pthreads/pthread_semaphore.c

   1 /*
   2  * Compile with:
   3  * gcc pthread_semaphore.c -o pthread_semaphore -lpthread -Wall -W -Wextra -ansi -pedantic
   4  */
   5
   6 #include <stdio.h>
   7 #include <stdlib.h>
   8 #include <pthread.h>
   9 #include <semaphore.h>
  10 #include <unistd.h>
  11
  12 #define NUM_THREADS 5
  13
  14 sem_t mutex;
  15 int myglobal = 0;               /* global shared variable */
  16
  17 /* Function prototypes */
  18 void *threadfun(void *arg);
  19 void diep(const char *s);
  20
  21 int main(void)
  22 {
  23     pthread_t tid[NUM_THREADS];
  24     int i;
  25
  26     /* Initialize the semaphore */
  27     if (sem_init(&mutex, 0, 1))
  28         diep("sem_init");
  29
  30     /* Create the threads */
  31     for (i = 0; i < NUM_THREADS; i++)
  32         if (pthread_create(&tid[i], NULL, threadfun, NULL))
  33             diep("pthread_create");
  34
  35     /* Make sure all threads are done */
  36     for (i = 0; i < NUM_THREADS; i++)
  37         if (pthread_join(tid[i], NULL))
  38             diep("pthread_join");
  39
  40     printf("myglobal = %d\n", myglobal);
  41
  42     return EXIT_SUCCESS;
  43 }
  44
  45 void *threadfun(void *arg)
  46 {
  47     int i, j;
  48
  49     for (i = 0; i < 5; i++) {
  50         sem_wait(&mutex);               /* begin critical region */
  51         j = myglobal;
  52         j++;
  53         sleep(1);
  54         myglobal = j;
  55         sem_post(&mutex);               /* end critical region */
  56     }
  57
  58     pthread_exit(NULL);
  59 }
  60
  61 void diep(const char *s)
  62 {
  63     perror(s);
  64     exit(EXIT_FAILURE);
  65 }