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.\" Copyright (c) 2021 The FreeBSD Foundation, Inc.
.\"
.\" 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.
.\"
.\" Parts of this documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
.\"
.\" 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. 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
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" 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
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd May 17, 2021
.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.
.Pp
The following mount options can be used when mounting
.Nm
filesystem:
.Bl -tag -width linrdlnk
.It Cm nodup
For file descriptors referencing vnodes, instead of the
.Xr dup 2
semantic described above, implement re-opening of the referenced vnode.
See below for more details.
.It Cm linrdlnk
Report the type of the
.Nm
vnode as
.Dv VLNK
instead of
.Fx
traditional
.Dv VCHR .
For
.Xr linux 4
ABI compatibility mount
.Nm
volume with the
.Cm linrdlnk
option.
.El
.Pp
For
.Nm
mounted without the
.Cm nodup
mount option,
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.
Flags to the
.Xr open 2
call other than
.Dv O_RDONLY ,
.Dv O_WRONLY
and
.Dv O_RDWR
are ignored.
.Pp
For
.Nm
mounted with the
.Cm nodup
option, and file descriptor referencing a vnode, the call:
.Bd -literal -offset indent
fd = open("/dev/fd/0", mode);
.Ed
.Pp
reopens the referenced vnode with the specified
.Fa mode .
In other words, the
.Fn open
call above is equivalent to
.Bd -literal -offset indent
fd = openat(0, "", O_EMPTY_PATH, mode);
.Ed
.Pp
In particular, if the file descriptor was opened with the
.Dv O_PATH
flag, then either
.Dv O_EMPTY_PATH
or
.Fn open
over
.Nm
mount with
.Cm nodup
option allows one to convert it to a regularly opened file,
assuming that the current permissions allow the requested
.Fa mode .
.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 none /dev/fd"
.Pp
For
.Xr linux 4
ABI compatibility:
.Pp
.Dl "mount -t fdescfs -o linrdlnk none /compat/linux/dev/fd"
.Pp
For substitute of
.Dv O_EMPTY_PATH
flag use:
.Pp
.Dl "mount -t fdescfs -o nodup none /dev/fdpath"
.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 Mt mpp@FreeBSD.org ,
and was based on the
manual page written by
.An Jan-Simon Pendry .
|
