pthreads/pthread_semaphore.c

   1 /* compile with:
   2    gcc pthread_semaphore.c -o pthread_semaphore -lpthread -Wall -W -Wextra -ansi -pedantic */
   3
   4 #include <stdio.h>
   5 #include <stdlib.h>
   6 #include <pthread.h>
   7 #include <semaphore.h>
   8 #include <unistd.h>
   9
  10 #define NUM_THREADS 5
  11
  12 sem_t mutex;
  13 int myglobal = 0;               /* global shared variable */
  14
  15 /* Function prototypes */
  16 void *threadfun(void *arg);
  17 void diep(const char *s);
  18
  19 int main(void)
  20 {
  21     pthread_t tid[NUM_THREADS];
  22     int i;
  23
  24     /* Initialize the semaphore */
  25     if (sem_init(&mutex, 0, 1))
  26         diep("sem_init");
  27
  28     /* Create the threads */
  29     for (i = 0; i < NUM_THREADS; i++)
  30         if (pthread_create(&tid[i], NULL, threadfun, NULL))
  31             diep("pthread_create");
  32
  33     /* Make sure all threads are done */
  34     for (i = 0; i < NUM_THREADS; i++)
  35         if (pthread_join(tid[i], NULL))
  36             diep("pthread_join");
  37
  38     printf("myglobal = %d\n", myglobal);
  39
  40     return EXIT_SUCCESS;
  41 }
  42
  43 void *threadfun(void *arg)
  44 {
  45     int i, j;
  46
  47     for (i = 0; i < 5; i++) {
  48         sem_wait(&mutex);               /* begin critical region */
  49         j = myglobal;
  50         j++;
  51         sleep(1);
  52         myglobal = j;
  53         sem_post(&mutex);               /* end critical region */
  54     }
  55
  56     pthread_exit(NULL);
  57 }
  58
  59 void diep(const char *s)
  60 {
  61     perror(s);
  62     exit(EXIT_FAILURE);
  63 }