The Linux Programming Interface

968 Chapter 47

Listing 47-1: Creating and operating on System V semaphores

––––––––––––––––––––––––––––––––––––––––––––––––––––––– svsem/svsem_demo.c

#include <sys/types.h>

#include <sys/sem.h>

#include <sys/stat.h>

#include "curr_time.h" /* Declaration of currTime() */

#include "semun.h" /* Definition of semun union */

#include "tlpi_hdr.h"

int

main(int argc, char *argv[])

{

int semid;

if (argc < 2 || argc > 3 || strcmp(argv[1], "--help") == 0)

usageErr("%s init-value\n"

" or: %s semid operation\n", argv[0], argv[0]);

if (argc == 2) { /* Create and initialize semaphore */

union semun arg;

semid = semget(IPC_PRIVATE, 1, S_IRUSR | S_IWUSR);

if (semid == -1)

errExit("semid");

arg.val = getInt(argv[1], 0, "init-value");

if (semctl(semid, /* semnum= */ 0, SETVAL, arg) == -1)

errExit("semctl");

printf("Semaphore ID = %d\n", semid);

} else { /* Perform an operation on first semaphore */

struct sembuf sop; /* Structure defining operation */

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

sop.sem_num = 0; /* Specifies first semaphore in set */

sop.sem_op = getInt(argv[2], 0, "operation");

/* Add, subtract, or wait for 0 */

sop.sem_flg = 0; /* No special options for operation */

printf("%ld: about to semop at %s\n", (long) getpid(), currTime("%T"));

if (semop(semid, &sop, 1) == -1)

errExit("semop");

printf("%ld: semop completed at %s\n", (long) getpid(), currTime("%T"));

}

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––– svsem/svsem_demo.c