pthreads/pthread_mutex.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <pthread.h>
   4 #include  <unistd.h>
   5
   6 #define NUM_THREADS 5
   7
   8 int myglobal = 0;               /* global shared variable */
   9 pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
  10
  11 /* function prototypes */
  12 void *threadfun(void *arg);
  13
  14 int main() {
  15    pthread_t tid[NUM_THREADS];
  16    int i;
  17
  18    /* create the threads */
  19    for (i=0; i<NUM_THREADS; i++) {
  20       if (pthread_create(&tid[i], NULL, threadfun, NULL)) {
  21          fprintf(stderr, "pthread_create() error\n");
  22          exit(EXIT_FAILURE);
  23       }
  24    }
  25
  26    /* make sure all threads are done */
  27    for (i=0; i<NUM_THREADS; i++) {
  28       if (pthread_join(tid[i], NULL)) {
  29          fprintf(stderr, "pthread_join() error\n");
  30          exit(EXIT_FAILURE);
  31       }
  32    }
  33
  34    printf("myglobal = %d\n", myglobal);
  35
  36    return EXIT_SUCCESS;
  37 }
  38
  39 void *threadfun(void *arg) {
  40    int i, j;
  41
  42    for (i=0; i<5; i++) {
  43       pthread_mutex_lock(&mymutex); /* Begin of critical region */
  44       j = myglobal;
  45       j++;
  46       sleep(1);                 /*  */
  47       myglobal = j;
  48       pthread_mutex_unlock(&mymutex); /* End of critical region */
  49    }
  50    pthread_exit(NULL);
  51 }
  52