1.\" Copyright (c) 2021 The FreeBSD Foundation, Inc. 2.\" 3.\" Copyright (c) 1996 4.\" Mike Pritchard <mpp@FreeBSD.org>. All rights reserved. 5.\" 6.\" Copyright (c) 1992, 1993, 1994 7.\" The Regents of the University of California. All rights reserved. 8.\" All rights reserved. 9.\" 10.\" This code is derived from software donated to Berkeley by 11.\" Jan-Simon Pendry. 12.\" 13.\" Parts of this documentation was written by 14.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship 15.\" from the FreeBSD Foundation. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 3. Neither the name of the University nor the names of its contributors 26.\" may be used to endorse or promote products derived from this software 27.\" without specific prior written permission. 28.\" 29.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 30.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 31.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 32.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 33.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 34.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 35.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 36.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 37.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 38.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 39.\" SUCH DAMAGE. 40.\" 41.Dd Jul 11, 2023 42.Dt FDESCFS 4 43.Os 44.Sh NAME 45.Nm fdescfs 46.Nd file-descriptor file system 47.Sh SYNOPSIS 48.Bd -literal 49fdescfs /dev/fd fdescfs rw 0 0 50.Ed 51.Sh DESCRIPTION 52The file-descriptor file system, or 53.Nm , 54provides access to the per-process file descriptor 55namespace in the global file system namespace. 56The conventional mount point is 57.Pa /dev/fd . 58.Pp 59The file system's contents 60appear as a list of numbered files 61which correspond to the open files of the process reading the 62directory. 63The files 64.Pa /dev/fd/0 65through 66.Pa /dev/fd/# 67refer to file descriptors which can be accessed through the file 68system. 69.Pp 70The following mount options can be used when mounting 71.Nm 72filesystem: 73.Bl -tag -width linrdlnk 74.It Cm nodup 75For file descriptors referencing vnodes, instead of the 76.Xr dup 2 77semantic described above, implement re-opening of the referenced vnode. 78See below for more details. 79.It Cm linrdlnk 80Report the type of the 81.Nm 82vnode as 83.Dv VLNK 84instead of 85.Fx 86traditional 87.Dv VCHR . 88For 89.Xr linux 4 90ABI compatibility mount 91.Nm 92volume with the 93.Cm linrdlnk 94option. 95.It Cm rdlnk 96Treat 97.Nm 98vnodes as symbolic links consistently, in particular, follow 99the resolved name for the name lookups. 100This option is strictly stronger than the 101.Cm linrdlnk 102option, it changes not only the type returned by 103.Xr stat 2 , 104but also causes the 105.Nm 106files to behave as symlinks. 107.El 108.Pp 109For 110.Nm 111mounted without the 112.Cm nodup 113mount option, 114if the file descriptor is open and the mode the file is being opened 115with is a subset of the mode of the existing descriptor, the call: 116.Bd -literal -offset indent 117fd = open("/dev/fd/0", mode); 118.Ed 119.Pp 120and the call: 121.Bd -literal -offset indent 122fd = fcntl(0, F_DUPFD, 0); 123.Ed 124.Pp 125are equivalent. 126Flags to the 127.Xr open 2 128call other than 129.Dv O_RDONLY , 130.Dv O_WRONLY 131and 132.Dv O_RDWR 133are ignored. 134.Pp 135For 136.Nm 137mounted with the 138.Cm nodup 139option, and file descriptor referencing a vnode, the call: 140.Bd -literal -offset indent 141fd = open("/dev/fd/0", mode); 142.Ed 143.Pp 144reopens the referenced vnode with the specified 145.Fa mode . 146In other words, the 147.Fn open 148call above is equivalent to 149.Bd -literal -offset indent 150fd = openat(0, "", O_EMPTY_PATH, mode); 151.Ed 152.Pp 153In particular, if the file descriptor was opened with the 154.Dv O_PATH 155flag, then either 156.Dv O_EMPTY_PATH 157or 158.Fn open 159over 160.Nm 161mount with 162.Cm nodup 163option allows one to convert it to a regularly opened file, 164assuming that the current permissions allow the requested 165.Fa mode . 166.Pp 167.Em "Note:" 168.Pa /dev/fd/0 , 169.Pa /dev/fd/1 170and 171.Pa /dev/fd/2 172files are created by default when devfs alone is mounted. 173.Nm 174creates entries for all file descriptors opened by the process. 175.Sh FILES 176.Bl -tag -width /dev/stderr -compact 177.It Pa /dev/fd/# 178.El 179.Sh EXAMPLES 180To mount a 181.Nm 182volume located on 183.Pa /dev/fd : 184.Pp 185.Dl "mount -t fdescfs none /dev/fd" 186.Pp 187For 188.Xr linux 4 189ABI compatibility: 190.Pp 191.Dl "mount -t fdescfs -o linrdlnk none /compat/linux/dev/fd" 192.Pp 193For substitute of 194.Dv O_EMPTY_PATH 195flag use: 196.Pp 197.Dl "mount -t fdescfs -o nodup none /dev/fdpath" 198.Sh SEE ALSO 199.Xr devfs 4 , 200.Xr mount 8 201.Sh HISTORY 202The 203.Nm 204file system first appeared in 205.Bx 4.4 . 206The 207.Nm 208manual page first appeared in 209.Fx 2.2 . 210.Sh AUTHORS 211.An -nosplit 212The 213.Nm 214manual page was written by 215.An Mike Pritchard Aq Mt mpp@FreeBSD.org , 216and was based on the 217manual page written by 218.An Jan-Simon Pendry . 219