mutex-test.c

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

pthread_mutex_t count_lock;
pthread_cond_t count_nonzero;
unsigned count;

void *decrement_count() {
  while(1)
  {
    pthread_mutex_lock(&count_lock);
    if (count == 0)
      pthread_cond_wait(&count_nonzero, &count_lock);

    count = count - 1;
    printf("dec count=%d\n", count);
    pthread_mutex_unlock(&count_lock);
  }
}

void *increment_count() {
  while(1)
  {
    pthread_mutex_lock(&count_lock);
    count = count + 1;
    printf("inc count=%d\n", count);

    if (count > 2) {
      printf("signal ...\n");
      pthread_cond_signal(&count_nonzero);
    }
    pthread_mutex_unlock(&count_lock);
  }
}

int main() {
  pthread_t dec, inc;

  pthread_create(&dec, NULL, decrement_count, NULL);
  pthread_create(&inc, NULL, increment_count, NULL);

  pthread_join(inc, NULL);
  pthread_join(dec, NULL);
  return 0;
}