$home/manuals/unix_v8/4/fd

FD(4)                      Kernel Interfaces Manual                      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  descriptors.   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, sub‐
       sequent  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 con‐
       ventionally 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.

                                                                         FD(4)