man/man4/fd.4

         Copyright (c) 1990 The Regents of the University of California.
 All rights reserved.

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 @(#)fd.4 5.1 (Berkeley) 05/10/90

 FD 4 ""
C 7  NAME
fd, stdin, stdout, stderr - file descriptor files
 DESCRIPTION
The files
 /dev/fd/0 through
 /dev/fd/# refer to file descriptors which can be accessed through the file
system.
If the file descriptor is open and the mode the file is being opened
with is a subset of the mode of the existing descriptor, the call:

fd = open("/dev/fd/0", mode);

and the call:

fd = fcntl(0, F_DUPFD, 0);

are equivalent.

Opening the files
 /dev/stdin ,  /dev/stdout and
 /dev/stderr is equivalent to the following calls:

fd = fcntl(STDIN_FILENO, F_DUPFD, 0);

fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);

fd = fcntl(STDERR_FILENO, F_DUPFD, 0);


Flags to the
 open (2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored.
 FILES
/dev/fd/#

/dev/stdin

/dev/stdout

/dev/stderr
 SEE ALSO
tty(4)