BSD 2.11 - man page for stderr (bsd section 4)

FD(4)											    FD(4)

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)

4th Berkeley Distribution		 February 4, 1997				    FD(4)