FD

Section: Devices and Network Interfaces (4)
Updated: EUCC
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NAME

/dev/fd/* - ``file descriptor'' driver

SYNOPSIS

pseudo-device fd

DESCRIPTION

/dev/fd/0 -
/dev/fd/19 are special files that reference the files associated with a process' open file descriptors. That is, opening the file: /dev/fd/N is equivalent to opening whatever file you opened to get the file descriptor N, even though you may not know the file's true name (or even if it has one). This can be used to force a program which opens files by name, to connect itself to open file descriptors (which you have thoughtfully provided) in weird and wonderful ways. A simple use would be to force a program which only accepts input from files, to read from its standard input. For instance:

cat /dev/fd/0

produces the same result as:

cat -

which would be useful if cat(1) didn't use the ``-'' convention.

This driver is particularly useful for enabling the ``process substitution'' mechanism in ksh(1).

If the open file descriptor references a socket, the driver fakes a dup(2) system call instead of actually opening a file. In this case it checks to see that the read/write mode you are attempting to open the file with, is compatible with the mode of the existing file descriptor. That is, if descriptor 5 refers to a socket and is open for writing, you cannot open /dev/fd/5 for reading.

As a notational convenience, the files /dev/stdin, /dev/stdout, and /dev/stderr are provided as hard links to /dev/fd/0, /dev/fd/1, and /dev/fd/2 respectively.

ERRORS

The named file descriptor will be ``opened'' as described above unless one of the following is true.

[ENXIO]: The file descriptor attempting to be reopened is larger than the maximum number of open files that a process may have. (E.g., attempting to open file descriptor 65 on a 4.3BSD system.)
[EBADF]: The file descriptor attempting to be reopened is not an open file.

Other errors as would be returned by access(2), and open(2).

AUTHOR

Fred Blonder <Fred@mimsy.umd.EDU>
NFS version by Arnold Robbins <arnold@emory.EDU>

FILES

/dev/fd/*, /dev/stdin, /dev/stdout, /dev/stderr

BUGS

Since, for sockets, the driver fakes a dup system call rather than actually re-opening the file, the new descriptor is a copy of the dup'ed descriptor, and shares its properties. Specifically: it will have the same read/write mode as the dup'ed descriptor. If descriptor 0 is open for reading and writing, opening /dev/fd/0 for reading will return a writable, as well as readable, file descriptor. Also: the descriptors share the same read/write pointer, so seeks, reads and writes on one will affect the other.

While not a bug in the driver specifically, use of these files can produce odd interactions with programs which don't expect to have their file descriptors surreptitiously aliased.

Having only 20 entries in the /dev/fd directory is an anachronism; modern Unix systems allow a process to have many more open file descriptors.

This document was created by man2html, using the manual pages.
Time: 06:30:35 GMT, December 12, 2024

FD