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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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proc_pid_fd(5) File Formats Manual proc_pid_fd(5)
/proc/pid/fd/ - file descriptors
/proc/pid/fd/
This is a subdirectory containing one entry for each file
which the process has open, named by its file descriptor,
and which is a symbolic link to the actual file. Thus, 0
is standard input, 1 standard output, 2 standard error, and
so on.
For file descriptors for pipes and sockets, the entries
will be symbolic links whose content is the file type with
the inode. A readlink(2) call on this file returns a
string in the format:
type:[inode]
For example, socket:[2248868] will be a socket and its
inode is 2248868. For sockets, that inode can be used to
find more information in one of the files under /proc/net/.
For file descriptors that have no corresponding inode
(e.g., file descriptors produced by bpf(2),
epoll_create(2), eventfd(2), inotify_init(2),
perf_event_open(2), signalfd(2), timerfd_create(2), and
userfaultfd(2)), the entry will be a symbolic link with
contents of the form
anon_inode:file-type
In many cases (but not all), the file-type is surrounded by
square brackets.
For example, an epoll file descriptor will have a symbolic
link whose content is the string anon_inode:[eventpoll].
In a multithreaded process, the contents of this directory
are not available if the main thread has already terminated
(typically by calling pthread_exit(3)).
Programs that take a filename as a command-line argument,
but don't take input from standard input if no argument is
supplied, and programs that write to a file named as a
command-line argument, but don't send their output to
standard output if no argument is supplied, can
nevertheless be made to use standard input or standard
output by using /proc/pid/fd files as command-line
arguments. For example, assuming that -i is the flag
designating an input file and -o is the flag designating an
output file:
$ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...
and you have a working filter.
/proc/self/fd/N is approximately the same as /dev/fd/N in
some UNIX and UNIX-like systems. Most Linux MAKEDEV
scripts symbolically link /dev/fd to /proc/self/fd, in
fact.
Most systems provide symbolic links /dev/stdin,
/dev/stdout, and /dev/stderr, which respectively link to
the files 0, 1, and 2 in /proc/self/fd. Thus the example
command above could be written as:
$ foobar -i /dev/stdin -o /dev/stdout ...
Permission to dereference or read (readlink(2)) the
symbolic links in this directory is governed by a ptrace
access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
Note that for file descriptors referring to inodes (pipes
and sockets, see above), those inodes still have permission
bits and ownership information distinct from those of the
/proc/pid/fd entry, and that the owner may differ from the
user and group IDs of the process. An unprivileged process
may lack permissions to open them, as in this example:
$ echo test | sudo -u nobody cat
test
$ echo test | sudo -u nobody cat /proc/self/fd/0
cat: /proc/self/fd/0: Permission denied
File descriptor 0 refers to the pipe created by the shell
and owned by that shell's user, which is not nobody, so cat
does not have permission to create a new file descriptor to
read from that inode, even though it can still read from
its existing file descriptor 0.
proc(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 proc_pid_fd(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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