fdesc(5)

NAME
fdesc - file-descriptor file system

SYNOPSIS
fdesc /dev fdesc rw 0 0

DESCRIPTION
The file-descriptor file system, or fdesc, provides access to the per-process file descriptor namespace in the global filesystem namespace. The conventional mount point is /dev and the filesystem should be union mounted in order to augment, rather than replace, the existing entries in /dev.

The contents of the mount point are fd, stderr, stdin, stdout and tty.

fd is a directory whose 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. 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.

The files /dev/stdin, /dev/stdout and /dev/stderr appear as symlinks to the relevant entry in the /dev/fd sub-directory. Opening them is equivalent to the following calls:

    fd = fcntl(STDIN_FILENO,  F_DUPFD, 0);
    fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
    fd = fcntl(STDERR_FILENO, F_DUPFD, 0);

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

The /dev/tty entry is an indirect reference to the current process's controlling terminal. It appears as a named pipe (FIFO) but behaves in exactly the same way as the real controlling terminal device.

FILES
/dev/fd/#
/dev/stdin
/dev/stdout
/dev/stderr
/dev/tty

SEE ALSO
tty(4), mount_devfs(8), mount_fdesc(8)

HISTORY
The fdesc filesystem first appeared in 4.4BSD. The fdesc manual page first appeared in FreeBSD 2.2.

AUTHOR
The fdesc manaul page was written by Mike Pritchard <mpp@FreeBSD.org>, and was based on the mount_fdesc(8) manual page written by Jan-Simon Pendry.