/*
PROCESS FORKING 		

  In Linux we have multiple processes executing at the same time. This
  program illustrates the fork() system call which creates a new child
  process of the calling process. Go through the comments carefully
  before you compile and execute the program.

*/

#include <stdio.h>
#include <sys/ipc.h>  /* You must include this file for all activities
			 related to creation and interprocess communication.
			 IPC stands for Inter-Process Communication */

/* 
   When the program begins executing from main(), it is a single process
   having a single process identifier (PID). When the fork() call is
   executed, a new process with a new PID is created. THE CODE OF THE NEW
   PROCESS IS IDENTICAL TO THE PARENT PROCESS, AND THE VALUES OF ALL
   VARIABLES IS A COPY OF THE VALUES OF THE VARIABLES OF THE PARENT AT THE
   TIME OF THE fork() CALL. The PIDs of the currently executing processes 
   and the PIDs of their parents can be seen by using the "ps -ef" command
   from the command prompt.
*/
 
main()
{
	int i;
	printf("Now there is a single process\n");
	printf("Observe what happens when we fork\n");
	printf("You can stop by pressing Control-c\n");
        printf("Press Enter to continue... \n"); getchar();

	/*
	   For the parent process, the fork() system call returns the 
	   PID of the newly created child process. On the other hand
           the newly created child process, whose execution starts from
           the fork() statement (that is, its control flow starts from
           the fork() statement) receives the value 0 as return value
           from the fork() statement.
        */
	if (fork() == 0) {

		/* 
		   We are here only if fork() returned zero. Therefore,
                   this part of the code is executed only by the child
		   process.
		*/
		while (1) {
		   for (i=0; i<100000; i++) ; /* Spending time */
		   printf("\t\t\t Child executing\n ");
		}
	}
	else {

		/*
		   We are here only if fork() returned a non-zero value.
		   Therefore this part of the code is executed only by
		   the original (or parent) process.
		*/
		while (1) {
		   for (i=0; i<100000; i++) ; /* Spending time */
		   printf("Parent executing\n");
		}
	}
}