PROCESS AND FORK

Basic fork program:

* The following variant of the Hello World program "forks off" a child process that prints a message and 

exits. 
* The parent process does not perform any useful work; it simply waits for the child process to finish.

* You can know what fork returns in the main process ( +ve number) and in child process (0) and during 

  error condition 

#include <sys/types.h> /* pid_t */
#include <sys/wait.h>  /* waitpid */
#include <stdio.h>     /* printf, perror */
#include <stdlib.h>    /* exit */
#include <unistd.h>    /* _exit, fork */
 
int main(void)
{
   pid_t pid = fork();
 
   if (pid == -1) {
      // When fork() returns -1, an error happened.
      perror("fork failed");
      exit(EXIT_FAILURE);
   }
   else if (pid == 0) {
      // When fork() returns 0, we are in the child process.
      printf("Hello from the child process!\n");
      _exit(EXIT_SUCCESS);  // exit() is unreliable here, so _exit must be used
   }
   else {
      // When fork() returns a positive number, we are in the parent process
      // and the return value is the PID of the newly created child process.
      int status;
      (void)waitpid(pid, &status, 0);
   }
   return EXIT_SUCCESS;
} 

Resource: 

en.wikipedia.org/wiki/Fork_(system_call)