

    /* Do some work so threads can alternate on mutex lock */
    for (j=0; j < 1000; j++)
      result = result + (double)random();
    }
  pthread_exit(NULL);
}

void *watch_count(void *idp)
{
  int *my_id = idp;

  printf("Starting watch_count(): thread %d\n", *my_id);

  /*
  Lock mutex and wait for signal.  Note that the pthread_cond_wait
  routine will automatically and atomically unlock mutex while it waits.
  Also, note that if COUNT_LIMIT is reached before this routine is run by
  the waiting thread, the loop will be skipped to prevent pthread_cond_wait
  from never returning.
  */
  pthread_mutex_lock(&count_mutex);
  while (count < COUNT_LIMIT) {
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %d Condition signal
           received.\n", *my_id);
    }
  pthread_mutex_unlock(&count_mutex);
  pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
  int i, rc;
  pthread_t threads[3];
  pthread_attr_t attr;

  /* Initialize mutex and condition variable objects */
  pthread_mutex_init(&count_mutex, NULL);
  pthread_cond_init (&count_threshold_cv, NULL);

  /*
  For portability, explicitly create threads in a joinable state
  so that they can be joined later.
  */
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  pthread_create(&threads[0], &attr, inc_count, (void *)&thread_ids[0]);
  pthread_create(&threads[1], &attr, inc_count, (void *)&thread_ids[1]);
  pthread_create(&threads[2], &attr, watch_count, (void *)&thread_ids[2]);

  /* Wait for all threads to complete */
  for (i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
  }
  printf ("Main(): Waited on %d  threads. Done.\n", NUM_THREADS);

  /* Clean up and exit */
  pthread_attr_destroy(&attr);
  pthread_mutex_destroy(&count_mutex);
  pthread_cond_destroy(&count_threshold_cv);
  pthread_exit(NULL);

}

See also http://www.llnl.gov/computing/tutorials/workshops/workshop/pthreads/MAIN.html ("POSIX thread programming")
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Author Comment

by:jewee
ID: 12088524
Why do I need 3 threads?
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LVL 86

Expert Comment

by:jkr
ID: 12088892
You don't need three threads - but that sample illustrates how to use threads to wait for a signal in general.
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