#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <ctype.h>

#define NUM	1

#define SEMAPHORE_KEY	2004

extern int errno;

int semid;
struct sembuf lock_sem_var = {0, -1, IPC_NOWAIT};
struct sembuf ulock_sem_var = {0, 1, IPC_NOWAIT};

union semun {
	int val;
	struct semid_ds *buf;
	unsigned short int *array;
	struct seminfo *__buf;
};

void init_semaphore()
{
	if((semid = semget(SEMAPHORE_KEY, NUM, 
					IPC_CREAT | 0666)) < 0) {
		printf("semget error: errno is %d\n", errno);
		exit(1);
	}
}

void set_sem_val()
{
	union semun semopts;
	semopts.val = 1;
	semctl(semid, 0, SETVAL, semopts);
}

int semaphore_lock(int flag)
{
	lock_sem_var.sem_num = flag;
	if(semop(semid, &lock_sem_var, 1) == -1) {
		return -1;
	}
	return 0;
}

int semaphore_unlock(int flag)
{
	ulock_sem_var.sem_num = flag;
	if(semop(semid, &ulock_sem_var, 1) == -1) {
		printf("unlock error. errno is %d flag is %d\n", errno, flag);
		return -1;
	}
	return 0;
}

void remove_semaphore()
{
	semctl(semid, 0, IPC_RMID);
}

void critical_resource()
{
	char c;
	printf("request from %d\n", getpid());
	while(1) {
		c = getchar();
		if(c != '\n') 
			break;
	}
	if(isdigit(c) != 0) {
		printf("found a number. Press CTL + C to exit\n");
		while(1) {}
	}
}

void process_for_write()
{
	int i;
	while(1) {
		i = semaphore_lock(0);
		if(i == 0) {
			printf("resource locked.......\n");
			critical_resource();
			semaphore_unlock(0);
			printf("resource unlocked.......\n");
		}
		sleep(1); /* Donot comment this. */
	}
}

void process_exit()
{
	remove_semaphore();
	exit(1);
}

void init_process_one()
{
	if(fork() == 0) {
		printf("pid is %d\n", getpid());
		signal(SIGINT, process_exit);
		process_for_write();
		while(1);
	}
}

void init_process_two()
{
	if(fork() == 0) {
		printf("pid is %d\n", getpid());
		signal(SIGINT, process_exit);
		process_for_write();
		while(1);
	}
}

int main()
{
	signal(SIGINT, SIG_DFL);
	signal(SIGINT, process_exit);
	init_semaphore();
	set_sem_val();
	init_process_one();
	init_process_two();
	while(1);
	return 0;
}