F_GETFL(2const)

✨ F_GETFL, F_SETFL — get/set file status flags ✨

okay bestie, so this is a page from section 2const of the manual — literally called F_GETFL(2const). it's the whole saga of F_GETFL and F_SETFL, and honestly? it's giving "tiny page, huge power". let's get into it 💅

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🎀 NAME 🎀

F_GETFL, F_SETFL — get/set file status flags

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📚 LIBRARY 📚

Standard C library (libc, link it with -lc)

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🦋 SYNOPSIS 🦋

#include <fcntl.h>

int fcntl(int fd, F_GETFL);
int fcntl(int fd, F_SETFL, int arg);

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💖 DESCRIPTION 💖

So every single open file description has its own little set of status flags, right? They get initialized by open(2) and then possibly modified later by fcntl(2). The important part — the main character energy part — is this: when you duplicate a file descriptor (with dup(2), F_DUPFD(2const), fork(2), etc.), all those copies point at the same open file description, which means they all share the same file status flags. One fd changes them, they all feel it. friendship bracelets basically 🫶

The file status flags themselves and what they actually mean are described over in open(2).

• F_GETFL — returns the file access mode and the file status flags, as the function result. arg is ignored, she's just there, she doesn't need to speak 💀
• F_SETFL — sets the file status flags to whatever value arg says. But bestie, listen: the file access mode (O_RDONLY, O_WRONLY, O_RDWR) and the file creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) that show up in arg are ignored. On Linux, this operation can only change the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. And you genuinely cannot change the O_DSYNC and O_SYNC flags — see BUGS, below. no cap 😭

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👑 RETURN VALUE 👑

• F_GETFL — the value of the file status flags.
• F_SETFL — zero.

And if something goes wrong: -1 gets returned, and errno is set to tell you what the error is. so dramatic 💀

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🙈 ERRORS 🙈

See fcntl(2).

• EPERM — you tried to clear the O_APPEND flag on a file that has the append-only attribute set. nope. not allowed. the file said no 🌸

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⭐ STANDARDS ⭐

POSIX.1-2024.

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🌈 HISTORY 🌈

SVr4, 4.3BSD, POSIX.1-1988. ancient icons fr fr.

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💀 BUGS 💀

F_SETFL

It is not possible to use F_SETFL to change the state of the O_DSYNC and O_SYNC flags. And here's the unhinged part — attempts to change the state of these flags are silently ignored. no error, no warning, just vibes and nothing happening 🙈

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🔥 SEE ALSO 🔥

fcntl(2), open(2)

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okay that's the whole page!! mwah 💖 this was from Linux man-pages 6.20, dated 2026-01-05 — so if you're reading this, shoutout to the man-pages girlies, they ate 🎀✨