FD(4)

📦 version 12.2, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 FD(4) 💖

okay bestie, we're doing the file descriptor files page from the Kernel Interfaces Manual, and honestly? it's giving ✨the filesystem is a lie but a useful one✨

✨ NAME ✨

fd, stdin, stdout, stderr — file descriptor files 🎀

like. literally the files that ARE your file descriptors. the way that's so iconic of them.

🌸 DESCRIPTION 🌸

okay so. the files /dev/fd/0 through /dev/fd/# refer to file descriptors that you can access through the file system. yes queen, your fds are just... files now. we love a crossover episode 💅

so here's the tea 🍓 — if the file descriptor is open, and the mode the file is being opened with is a subset of the mode of the existing descriptor, then this call:

fd = open("/dev/fd/0", mode);

and this call:

fd = fcntl(0, F_DUPFD, 0);

are equivalent. same vibe, same result, no cap. 🔥

and opening /dev/stdin, /dev/stdout and /dev/stderr? that's equivalent to these three calls right here:

fd = fcntl(STDIN_FILENO,  F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);

so cute. so consistent. 🦋

⚠️ okay important bestie: flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. like, straight up not read. don't be out here expecting them to do something, because they will not, and that's just how it is. 🙈

🧚 IMPLEMENTATION NOTES 🧚

so by default, /dev/fd is provided by devfs(5), which provides nodes for the first three file descriptors. that's your main character trio right there, we stan. 👑

some sites may require nodes for additional file descriptors though — and those can be made available by mounting fdescfs(5) on /dev/fd. so if you need more than three, that's the move. slay. 💜

🌈 FILES 🌈

  • /dev/fd/#
  • /dev/stdin
  • /dev/stdout
  • /dev/stderr

the whole squad, assembled. 🫶

⭐ SEE ALSO ⭐

go read those next bestie, they're part of the lore 📚

okay so that's the whole page!! shoutout to the Debian girlies for shipping this one way back on June 9, 1993 — literally older than all of us and still lowkey essential. periodt 💖✨🎀

📦 where this page comes from