.\" 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.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    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.
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.\" $FreeBSD$
.\"
.Dd September 18, 2010
.Dt FDESCFS 5
.Os
.Sh NAME
.Nm fdescfs
.Nd file-descriptor file system
.Sh SYNOPSIS
.Bd -literal
fdescfs	/dev/fd	fdescfs rw 0 0
.Ed
.Sh DESCRIPTION
The file-descriptor file system, or
.Nm ,
provides access to the per-process file descriptor
namespace in the global file system namespace.
The conventional mount point is
.Pa /dev/fd .
.Pp
The file system's 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
Flags to the
.Xr open 2
call other than
.Dv O_RDONLY ,
.Dv O_WRONLY
and
.Dv O_RDWR
are ignored.
.Pp
.Em "Note:"
.Pa /dev/fd/0 ,
.Pa /dev/fd/1
and
.Pa /dev/fd/2
files are created by default when devfs alone is mounted.
.Nm
creates entries for all file descriptors opened by the process.
.Sh FILES
.Bl -tag -width /dev/stderr -compact
.It Pa /dev/fd/#
.El
.Sh EXAMPLES
To mount a
.Nm
volume located on
.Pa /dev/fd :
.Pp
.Dl "mount -t fdescfs null /dev/fd"
.Sh SEE ALSO
.Xr devfs 5 ,
.Xr mount 8
.Sh HISTORY
The
.Nm
file system 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
manual page written by
.An Jan-Simon Pendry .