do_all_file_descriptor_in_linux_represent_io - nutthawit/alpine-dotfile GitHub Wiki

Excellent and subtle question — the answer is “almost all, but not strictly only I/O in the traditional sense.”

Let’s go step by step 👇

🧩 1. What a File Descriptor (FD) is

In Linux (and all Unix-like OSes):

A file descriptor (FD) is simply a small integer that identifies an open kernel object associated with a process.

Each process has its own FD table, mapping numbers (0, 1, 2, 3, …) → to kernel objects.

So yes, most of them represent I/O endpoints, but not all are “files” in the usual sense.

🧱 2. Common categories of FDs

Category Example Description I/O-like?
Regular files /etc/passwd, logs, etc. Classic disk I/O ✅ Yes
Pipes / FIFOs Anonymous or named pipes Data streaming between processes ✅ Yes
Sockets TCP/UDP/UNIX sockets Network or IPC communication ✅ Yes
Terminals / PTYs /dev/tty, pseudo-terminals Used by shells, SSH, etc. ✅ Yes
Character devices /dev/input/event0, /dev/urandom Interface to hardware devices ✅ Yes
Block devices /dev/sda Used for disk I/O ✅ Yes
Event FDs eventfd, timerfd, signalfd, inotify Used for signaling / events ⚙️ Not "data I/O", but I/O-ready
epoll FDs Returned by epoll_create() Used to monitor other FDs ⚙️ Special kernel event interface
memfd / tmpfs Anonymous memory-backed “files” Shared memory / IPC ✅ (acts like file I/O)
fanotify / netlink File change or networking notification Kernel messaging channel ⚙️ System event I/O

🧠 3. Key idea: everything is I/O-like

Even though some FDs (like eventfd, timerfd, epoll) don’t represent "reading or writing data" in the traditional sense, they still expose an I/O interface:

  • You can read() them (to consume events),
  • poll() or epoll_wait() can wait until they’re “readable.”

That’s why calloop and other event loops can monitor them all the same way — they all fit into the I/O readiness model (readable, writable, error).

📚 Example: timerfd

You can create a timer like this:

int fd = timerfd_create(CLOCK_MONOTONIC, 0);

Then you can use read(fd, &expirations, 8) to get how many times it fired. No "real data" is transferred — yet it behaves exactly like a readable file.

So it’s not “I/O” in the sense of disk or network data, but it is I/O in the sense of the Unix I/O abstraction — a file descriptor you can read(), poll(), or select().

⚙️ 4. Why Linux does this

This design is intentional and fundamental to Unix philosophy:

“Everything is a file.”

It means:

  • Devices, sockets, timers, events — all are accessible through the same API: read, write, poll, etc.
  • You can multiplex them with the same event loop (like calloop, epoll, select).

That’s why in Rust and other languages, everything that implements AsFd / AsRawFd can integrate seamlessly with I/O polling.

✅ TL;DR Summary

Statement True? Explanation
“All FDs are for I/O.” ✅ Mostly true They all expose an I/O interface (read/write/poll), even if not classic data I/O
“All FDs represent files.” ✅ Conceptually yes In Unix, everything is modeled as a file
“Some FDs are for events, not data.” ✅ e.g., timerfd, signalfd, epoll
“FDs are always readable/writable.” ⚠️ Some are only readable (like timerfd), some only writable

If you’d like, I can show you a categorized map of all FD types available in /proc/<pid>/fd, and how each fits into the I/O model (read/write/poll behavior). Would you like that?

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