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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 de-
     scriptor namespace to the global filesystem namespace.  The conventional
     mount point is /dev and the filesystem should be union mounted in order
     to augment, rather than replace, the existing entries in /dev. This com-
     mand 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 separat-
             ed string of options.  See the mount(8) man page for possible op-
             tions 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 exist-
     ing 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 equiva-
     lent 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 con-
     trolling terminal.  It appears as a named pipe (FIFO) but behaves in ex-
     actly 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 di-
     rectory.  This makes sense in the context of this filesystem, but is in-
     consistent with usual filesystem conventions.  However, it is still pos-
     sible 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                               2