F_GETFD(2const)

✨ F_GETFD ✨

okay bestie, this is a 2const page — meaning it's one lil slice of the big fcntl(2) family — and it's all about getting and setting the flags on a file descriptor. That's it. That's the whole vibe. 💅

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🎀 NAME 🎀

F_GETFD, F_SETFD — get/set file descriptor flags

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📚 LIBRARY 📚

Standard C library (libc, -lc) — your everyday ride-or-die, no special linking era needed.

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🌸 SYNOPSIS 🌸

#include <fcntl.h>

int fcntl(int fd, F_GETFD);
int fcntl(int fd, F_SETFD, int arg);

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💖 DESCRIPTION 💖

So these two operations (they're commands you hand to fcntl(2)) manipulate the flags associated with a file descriptor. Like, the descriptor itself has its own lil flag collection. Not the file's flags — the descriptor's. Important distinction, periodt.

Currently, only one such flag is even defined: FD_CLOEXEC, a.k.a. the close-on-exec flag. Only one. That's the whole roster. 👑

Here's the tea:

• If the FD_CLOEXEC bit is set, the file descriptor will automatically be closed during a successful execve(2). ✨ (But if the execve(2) fails, the file descriptor is left open. Just so you know. Not me being extra, that's literally what it says.)
• If the FD_CLOEXEC bit is not set, the file descriptor will remain open across an execve(2).

Now the two operations themselves:

F_GETFD

Return (as the function result) the file descriptor flags. And arg? Ignored. It's giving "you didn't need to bring that." 🙈

F_SETFD

Set the file descriptor flags to the value specified by arg. Slay, you're in charge now.

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🔥 RETURN VALUE 🔥

F_GETFD

Value of file descriptor flags. (The flags come back as the return value itself, bestie.)

F_SETFD

Zero.

And if things go wrong — on error, -1 is returned, and errno is set to indicate the error. 💀

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💀 ERRORS 💀

See fcntl(2). That's where all the error deets live, we're just pointing you there. 🫶

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👑 STANDARDS 👑

POSIX.1-2024. Certified. Official. Iconic.

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🦋 HISTORY 🦋

SVr4, 4.3BSD, POSIX.1-1988. Ancient lineage fr fr, this one's been around.

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⚠️ CAVEATS ⚠️

okay this part matters, listen up. 🚨

In multithreaded programs, using fcntl(2) F_SETFD to set the close-on-exec flag at the same time as another thread performs a fork(2) plus execve(2) is vulnerable to a race condition that may unintentionally leak the file descriptor to the program executed in the child process.

Like — you did NOT mean to hand that fd to the child, and yet. 😭

See the discussion of the O_CLOEXEC flag in open(2) for details and a remedy to the problem. (Yes there's a fix, go read it, iykyk.)

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🌈 SEE ALSO 🌈

• fcntl(2)

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that's the whole page bestie 💜 every flag, every caveat, every last return value — kept it all, just made it cute. this one's from Linux man-pages 6.20, dated 2026-01-05, and honestly? obsessed with her. 🎀✨