#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

/*  sem_block.c  - a program that creates and blocks on a semaphore **/

/*  usage: sem_block key amt
 * action: program creates a semaphore with key `key', sets its initial
 *	   value to zero then blocks waiting to decrement it by amt
 */

#define oops(m,x) { perror(m); exit(x); }
union semun { int val; struct semid_ds *buf; ushort *array; };

main(int ac, char *av[])
{
	int	     semid;		/* an id		*/
	union semun  sem_info;		/* some properties	*/
	struct sembuf action;		/* action for the sem   */
	int	     amount;	  	/* amount to decrement  */

	if ( ac != 3 ){
		printf("usage: sem_block key\n"); exit(1);
	}

	amount = atoi(av[2]);

	/*
	 * create or connect to a semset with 1 semaphore in it
	 */

	semid = semget( atoi(av[1]), 1, IPC_CREAT|0666 );
	if( semid == -1 )
		oops("semget",2);

	/*
	 * initialize value of semaphore number 0 to 0
	 */

	sem_info.val = 0;
	if ( semctl(semid, 0, SETVAL, sem_info) == -1 )
		oops("semctl",3);

	/*
	 * prepare the block-to-decrement command
	 */

	action.sem_num = 0;		/* action for semaphore 0 */
	action.sem_flg = SEM_UNDO;	/* auto clean up	  */
	
	action.sem_op  = -amount;	/* DECREMENT SEMAPHORE */

	/*
	 * about to block on request to decrement...
	 */

	printf("sem_block: waiting to decrement semaphore by %d\n", amount);
	if ( semop( semid, &action, 1) == -1 )
		oops("semop",10);
	printf("sem_block DONE!\n");
	semctl(semid, 0, IPC_RMID, 0);	/* delete it */
}