---
name: "proc_pid_fd"
section: "5"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/proc_pid_fd"
---

# proc\_pid\_fd(5)

file descriptors

## NAME

/proc/pid/fd/ - file descriptors

## DESCRIPTION

- */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)](/readlink/2.md) 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)](/bpf/2.md), [**epoll\_create**(2)](/epoll_create/2.md), [**eventfd**(2)](/eventfd/2.md), [**inotify\_init**(2)](/inotify_init/2.md), [**perf\_event\_open**(2)](/perf_event_open/2.md), [**signalfd**(2)](/signalfd/2.md), [**timerfd\_create**(2)](/timerfd_create/2.md), and [**userfaultfd**(2)](/userfaultfd/2.md)), 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)](/pthread_exit/3.md)).

-

  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)](/readlink/2.md)) the symbolic links in this directory is governed by a ptrace access mode **PTRACE\_MODE\_READ\_FSCREDS** check; see [**ptrace**(2)](/ptrace/2.md).

-

  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.

## SEE ALSO

[**proc**(5)](/proc/5.md)