[10/17/2011]

Addendum to 10/14/2011 notes, in preparation for the Midterm Exam....
=====================================================================

Here's Sample Question #2 again -- note the comments showing how
 the file descriptor changes as close() and dup2() are called.

/* SAMPLE QUESTION #2 */
#include <stdio.h>
#include <unistd.h>

/*    FILE DESCRIPTOR TABLE:
 *      +---------+
 *    0 | stdin   |
 *      +---------+
 *    1 | stdout  |
 *      +---------+
 *    2 | stderr  |
 *      +---------+
 */

int main()
{
  int rc;
  printf( "ONE\n" );
  fprintf( stderr, "ERROR: ONE\n" );
  rc = close( 1 );
  /*    FILE DESCRIPTOR TABLE:
   *      +---------+
   *    0 | stdin   |
   *      +---------+
   *    1 |         |
   *      +---------+
   *    2 | stderr  |
   *      +---------+
   */

  printf( "TWO\n" );
  fprintf( stderr, "ERROR: TWO\n" );
  rc = dup2( 2, 1 );
  /*    FILE DESCRIPTOR TABLE:
   *      +---------+
   *    0 | stdin   |
   *      +---------+
   *    1 | stderr  |
   *      +---------+
   *    2 | stderr  |
   *      +---------+
   *
   *   dup2() copied entry for fd 2 to fd 1
   */
  
  printf( "THREE\n" );
  fprintf( stderr, "ERROR: THREE\n" );
  close( 2 );
  /*    FILE DESCRIPTOR TABLE:
   *      +---------+
   *    0 | stdin   |
   *      +---------+
   *    1 | stderr  |
   *      +---------+
   *    2 |         |
   *      +---------+
   *
   *   At this point, anything going to stdout via printf()
   *    is redirected to stderr (so it also appears as part
   *     of this program's output)
   */

  printf( "FOUR\n" );
  fprintf( stderr, "ERROR: FOUR\n" );
  return 0;
}

=====================================================================

Note the correction to the comment after close( 1 ) and close( 2 ).
 Though stdout is buffered, stderr is not.  This will be irrelevant
  on the Midterm Exam.

/* SAMPLE QUESTION #4 */
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>

int main()
{
  int rc;
  close( 1 );   /* closing this descriptor only "hides" the output */
  close( 2 );   /*  any printf() output is still in mem until      */
                /*   fflush() flushes it away.  NOTE that this is  */
                /*    not true of stderr, though!  No buffering    */
                /*     of stderr occurs.                           */
  printf( "HI\n" );
/* fflush( stdout ); */
  fprintf( stderr, "ERROR\n" );
  open( "newfile.txt", O_WRONLY | O_CREAT | O_TRUNC, 0600 );
  printf( "WHAT?\n" );
/* fflush( stdout ); */
  fprintf( stderr, "ERROR AGAIN\n" );
  rc = fork();
  if ( rc == 0 )
  {
    printf( "AGAIN?\n" );
    fprintf( stderr, "ERROR ERROR\n" );
  }
  else
  {
    wait( NULL );
  }
  printf( "BYE\n" );
  fprintf( stderr, "HELLO\n" );
  return 0;
}