//Here's seminit.c (run this first!):
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int main(void)
{	key_t key;
	int semid;
	union semun arg;
	if((key = ftok("semdemo.c", 'J')) == -1)
	{	perror("ftok");
		exit(1);
	}
	/* create a semaphore set with 1 semaphore: */
	if((semid = semget(key, 1, 0666 | IPC_CREAT)) == -1)
	{	perror("semget");
		exit(1);
	}
	/* initialize semaphore #0 to 1: */
	arg.val = 1;
	if(semctl(semid, 0, SETVAL, arg) == -1)
	{	perror("semctl");
		exit(1);
	}
	return 0;
}
//Here's semdemo.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int main(void)
{	key_t key;
	int semid;
	struct sembuf sb = {0, -1, 0}; /* set to allocate resource */
	if((key = ftok("semdemo.c", 'J')) == -1)
	{	perror("ftok");
		exit(1);
	}
	/* grab the semaphore set created by seminit.c: */
	if((semid = semget(key, 1, 0)) == -1)
	{	perror("semget");
		exit(1);
	}
	printf("Press return to lock: ");
	getchar();
	printf("Trying to lock...\n");
	if(semop(semid, &sb, 1) == -1)
	{	perror("semop");
		exit(1);
	}
	printf("Locked.\n");
	printf("Press return to unlock: ");
	getchar();
	sb.sem_op = 1; /* free resource */
	if(semop(semid, &sb, 1) == -1)
	{	perror("semop");
		exit(1);
	}
	printf("Unlocked\n");
	return 0;
}
//Here's semrm.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int main(void)
{	key_t key;
	int semid;
	union semun arg;
	if((key = ftok("semdemo.c", 'J')) == -1)
	{	perror("ftok");
		exit(1);
	}
	/* grab the semaphore set created by seminit.c: */
	if((semid = semget(key, 1, 0)) == -1)
	{	perror("semget");
		exit(1);
	}
	/* remove it: */
	if(semctl(semid, 0, IPC_RMID, arg) == -1)
	{	perror("semctl");
		exit(1);
	}
	return 0;
}