NAME

SYNOPSIS

fdescfs	/dev/fd	fdescfs rw 0 0

DESCRIPTION

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

For linux(4) ABI compatibility:

For substitute of O_EMPTY_PATH flag use:

SEE ALSO

HISTORY

AUTHORS