[09/20/2011]

file descriptors

            C++     C                      Java
0 STDIN     cin    scanf()               System.in
1 STDOUT    cout   printf()              System.out
2 STDERR    cerr   fprintf(stderr,...)   System.err



file descriptor table (e.g. fd.c):

    +-------------+
  0 |  stdin      |
    +-------------+
  1 | newfile.txt |
    +-------------+
  2 | newfile.txt |
    +-------------+


the file descriptor table is copied as-is from parent to child
   (and also through an exec() system call)









BEFORE
    +-------------+
  0 |  stdin      |
    +-------------+
  1 |  stdout     |
    +-------------+
  2 |  stderr     |
    +-------------+

AFTER we close( 1 ) and open()
    +--------------+
  0 |  stdin       |
    +--------------+
  1 |  newfile.txt |
    +--------------+
  2 |  stderr      |
    +--------------+

AFTER dup2( 1, 2 )
    +--------------+
  0 |  stdin       |
    +--------------+
  1 |  newfile.txt |
    +--------------+
  2 |  newfile.txt |
    +--------------+











PIPES
after the pipe() system call:
    +-------------+
  0 |  stdin      |
    +-------------+
  1 |  stdout     |
    +-------------+
  2 |  stderr     |
    +-------------+
  3 |  read--     |
    +-------------+
  4 |  write--    |
    +-------------+

 in CHILD process, after dup2() and close()
    +-------------+
  0 |  stdin      |
    +-------------+
  1 |  write--    |
    +-------------+
  2 |  stderr     |
    +-------------+
  3 |             |
    +-------------+
  4 |  write--    |
    +-------------+

 in PARENT process, after dup2() and close()
    +-------------+
  0 |  read--     |
    +-------------+
  1 |  stdout     |
    +-------------+
  2 |  stderr     |
    +-------------+
  3 |  read--     |
    +-------------+
  4 |             |
    +-------------+