.\" Copyright (c) 2021 The FreeBSD Foundation, Inc. .\" .\" Copyright (c) 1996 .\" Mike Pritchard . 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 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. .\" .Dd Jul 11, 2023 .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. .It Cm rdlnk Treat .Nm vnodes as symbolic links consistently, in particular, follow the resolved name for the name lookups. This option is strictly stronger than the .Cm linrdlnk option, it changes not only the type returned by .Xr stat 2 , but also causes the .Nm files to behave as symlinks. .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 .