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

#include <errno.h>

union semun { int val ; struct semid_ds *buf ; ushort *array; };


initsem(key_t semkey)
{
	int status = 0, semid;


	/*
	 * try to create it.
	 * If we do, then initialize it
	 *  otherwise just get it
 	 */

	semid = semget(semkey, 1, 0666|IPC_CREAT|IPC_EXCL);
	if ( semid == -1 ){
		if ( errno == EEXIST )
			semid = semget(semkey, 1, 0);
	}
	else
	{
		union semun arg;
		arg.val = 1;
		status = semctl(semid, 0, SETVAL, arg);
	}

	if ( semid == -1 || status == -1 ){
		perror("initsem");
		return -1 ;
	}
	return semid;
}

int waitfor(int semid)
{
	struct sembuf action;

	action.sem_num = 0;
	action.sem_op  = -1;		/* try to decrement */
	action.sem_flg = SEM_UNDO ;

	if ( semop(semid, &action, 1) == -1 ){
		perror("semop in waitfor failed");
		exit(1);
	}
	return 0;
}
int release(int semid)
{
	struct sembuf action;

	action.sem_num = 0;
	action.sem_op  = 1;		/* try to decrement */
	action.sem_flg = SEM_UNDO ;

	if ( semop(semid, &action, 1) == -1 ){
		perror("semop in waitfor failed");
		exit(1);
	}
	return 0;
}

main()
{
	key_t semkey = 5566;
	int   i;

	for(i = 0 ; i<3 ; i++ )
	{
		if ( fork() == 0 )
			handlesem(semkey);
	}
}

handlesem(key_t skey)
{
	int semid;
	int pid = getpid();

	semid = initsem(skey);
	if ( semid < 0 ){
		exit(1);
	}

	printf("\nPID %d before critical section\n", pid);
	waitfor(semid);

	printf("PID %d in critical section\n", pid);
	sleep(10);

	printf("PID %d leaving critical section\n", pid);

	release(semid);

	printf("PID %d done\n", pid);
	exit(0);
}