proc_pid_fd(5)

👑 proc_pid_fd — the manual page, but make it cute ✨

okay so bestie, we're in the File Formats Manual section today and the page is called proc_pid_fd(5). buckle up, it's actually so good 💖

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🌸 NAME 🌸

/proc/pid/fd/ — file descriptors. that's it. that's the whole vibe. 🎀

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✨ DESCRIPTION ✨

🦋 the directory itself

/proc/pid/fd/ is a little subdirectory with one entry per file the process has open, and each entry is named after its file descriptor. every single entry is a symbolic link pointing at the actual file. so like, literally: 0 is standard input, 1 is standard output, 2 is standard error, and so on forever. main character numbering fr fr 💅

🎀 pipes and sockets

for file descriptors that belong to pipes and sockets, the entries are symbolic links whose content is the file type plus the inode. a readlink(2) call on one of these gives you a string shaped like this:

type:[inode]

so for example socket:[2248868] is lowkey a socket and its inode is 2248868. and for sockets specifically, you can take that inode and go find more info in one of the files under /proc/net/. iykyk 🔥

🌈 the no-inode girlies (anon inodes)

for file descriptors that have no corresponding inode — like the ones 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 is a symbolic link whose contents look like:

anon_inode:file-type

and in many cases (but not all), that file-type is wrapped in square brackets. obsessed with the inconsistency honestly 😭

for example, an epoll file descriptor has a symbolic link whose content is the string anon_inode:[eventpoll]. iconic.

💀 multithreaded reality check

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). so like, don't come for her after the main thread has left the group chat. not me finding this out the hard way 🙈

👑 using fd files as command-line args (the hack that ate)

okay this is the good part. some programs take a filename as a command-line argument but won't read from standard input when no argument is given. same deal the other way: some programs write to a file named as a command-line argument but won't send output to standard output when no argument is given. whatever, we can still make them use standard input or standard output by literally passing /proc/pid/fd files as command-line arguments. slay.

so assuming -i is the flag for an input file and -o is the flag for an output file:

$ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...

and boom, you have a working filter. periodt. ✨

🧚 /dev/fd and friends

/proc/self/fd/N is approximately the same as /dev/fd/N in some UNIX and UNIX-like systems. and most Linux MAKEDEV scripts symbolically link /dev/fd to /proc/self/fd, in fact. 💜

most systems also provide the symbolic links /dev/stdin, /dev/stdout, and /dev/stderr, which respectively link to the files 0, 1, and 2 in /proc/self/fd. which means the example command above could be written as:

$ foobar -i /dev/stdin -o /dev/stdout ...

so much cuter, no notes 🫶

🌸 permissions, the serious bit

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). keep that in your notes bestie.

also, 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 the owner may differ from the user and group IDs of the process. an unprivileged process may simply lack permission to open them. like, watch:

$ 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. it's giving "you already have the thing but you can't make a new one." the audacity honestly 😭

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⭐ SEE ALSO ⭐

• proc(5) — go read it, bestie, it's the whole neighborhood 🌈

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lowkey reminder: every path, flag, syscall, inode number and string in here is copied exactly, no cap. the vibes are mine, the facts are the kernel's. 💅

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and that's the page!! this one came from Linux man-pages 6.20, dated 2026-02-08. mwah, go read your own file descriptors, they're literally right there and they love you 🎀✨💖