fcntl(2)

💅 fcntl — manipulate file descriptor

okay bestie, pull up a chair 🎀 this is <fcntl.h>, the header you include before you get to play. and yes, the manual calls this thing fcntl(2) — a system call, not a library page, so jot that down. it lives in the Standard C library (libc, link with -lc). we're doing file descriptor manipulation today and honestly? she ate. 🔥

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✨ SYNOPSIS ✨

#include <fcntl.h>

int fcntl(int fd, int op, ...);

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🌸 DESCRIPTION 🌸

so <pink>fcntl()does **one of the operations described below** on the open file descriptorfd. which one? that's literally what opis for, bestie.op` picks the operation. periodt. 👑

now real talk — some of these operations only exist since a particular Linux kernel version. and the preferred way to check whether your host kernel even supports a particular operation? you just call fcntl() with the op value you want, and then test whether the call failed with EINVAL — because EINVAL means the kernel doesn't recognize this value. that's the vibe check. 🫶

🦋 Duplicating a file descriptor

• F_DUPFD(2const)
• F_DUPFD_CLOEXEC(2const)

🦋 File descriptor flags

• F_GETFD(2const)
• F_SETFD(2const)

🦋 File status flags

• F_GETFL(2const)
• F_SETFL(2const)

🦋 Advisory record locking

• F_SETLK(2const)
• F_SETLKW(2const)
• F_GETLK(2const)

🦋 Open file description locks (non-POSIX)

• F_OFD_SETLK(2const)
• F_OFD_SETLKW(2const)
• F_OFD_GETLK(2const)

🦋 Managing signals

• F_GETOWN(2const)
• F_SETOWN(2const)
• F_GETOWN_EX(2const)
• F_SETOWN_EX(2const)
• F_GETSIG(2const)
• F_SETSIG(2const)

🦋 Leases

• F_SETLEASE(2const)
• F_GETLEASE(2const)

🦋 Delegations

• F_SETDELEG(2const)
• F_GETDELEG(2const)

🦋 File and directory change notification (dnotify)

• F_NOTIFY(2const)

🦋 Changing the capacity of a pipe

• F_SETPIPE_SZ(2const)
• F_GETPIPE_SZ(2const)

🦋 File Sealing

• F_ADD_SEALS(2const)
• F_GET_SEALS(2const)

🦋 File read/write hints

• F_GET_RW_HINT(2const)
• F_SET_RW_HINT(2const)
• F_GET_FILE_RW_HINT(2const)
• F_SET_FILE_RW_HINT(2const)

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💖 RETURN VALUE 💖

good news: on success, the return value depends on the operation. it's giving ✨situational✨. no single answer, sorry bestie.

bad news: on error, -1 is returned, and errno is set to indicate the error. 😭

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💀 ERRORS 💀

• EACCES or EAGAIN — Operation is prohibited by locks held by other processes. somebody else is holding the lock, and it is not your turn. 🙈
• EAGAIN — The operation is prohibited because the file has been memory-mapped by another process.
• EBADF — fd is not an open file descriptor. ma'am. that's not a real fd. 💀
• EINVAL — The value specified in op is not recognized by this kernel.

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🌈 VERSIONS 🌈

POSIX.1-2024 specifies FD_CLOFORK and F_DUPFD_CLOFORK, but Linux doesn't support them. not us, not our house, we don't have those. 🙅♀️

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⭐ STANDARDS ⭐

POSIX.1-2024. iykyk. 💅

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🎀 HISTORY 🎀

4.3BSD, SVr4, POSIX.1-1988. she's got lore. she's been around. iconic behavior honestly. 🫶

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👑 SEE ALSO 👑

• dup2(2)
• flock(2)
• open(2)
• socket(2)
• lockf(3)
• capabilities(7)
• feature_test_macros(7)
• lslocks(8)

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ok that's the whole page bestie, all 40-ish of those F_* operations, every flag, every error, every standard, nothing skipped, no cap. go forth and manipulate some file descriptors responsibly. 🌸💖

and before you go — this lil masterpiece is from the Linux man-pages 6.20, dated 2026-02-08. slay. ✨