The Linux Programming Interface

System V Semaphores 975

if (argc < 3 || strcmp(argv[1], "--help") == 0)

usageErr("%s semid val...\n", argv[0]);

semid = getInt(argv[1], 0, "semid");

/* Obtain size of semaphore set */

arg.buf = &ds;

if (semctl(semid, 0, IPC_STAT, arg) == -1)

errExit("semctl");

if (ds.sem_nsems != argc - 2)

cmdLineErr("Set contains %ld semaphores, but %d values were supplied\n",

(long) ds.sem_nsems, argc - 2);

/* Set up array of values; perform semaphore initialization */

arg.array = calloc(ds.sem_nsems, sizeof(arg.array[0]));

if (arg.array == NULL)

errExit("calloc");

for (j = 2; j < argc; j++)

arg.array[j - 2] = getInt(argv[j], 0, "val");

if (semctl(semid, 0, SETALL, arg) == -1)

errExit("semctl-SETALL");

printf("Semaphore values changed (PID=%ld)\n", (long) getpid());

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––– svsem/svsem_setall.c

47.5 Semaphore Initialization

According to SUSv3, an implementation is not required to initialize the values of

the semaphores in a set created by semget(). Instead, the programmer must explicitly

initialize the semaphores using the semctl() system call. (On Linux, the semaphores

returned by semget() are in fact initialized to 0, but we can’t portably rely on this.) As

stated earlier, the fact that semaphore creation and initialization must be per-

formed by separate system calls, instead of in a single atomic step, leads to possible

race conditions when initializing a semaphore. In this section, we detail the nature

of the race and look at a method of avoiding it based on an idea described in

[Stevens, 1999].

Suppose that we have an application consisting of multiple peer processes

employing a semaphore to coordinate their actions. Since no single process is guar-

anteed to be the first to use the semaphore (this is what is meant by the term peer),

each process must be prepared to create and initialize the semaphore if it doesn’t

already exist. For this purpose, we might consider employing the code shown in

Listing 47-5.