#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <sys/wait.h>   /* Wait for Process Termination */
#include <stdlib.h>     /* General Utilities */

int main() {

  pid_t childpid; /* variable to store the child's process id (basically an int) */
  int retval;     /* child process: user-provided return code */
  int status;     /* parent process: child's exit status */

  printf("Process started, sleeping for 3 seconds...\n");
  sleep(3);
  printf("ok, forking...\n");

  /* now create new process */
  childpid = fork();

  if (childpid >= 0) /* fork succeeded */ {

    if (childpid == 0) /* fork() returns 0 to the child process */ {

      printf("CHILD: I am the child process!\n");
      printf("CHILD: Here's my PID: %d\n", getpid());
      printf("CHILD: My parent's PID is: %d\n", getppid());
      printf("CHILD: The value of my copy of childpid is: %d\n", childpid);
      printf("CHILD: Sleeping for 5 seconds...\n");
      sleep(5); /* sleep for 5 seconds */
      printf("CHILD: Enter an exit value (0 to 255): ");
      scanf(" %d", &retval);
      printf("CHILD: Goodbye!\n");    
      exit(retval); /* child exits with user-provided return code */

    } else /* fork() returns new pid to the parent process */ {

      printf("PARENT: I am the parent process!\n");
      printf("PARENT: Here's my PID: %d\n", getpid());
      printf("PARENT: The value of my copy of childpid is %d\n", childpid);
      printf("PARENT: I will now wait for my child to exit.\n");
      wait(&status); /* wait for child to exit, and store its status */
      printf("PARENT: Child's exit code is: %d\n", WEXITSTATUS(status));
      printf("PARENT: Goodbye!\n");             
      exit(0);  /* parent exits */       

    }
  } else /* fork returns -1 on failure */ {
    perror("failed to fork"); /* display error message */
    exit(0); 
  }
}