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

FD(4)					       Kernel Interfaces Manual 					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)