svsem/svsem_good_init.c

+/* svsem_good_init.c
+
+   Show how to initialize System V semaphores in a manner that avoids
+   race conditions. (Compare with svsem_bad_init.c.)
+*/
 #include <sys/types.h>
 #include <sys/sem.h>
 #include <sys/stat.h>
 #include "semun.h"                      /* Definition of semun union */
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     int semid, key, perms;
     struct sembuf sops[2];
 
     if (argc != 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s sem-op\n", argv[0]);
 
     key = 12345;
     perms = S_IRUSR | S_IWUSR;
 
     semid = semget(key, 1, IPC_CREAT | IPC_EXCL | perms);
 
     if (semid != -1) {                  /* Successfully created the semaphore */
         union semun arg;
         struct sembuf sop;
 
+        sleep(5);
+        printf("%ld: created semaphore\n", (long) getpid());
+
         arg.val = 0;                    /* So initialize it to 0 */
         if (semctl(semid, 0, SETVAL, arg) == -1)
             errExit("semctl 1");
+        printf("%ld: initialized semaphore\n", (long) getpid());
 
         /* Perform a "no-op" semaphore operation - changes sem_otime
            so other processes can see we've initialized the set. */
 
         sop.sem_num = 0;                /* Operate on semaphore 0 */
         sop.sem_op = 0;                 /* Wait for value to equal 0 */
         sop.sem_flg = 0;
         if (semop(semid, &sop, 1) == -1)
             errExit("semop");
+        printf("%ld: completed dummy semop()\n", (long) getpid());
 
     } else {                            /* We didn't create the semaphore set */
 
         if (errno != EEXIST) {          /* Unexpected error from semget() */
             errExit("semget 1");
 
         } else {                        /* Someone else already created it */
             const int MAX_TRIES = 10;
             int j;
             union semun arg;
             struct semid_ds ds;
 
             semid = semget(key, 1, perms);      /* So just get ID */
             if (semid == -1)
                 errExit("semget 2");
 
+            printf("%ld: got semaphore key\n", (long) getpid());
             /* Wait until another process has called semop() */
 
             arg.buf = &ds;
             for (j = 0; j < MAX_TRIES; j++) {
+                printf("Try %d\n", j);
                 if (semctl(semid, 0, IPC_STAT, arg) == -1)
                     errExit("semctl 2");
 
                 if (ds.sem_otime != 0)          /* Semop() performed? */
                     break;                      /* Yes, quit loop */
                 sleep(1);                       /* If not, wait and retry */
             }
 
             if (ds.sem_otime == 0)              /* Loop ran to completion! */
                 fatal("Existing semaphore not initialized");
         }
     }
 
     /* Now perform some operation on the semaphore */
 
     sops[0].sem_num = 0;                        /* Operate on semaphore 0... */
     sops[0].sem_op = getInt(argv[1], 0, "sem-op");
     sops[0].sem_flg = 0;
     if (semop(semid, sops, 1) == -1)
         errExit("semop");
 
     exit(EXIT_SUCCESS);
 }