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 Copyright (c) 1992, 1993
 The Regents of the University of California. All rights reserved.
 All rights reserved.

 This code is derived from software donated to Berkeley by
 Jan-Simon Pendry.

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 must display the following acknowledgement:
 This product includes software developed by the University of
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 from: @(#)mount_fdesc.8 8.1 (Berkeley) 6/9/93
 $Id: mount_fdesc.8,v 1.2 1994/01/12 19:01:12 cgd Exp $

.Dd June 9, 1993
.Dt MOUNT_FDESC 8
.Os BSD 4.4
.Sh NAME
.Nm mount_fdesc
.Nd mount the file-descriptor file system
.Sh SYNOPSIS
.Nm mount_fdesc
.Op Fl F Ar fsoptions

a fdesc

a mount_point
.Sh DESCRIPTION
The
.Nm mount_fdesc
command attaches an instance of the per-process file descriptor
namespace to the global filesystem namespace.
The conventional mount point is

a /dev
and the filesystem should be union mounted in order to augment,
rather than replace, the existing entries in

a /dev .
This command is normally executed by
.Xr mount 8
at boot time.

p
The contents of the mount point are

a fd ,

a stderr ,

a stdin ,

a stdout 
and

a tty .

p

a 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

a /dev/fd/0
through

a /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:
d -literal -offset indent fd = open("/dev/fd/0", mode);
.Ed

p
and the call:
d -literal -offset indent fd = fcntl(0, F_DUPFD, 0);
.Ed

p
are equivalent.

p
The files

a /dev/stdin ,

a /dev/stdout
and

a /dev/stderr
appear as symlinks to the relevant entry in the

a /dev/fd
sub-directory.
Opening them is equivalent to the following calls:
d -literal -offset indent fd = fcntl(STDIN_FILENO, F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
.Ed

p
Flags to the
.Xr open 2
call other than
.Dv O_RDONLY , 
.Dv O_WRONLY
and
.Dv O_RDWR
are ignored.

p
The

a /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.
.Sh FILES
l -tag -width /dev/stderr -compact t Pa /dev/fd/# t Pa /dev/stdin t Pa /dev/stdout t Pa /dev/stderr t Pa /dev/tty .El
.Sh SEE ALSO
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr tty 4 ,
.Xr fstab 5
.Sh CAVEATS
No

a .
and

a ..
entries appear when listing the contents of the

a /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

a .
and 

a ..
in a pathname.

p
This filesystem may not be NFS-exported.
.Sh HISTORY
The
.Nm mount_fdesc
utility first appeared in 4.4BSD.