osx man page for mount_fdesc

MOUNT_FDESC(8)						    BSD System Manager's Manual 					    MOUNT_FDESC(8)

NAME
     mount_fdesc -- mount the file-descriptor file system

SYNOPSIS
     mount_fdesc [-o options] fdesc mount_point

DESCRIPTION
     The mount_fdesc command attaches an instance of the per-process file descriptor namespace to the global filesystem namespace.  The conven-
     tional mount point is /dev and the filesystem should be union mounted in order to augment, rather than replace, the existing entries in /dev.
     This command is normally executed by mount(8) at boot time.

     The options are as follows:

     -o      Options are specified with a -o flag followed by a comma separated string of options.  See the mount(8) man page for possible options
	     and their meanings.

     The contents of the mount point are fd, stderr, stdin, stdout and tty.

     fd is a directory whose contents appear as a list of numbered files which correspond to the open files of the process reading the directory.
     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.

     The files /dev/stdin, /dev/stdout and /dev/stderr appear as symlinks to the relevant entry in the /dev/fd sub-directory.  Opening them 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.

     The /dev/tty entry is an indirect reference to the current process's controlling terminal.  It appears as a named pipe (FIFO) but behaves in
     exactly the same way as the real controlling terminal device.

FILES
     /dev/fd/#
     /dev/stdin
     /dev/stdout
     /dev/stderr
     /dev/tty

SEE ALSO
     mount(2), unmount(2), tty(4), fstab(5), mount(8)

CAVEATS
     No ~.  and .. entries appear when listing the contents of the /dev/fd directory.  This makes sense in the context of this filesystem, but is
     inconsistent with usual filesystem conventions.  However, it is still possible to refer to both ~.  and .. in a pathname.

     This filesystem may not be NFS-exported.

HISTORY
     The mount_fdesc utility first appeared in 4.4BSD.

4.4BSD								  March 27, 1994							    4.4BSD