/* fork.c - demo of fork and exec
 *
 * Note that exec takes 2 parameters:  name of binary; argument list
 *
 * Modifications to experiment with:
 * 1.  Change the name of the file we wish to display to a nonexistent one.
 * 2.  Instead of running cat, run a "bad" program.
 */
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

main(int argc, char **argv)
{
  char *newargv[3];
  int i, j;

  /* We can change name of the file to display to a nonexistent file.
  */
/*
  newargv[0] = "cat";
  newargv[1] = "f85.txt";
  newargv[2] = NULL;
*/

  newargv[0] = "./bad";
  newargv[1] = NULL;


  for (j = 1; j <= 4; ++j)
  {
    /* Print out Process ID.  Used to prove which process is talking.
     */
    printf("I am process %d.  Iteration %d\n", getpid(), j);


    /* Try to fork and create a new process address space, then
     * in the child:  attempt to exec the 'cat' program/command.
     */
    if (fork() == 0)
    {
      printf ("      Child process %d has just been born\n", getpid());

      i = execv("/bin/cat", newargv);
      printf ("      (child) pid = %d:  execv returned %d\n", getpid(), i);
    }
    else
    {
      /* The parent:  if we successfully called cat, we are going to "wait"
       * until it finishes and then only move on from there.
       */
      printf ("   (parent) pid = %d:  i = %d    ", getpid(), i);
      printf ("wait returns %d    ", wait(&i));
      printf ("wait changed i to %d\n\n", i);

      /* i is the status.  Documentation on the wait system call is given
       * in "man -s 2 wait".  It says we can test the value of the status
       * parameter i using various macros to see what happened.
       */
      printf("   WIFEXITED:  terminated normally?    %d\n", WIFEXITED(i));
      printf("   WEXITSTATUS:  child's exit status   %d\n", WEXITSTATUS(i));
      printf("   WIFSIGNALED:  signal killed child?  %d\n", WIFSIGNALED(i));
      if (WIFSIGNALED(i))
      {
        printf("   WTERMSIG:  which signal:  %d\n", WTERMSIG(i));
        printf("   WCOREDUMP:  core dump?    %d\n", WCOREDUMP(i));
      }
    }
  }
}