.\"
.\" Copyright (c) 1992, 1993, 1994
.\"	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.
.\"
.\" %sccs.include.redist.roff%
.\"
.\"	@(#)mount_fdesc.8	8.2 (Berkeley) 03/27/94
.\"
.\"
.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 o Ar options
.Ar fdesc
.Ar 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 options are as follows:
.Bl -tag -width indent
.It Fl o
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.El
.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 ,
.Xr mount 8
.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 mount_fdesc
utility first appeared in 4.4BSD.