/*
 * Locking example using the simpler semaphore operations.
 */

#include	<sys/types.h>

#define	SEQFILE		"seqno"
#define	SEMKEY		((key_t) 23456L)
#define	MAXBUFF	   	100

main()
{
	int	fd, i, n, pid, seqno, semid;
	char	buff[MAXBUFF];

	pid = getpid();
	if ( (fd = open(SEQFILE, 2)) < 0)
		err_sys("can't open %s", SEQFILE);
	if ( (semid = sem_create(SEMKEY, 1)) < 0)
		err_sys("can't open semaphore");

	for (i = 0; i < 20; i++) {
		sem_wait(semid);		/* get the lock */

		lseek(fd, 0L, 0);		/* rewind before read */
		if ( (n = read(fd, buff, MAXBUFF)) <= 0)
			err_sys("read error");
		buff[n] = '\0';		/* null terminate for sscanf */

		if ( (n = sscanf(buff, "%d\n", &seqno)) != 1)
			err_sys("sscanf error");
		printf("pid = %d, seq# = %d\n", pid, seqno);

		seqno++;

		sprintf(buff, "%03d\n", seqno);
		n = strlen(buff);
		lseek(fd, 0L, 0);		/* rewind before write */
		if (write(fd, buff, n) != n)
			err_sys("write error");

		sem_signal(semid);		/* release the lock */
	}
	sem_close(semid);
}