FDESCFS(5)                FreeBSD File Formats Manual               FDESCFS(5)

NAME
     fdescfs - file-descriptor file system

SYNOPSIS
     fdescfs /dev/fd fdescfs rw 0 0

DESCRIPTION
     The file-descriptor file system, or fdescfs, provides access to the per-
     process file descriptor namespace in the global file system namespace.
     The conventional mount point is /dev/fd.

     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 /dev/fd/0 through /dev/fd/# refer to file descriptors which can be
     accessed through the file system.

     The following mount options can be used when mounting fdescfs filesystem:

     nodup     For file descriptors referencing vnodes, instead of the dup(2)
               semantic described above, implement re-opening of the
               referenced vnode.  See below for more details.

     linrdlnk  Report the type of the fdescfs vnode as VLNK instead of FreeBSD
               traditional VCHR.  For linux(4) ABI compatibility mount fdescfs
               volume with the linrdlnk option.

     For fdescfs mounted without the 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:

           fd = open("/dev/fd/0", mode);

     and the call:

           fd = fcntl(0, F_DUPFD, 0);

     are equivalent.  Flags to the open(2) call other than O_RDONLY, O_WRONLY
     and O_RDWR are ignored.

     For fdescfs mounted with the nodup option, and file descriptor
     referencing a vnode, the call:

           fd = open("/dev/fd/0", mode);

     reopens the referenced vnode with the specified mode.  In other words,
     the open() call above is equivalent to

           fd = openat(0, "", O_EMPTY_PATH, mode);

     In particular, if the file descriptor was opened with the O_PATH flag,
     then either O_EMPTY_PATH or open() over fdescfs mount with nodup option
     allows one to convert it to a regularly opened file, assuming that the
     current permissions allow the requested mode.

     Note: /dev/fd/0, /dev/fd/1 and /dev/fd/2 files are created by default
     when devfs alone is mounted.  fdescfs creates entries for all file
     descriptors opened by the process.

FILES
     /dev/fd/#

EXAMPLES
     To mount a fdescfs volume located on /dev/fd:

           mount -t fdescfs none /dev/fd

     For linux(4) ABI compatibility:

           mount -t fdescfs -o linrdlnk none /compat/linux/dev/fd

     For substitute of O_EMPTY_PATH flag use:

           mount -t fdescfs -o nodup none /dev/fdpath

SEE ALSO
     devfs(5), mount(8)

HISTORY
     The fdescfs file system first appeared in 4.4BSD.  The fdescfs manual
     page first appeared in FreeBSD 2.2.

AUTHORS
     The fdescfs manual page was written by Mike Pritchard <mpp@FreeBSD.org>,
     and was based on the manual page written by Jan-Simon Pendry.

FreeBSD 13.1-RELEASE-p6          May 17, 2021          FreeBSD 13.1-RELEASE-p6