FDESCFS(4)

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.

rdlnk
    Treat fdescfs vnodes as symbolic links consistently, in particular, follow the resolved name for the name lookups. This option is strictly stronger than the linrdlnk option, it changes not only the type returned by stat(2), but also causes the fdescfs files to behave as symlinks.

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(4), mount(8)

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

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