pthread_semaphore.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <pthread.h>
   4 #include <semaphore.h>
   5 #include <unistd.h>
   6
   7 #define NUM_THREADS 5
   8
   9 sem_t mutex;
  10 int myglobal = 0;               /* global shared variable */
  11
  12 /* function prototypes */
  13 void *threadfun(void *arg);
  14
  15 int main(void) {
  16    pthread_t tid[NUM_THREADS];
  17    int i;
  18
  19    /* initialize the semaphore */
  20    if (sem_init(&mutex, 0, 1)) {
  21       fprintf(stderr, "sem_init() error\n");
  22       exit(EXIT_FAILURE);
  23    }
  24
  25    /* create the threads */
  26    for (i=0; i<NUM_THREADS; i++) {
  27       if (pthread_create(&tid[i], NULL, threadfun, NULL)) {
  28          fprintf(stderr, "pthread_create() error\n");
  29          exit(EXIT_FAILURE);
  30       }
  31    }
  32
  33    /* make sure all threads are done */
  34    for (i=0; i<NUM_THREADS; i++)
  35       if (pthread_join(tid[i], NULL)) {
  36          fprintf(stderr, "pthread_join() error\n");
  37          exit(EXIT_FAILURE);
  38       }
  39
  40    printf("myglobal = %d\n", myglobal);
  41
  42    return EXIT_SUCCESS;
  43 }
  44
  45 void *threadfun(void *arg) {
  46    int i, j;
  47
  48    for (i=0; i<5; i++) {
  49       sem_wait(&mutex);         /* begin critical region */
  50       j = myglobal;
  51       j++;
  52       sleep(1);
  53       myglobal = j;
  54       sem_post(&mutex);         /* end critical region */
  55    }
  56
  57    pthread_exit(NULL);
  58 }