/* this program illustrates the use of message passing in UNIX */

#include <sys/types.h>  /* Include files necessary for message passing */
#include <errno.h>
#include <sys/ipc.h>
#include <sys/msg.h>


#include <sys/wait.h>  /* Include file for waitpid system call */

#define MSG_MODE (MSG_R | MSG_W |  IPC_CREAT)  /*  Set mode bits on message queue */

main()
{
	int msgid, pid;
	struct mymesg *ptr;

/*	union wait status; */
	int status;


	struct mymesg {   /*  Structure definition of messages */
                          /*  that we will pass                */
		long mtype;
	    	char mtext[512];
                	};

  /*   System call to create message queue */
	if ((msgid = msgget(IPC_PRIVATE,MSG_MODE)) == -1)       {
		printf("Unsuccessful message queue creation \n");
		printf("Errno was %d \n",errno);
		exit(-1);
	}

	if ((pid = fork()) != 0) {   /*  Fork out a child process */
	        /* This is the parent process */
		
		ptr = (struct mymesg *)malloc(sizeof(struct mymesg));

		ptr->mtype = 1;  /*  Set message type to one */
		strcpy(ptr->mtext,"Are you home");

                /*  System call to send message */
		if (msgsnd(msgid,ptr,sizeof(struct mymesg),0) == -1) {
			printf("Unsuccessful message send \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		}

                /*  System call to receive a message of type 2 */
		if (msgrcv(msgid,ptr,sizeof(struct mymesg),2,0) == -1) {
			printf("Unsuccessful message send \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		}

		printf("Parent: %s\n", ptr->mtext);  /* Print the 
                                                    received message */

		waitpid (pid, &status, 0);

                /*  System call to destroy message queue  */

		if (msgctl(msgid,IPC_RMID,0) == -1) {
			printf ("Error removing message queue.\n");
			printf ("Errno was %d.\n", errno);
			exit(-1);
		}
	}
	else {
	
                /*  This is the child process */

		ptr = (struct mymesg *)malloc(sizeof(struct mymesg));

                /*  System call to receive a message of type 1 */
		if (msgrcv(msgid,ptr,sizeof(struct mymesg),1,0) == -1) {
			printf("Unsuccessful message send \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		}
		printf("Child: %s \n", ptr->mtext);  /* Print message
                                                        received */

		sleep(5);  /*  Sleep for 5 seconds */

		ptr->mtype = 2;  /* Set message type to 2 */
		strcpy(ptr->mtext,"Yes I am home\n");

	
                /*  System call to send message */
		if (msgsnd(msgid,ptr,sizeof(struct mymesg),0) == -1) {
			printf("Unsuccessful message send \n");
			printf("Errno was %d \n",errno);
			exit(-1);
		}
	}
}