psem/thread_incr_psem.c

This is psem/thread_incr_psem.c (Listing 53-6, page 1101), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* 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);
 }

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