/* this is an example on the use of semaphores in UNIX  */

#include <sys/types.h>   /*  The include files necessary for using semaphores */
#include <errno.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#include <sys/wait.h>   /*  Header file needed to use waitpid system call */

#define SEM_MODE (SEM_A | SEM_R |  IPC_CREAT) 
#define NUM_OF_SEMAPHORES 3 /*  The number of semaphores that semget will create
                             */

#define DOWN -1   /*  Constant values for the semaphore operation */
#define UP 1
main()
{
	int semid, pid;
	int *ptr;


	/* union wait status; */
	int status;

	struct sembuf *ksems;

/*  System call to create an array of semaphores */
/*  Unix initializes all semaphores to 0 */

	if ((semid = semget(IPC_PRIVATE,NUM_OF_SEMAPHORES ,SEM_MODE)) == -1)            {
		printf("Unsuccessful semaphore creation \n");
		printf("Errno was %d \n",errno);
		exit(-1);
	}

	if ((pid = fork()) != 0) {
	  /*  This is the parent */
		
		ksems = (struct sembuf *)malloc(sizeof(struct sembuf));

		ksems->sem_num = 1;  /*  Operate on semaphore 1 */
		ksems->sem_op =  DOWN;

		printf ("Parent:  I am doing a down on semaphore 1.\n\n");

	/*  System call to apply operations to semaphores */
		if (semop(semid,ksems,1) == -1)  {
			printf("error modifing semaphore \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		} 


		printf("Parent:  Ok, I can enter my critical section.\n\n");

		sleep(10);  /*  Make believe critical section */

		ksems->sem_num = 1;  /*  Operate on semaphore 1 */
		ksems->sem_op =  UP;

		printf ("Parent: I am doing an up on semaphore 1 to exit my\n");
		printf ("Parent:  critical section.\n");

	/*  System call to apply operations to semaphores */
		if (semop(semid,ksems,1) == -1)  {
			printf("error modifing semaphore \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		} 

		waitpid(pid, &status, 0);  /* Make sure that child
                                              process has completed */

	/*  System call to destroy semaphore array */
		if  (semctl(semid, IPC_RMID, 0) == -1)  {
			printf("error destroying semaphore array.\n");
			printf("Errno was %d. \n", errno);
			exit(-1);
		}

	}
	else {
 	  /* This is the child */

		sleep (5);

		ksems = (struct sembuf *)malloc(sizeof(struct sembuf));

		ksems->sem_num = 1;  /*  Operate on semaphore 1 */
		ksems->sem_op =  UP;

		printf ("Child:   I am doing an up on semaphore 1.\n\n");

	/*  System call to apply operations to semaphores */
		if (semop(semid,ksems,1) == -1)  {
			printf("error modifing semaphore \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		} 

	}
		
}