locksvsem.c

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#include	"unpipc.h"

#define	LOCK_PATH	"/tmp/svsemlock"
#define	MAX_TRIES	10

int		semid, initflag;
struct sembuf	postop, waitop;

void
my_lock(int fd)
{
	int		oflag, i;
	union semun	arg;
	struct semid_ds	seminfo;

	if (initflag == 0) {
		oflag = IPC_CREAT | IPC_EXCL | SVSEM_MODE;
		if ( (semid = semget(Ftok(LOCK_PATH, 0), 1, oflag)) >= 0) {
				/* 4success, we're the first so initialize */
			arg.val = 1;
			Semctl(semid, 0, SETVAL, arg);

		} else if (errno == EEXIST) {
				/* 4someone else has created; make sure it's initialized */
			semid = Semget(Ftok(LOCK_PATH, 0), 1, SVSEM_MODE);
			arg.buf = &seminfo;
			for (i = 0; i < MAX_TRIES; i++) {
				Semctl(semid, 0, IPC_STAT, arg);
				if (arg.buf->sem_otime != 0)
					goto init;
				sleep(1);
			}
			err_quit("semget OK, but semaphore not initialized");

		} else
			err_sys("semget error");
init:
		initflag = 1;
		postop.sem_num = 0;		/* and init the two semop() structures */
		postop.sem_op  = 1;
		postop.sem_flg = SEM_UNDO;
		waitop.sem_num = 0;
		waitop.sem_op  = -1;
		waitop.sem_flg = SEM_UNDO;
	}
	Semop(semid, &waitop, 1);		/* down by 1 */
}

void
my_unlock(int fd)
{
	Semop(semid, &postop, 1);		/* up by 1 */
}