FD(4)                                                       FD(4)

     NAME
          fd, stdin, stdout, stderr, tty - file descriptor files

     DESCRIPTION
          These files, conventionally called /dev/fd/0, /dev/fd/1, ...
          /dev/fd/127, refer to files accessible through file descrip-
          tors.  If file descriptor n is open, these two system calls
          have the same effect:

               fd = open("/dev/fd/n", mode);
               fd = dup(n);

          Creat(2) is equivalent to open, and mode is ignored.  As
          with dup, subsequent IO on fd fails unless the original file
          descriptor allows the read or write operation.

          Entry /dev/fd/3 is conventionally the `control terminal' of
          the process group to which the present process belongs; see
          stream(4).

          The entries /dev/stdin, /dev/stdout, /dev/stderr, and
          /dev/tty are conventionally linked to /dev/fd/0, /dev/fd/1,
          /dev/fd/2, and /dev/fd/3 respectively.

     SEE ALSO
          open(2), dup(2), stream(4)

     DIAGNOSTICS
          Open returns -1 if the related file descriptor is not open.