#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/fcntl.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>

#define SEMNAME "/tmp/license"
#define N 8

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

int acquire(int);
int release(int);

int
main()
{
    int key, id, i, status;
    int pid, num_waiting;
    int uid = getuid();
    union semun argument;

    /* Only 3 processes can execute the program at a time. */
    argument.val = 3;

    if((open(SEMNAME, O_CREAT)) < 0) {
	perror("Could not create file\n");
    }
    if((key = ftok(SEMNAME, uid )) < 0) {
	perror("ftok");
	exit(1);
    }
    id = semget(key, 1, 0666 | IPC_CREAT);
    if(id < 0) {
	fprintf(stderr, "Unable to obtain semaphore\n");
    }

    /* Note that only one process initializes the semaphore */
    if( semctl(id, 0, SETVAL, argument) < 0)  {
	fprintf( stderr, "Cannot set semaphore value.\n");
    } else {
	fprintf(stderr, "Semaphore %d initialized.\n", key);
    }
    

    for(i = 0; i < N; i++) {
	
	if( (pid = fork()) == 0 ) {
	    if((acquire(id)) == 0) {
		execlp("date", 0);
	    } else {
		exit(1);
	    }
	} else {
	    num_waiting = semctl(id, 0, GETNCNT);
	    printf("There are %d processes waiting for the license\n", 
		   num_waiting);

	}
    }

    while((pid = wait(&status)) > 0) {
	if((release(id)) < 0) {
	    printf("Error: could not release license for %d\n", pid);
	} else {
	    printf("Process %d no longer holds license\n", pid);
	}
	num_waiting = semctl(id, 0, GETNCNT);
	printf("Now there are %d processes waiting for the license\n", 
	       num_waiting);
    }

    /* Remove the semaphore */
    if((semctl(id, 0, IPC_RMID)) < 0) {
	perror("semctl");
    } else {
	printf("Removed semaphore\n");
    }
    
    /* Remove the file */
    if((unlink("/tmp/license")) < 0 ) {
	perror("Remove File");
	exit(1);
    }

    return (0);
}

int acquire(int id)
{
    int retval;
    struct sembuf operations[1];
    /* Set up the sembuf structure. */
    operations[0].sem_num = 0;  /* index into semaphore array */
    operations[0].sem_op = -1;  
    operations[0].sem_flg = 0;  /* wait if semaphore is <=0 */

    retval = semop(id, operations, 1);

    if(retval != 0) {
	printf("acquire operation did not succeed\n");
        perror("REASON");
    }
    return retval;
}

int release(int id)
{
    int retval;
    struct sembuf operations[1];
   /* Set up the sembuf structure. */
    operations[0].sem_num = 0;
    operations[0].sem_op = 1;
    operations[0].sem_flg = 0;

    retval = semop(id, operations, 1);

    if(retval != 0)  {
	printf("Release did not succeed.\n");
        perror("REASON");
    }

    return retval;
}