psem/thread_incr_psem.c

+/* thread_incr_psem.c
+
+   Use a POSIX unnamed semaphore to synchronize access by two threads to
+   a global variable.
+
+   See also thread_incr.c and thread_incr_mutex.c.
+*/
 #include <semaphore.h>
 #include <pthread.h>
 #include "tlpi_hdr.h"
 
 static int glob = 0;
 static sem_t sem;
 
 static void *                   /* Loop 'arg' times incrementing 'glob' */
 threadFunc(void *arg)
 {
     int loops = *((int *) arg);
     int loc, j;
 
     for (j = 0; j < loops; j++) {
         if (sem_wait(&sem) == -1)
             errExit("sem_wait");
 
         loc = glob;
         loc++;
         glob = loc;
 
         if (sem_post(&sem) == -1)
             errExit("sem_post");
     }
 
     return NULL;
 }
 
 int
 main(int argc, char *argv[])
 {
     pthread_t t1, t2;
     int loops, s;
 
     loops = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-loops") : 10000000;
 
     /* Initialize a semaphore with the value 1 */
 
     if (sem_init(&sem, 0, 1) == -1)
         errExit("sem_init");
 
     /* Create two threads that increment 'glob' */
 
     s = pthread_create(&t1, NULL, threadFunc, &loops);
     if (s != 0)
         errExitEN(s, "pthread_create");
     s = pthread_create(&t2, NULL, threadFunc, &loops);
     if (s != 0)
         errExitEN(s, "pthread_create");
 
     /* Wait for threads to terminate */
 
     s = pthread_join(t1, NULL);
     if (s != 0)
         errExitEN(s, "pthread_join");
     s = pthread_join(t2, NULL);
     if (s != 0)
         errExitEN(s, "pthread_join");
 
     printf("glob = %d\n", glob);
     exit(EXIT_SUCCESS);
 }