.\" Copyright (c) 1996
.\"	Mike Pritchard <mpp@FreeBSD.org>.  All rights reserved.
.\"
.\" 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.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\"
.\" $FreeBSD: src/share/man/man5/fdesc.5,v 1.6.2.4 2001/12/20 16:50:57 ru Exp $
.\" $DragonFly: src/share/man/man5/fdesc.5,v 1.2 2003/06/17 04:37:00 dillon Exp $
.\"
.Dd December 14, 1996
.Dt FDESC 5
.Os
.Sh NAME
.Nm fdesc
.Nd file-descriptor file system
.Sh SYNOPSIS
fdesc	/dev	fdesc rw 0 0
.Sh DESCRIPTION
The file-descriptor file system, or
.Nm ,
provides access to the per-process file descriptor
namespace in 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 .
.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 tty 4 ,
.Xr mount_fdesc 8
.Sh HISTORY
The
.Nm
filesystem first appeared in
.Bx 4.4 .
The
.Nm
manual page first appeared in
.Fx 2.2 .
.Sh AUTHORS
.An -nosplit
The
.Nm
manual page was written by
.An Mike Pritchard Aq mpp@FreeBSD.org ,
and was based on the
.Xr mount_fdesc 8
manual page written by
.An Jan-Simon Pendry .