.\"
.\" Copyright (c) 1992 The Regents of the University of California
.\" Copyright (c) 1990, 1992 Jan-Simon Pendry
.\" All rights reserved.
.\"
.\" This code is derived from software donated to Berkeley by
.\" Jan-Simon Pendry.
.\"
.\" %sccs.include.redist.roff%
.\"
.\"	@(#)mount_fdesc.8	5.3 (Berkeley) 05/28/93
.\"
.\"
.Dd 
.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
.Pa fdesc
.Pa 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
.Pa /dev
and the filesystem should be union mounted in order to augment,
rather than replace, the existing entries in
.Pa /dev .
This command is normally executed by
.Xr mount 8
at boot time.
.Pp
The contents of the mount point are
.Pa fd ,
.Pa stderr ,
.Pa stdin ,
.Pa stdout 
and
.Pa tty .
.Pp
.Pa 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
.Pa /dev/fd/0
through
.Pa /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:
.Bd -literal -offset indent
fd = open("/dev/fd/0", mode);
.Ed
.Pp
and the call:
.Bd -literal -offset indent
fd = fcntl(0, F_DUPFD, 0);
.Ed
.Pp
are equivalent.
.Pp
The files
.Pa /dev/stdin ,
.Pa /dev/stdout
and
.Pa /dev/stderr
appear as symlinks to the relevant entry in the
.Pa /dev/fd
sub-directory.
Opening them is equivalent to the following calls:
.Bd -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
.Pp
Flags to the
.Xr open 2
call other than
.Dv O_RDONLY , 
.Dv O_WRONLY
and
.Dv O_RDWR
are ignored.
.Pp
The
.Pa /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
.Bl -tag -width /dev/stderr -compact
.It Pa /dev/fd/#
.It Pa /dev/stdin
.It Pa /dev/stdout
.It Pa /dev/stderr
.It Pa /dev/tty
.El
.Sh SEE ALSO
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr tty 4 ,
.Xr fstab 5
.Sh CAVEATS
No
.Pa .
and
.Pa ..
entries appear when listing the contents of the
.Pa /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
.Pa .
and 
.Pa ..
in a pathname.
.Pp
This filesystem may not be NFS-exported.
.Sh HISTORY
The
.Nm
descriptor filesystem
is
.Ud