#include <sys/types.h>       /* needed to use pid_t, etc. */         
#include <sys/wait.h>        /* needed to use wait() */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>         /*  LINUX constants and functions (fork(), etc.) */

int main()
{
 pid_t pid;

 pid = fork();
  
 if (pid < 0)
  {
   printf("A fork error has occurred.\n");
   exit(-1);
  }
 else 
  if (pid == 0) /* We are in the child. */
   {
    printf("I am the child, about to call ps using system.\n");
    system("ls");
    /* Notice that we do continue in the child after the call to system(). */
    printf("I am the child, about to exit.\n");
    exit(127);
   }
  else  /* We are in the parent. */
   {
    wait(0);               /* Wait for the child to terminate. */
    printf("I am the parent.  The child just ended.  I will now exit.\n");
    exit(0);
   }

 return(0);  
}