dirfd(3)

✨ dirfd(3) — Library Functions Manual ✨

okay bestie, gather round, because today we're talking about dirfd from the library functions club 💅 this one's a little cutie that scoops the file descriptor out of a directory stream. yes, that file descriptor. let's go 🦋

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🌸 NAME 🌸

dirfd — get directory stream file descriptor

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💖 LIBRARY 💖

Standard C library (libc, -lc) — the classic, the icon, the one that's already linked in.

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✨ SYNOPSIS ✨

#include <sys/types.h>
#include <dirent.h>

int dirfd(DIR *dirp);

🎀 Feature Test Macro Requirements for glibc 🎀

(psst, go check feature_test_macros(7) if you're lost 💜)

dirfd():
    /* Since glibc 2.10: */ _POSIX_C_SOURCE >= 200809L
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

so it's either the POSIX one on newer glibc, or _BSD_SOURCE / _SVID_SOURCE if you're on glibc 2.19 or older. vintage behavior, honestly iconic.

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🫶 DESCRIPTION 🫶

dirfd() hands you back the file descriptor that's chilling inside the directory stream dirp. that's it, that's the whole job, and it ate 🔥

here's the tea though: this file descriptor is the one used internally by the directory stream. so it's only useful for functions that don't depend on, or mess with, the file position — think fstat(2) and fchdir(2). and the moment you call closedir(3), it gets closed automatically. no take-backs, no manual cleanup, it just goes 🌈

not me reminding you again: do not touch the file position with this fd. it's not yours, it's the directory stream's. respect the boundary bestie 😭

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💜 RETURN VALUE 💜

• slay — on success, dirfd() returns a file descriptor, which is a nonnegative integer ✨
• not slay — on error it returns -1, and errno gets set to tell you what went wrong 💀

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👑 ERRORS 👑

POSIX.1-2008 specifies two errors here... and plot twist: neither of them is returned by the current implementation. so these are basically the lore, not the reality 🙈

• EINVAL — dirp does not refer to a valid directory stream.
• ENOTSUP — the implementation does not support the association of a file descriptor with a directory.

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🌈 ATTRIBUTES 🌈

for the whole explanation of what these terms even mean, go peek at attributes(7). here's the rundown:

• Interface: dirfd()
• Attribute: Thread safety
• Value: MT-Safe

so yeah, MT-Safe, meaning she's thread-safe and thriving 🧚

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⭐ STANDARDS ⭐

POSIX.1-2008.

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🦋 HISTORY 🦋

4.3BSD-Reno — and not in 4.2BSD. a glow-up era, we love to see it 💖

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🔥 SEE ALSO 🔥

open(2), openat(2), closedir(3), opendir(3), readdir(3), rewinddir(3), scandir(3), seekdir(3), telldir(3)

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and that's the whole page, no cap 🫶 this was brought to you by Linux man-pages 6.20, dated 2026-02-08, serving dirfd(3) realness. go call her, she's waiting 💅✨