FUTEX_WAKE(2const)

✨ FUTEX_WAKE ✨ (that's the 2const page bestie, section 2 constants, iykyk)

okay so. you know how threads be Waiting and Just Sitting There? yeah. this is the lil syscall constant that goes "WAKE UP GIRL 💖" to the ones chilling inside FUTEX_WAIT. it's giving alarm clock but for kernel sleepers.

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🌸 LIBRARY 🌸

Standard C library (libc, -lc) 💅

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🎀 SYNOPSIS 🎀

#include <linux/futex.h>  /* Definition of FUTEX_* constants */
#include <sys/syscall.h>  /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(SYS_futex, uint32_t *uaddr, FUTEX_WAKE, uint32_t val);

like yes you do have to include all three headers, don't skip, the comments literally tell you why 🫶

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🦋 DESCRIPTION 🦋

So this operation wakes at most val of the waiters that are waiting (for example, the ones sitting inside FUTEX_WAIT(2const))) on the futex word at the address uaddr. At most, bestie. Not "all of them", not "exactly val" — at most. 📌

Most commonly, val is specified as either 1 (wake up a single waiter, cute, lowkey, one-on-one energy) or INT_MAX (wake up all waiters, main character group-chat notification 🎉).

No guarantee is provided about which waiters are awoken (e.g., a waiter with a higher scheduling priority is not guaranteed to be awoken in preference to a waiter with a lower priority).

fr fr. the kernel does not care who you're besties with. priority does NOT earn you a fast pass here. it's a vibe check, not a ranking. 😭

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🌈 RETURN VALUE 🌈

• On error, -1 is returned, and errno is set to indicate the error. 💀
• On success, FUTEX_WAKE returns the number of waiters that were woken up. like, an actual count, so you can see how many people showed up to the party ✨

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💜 ERRORS 💜

See futex(2).

• EINVAL — the kernel detected an inconsistency between the user-space state at uaddr and the kernel state — that is, it detected a waiter which waits in FUTEX_LOCK_PI(2const) or FUTEX_LOCK_PI2(2const) on uaddr. 🚨 that's a mismatch, that's messy, the kernel is not having it.

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👑 STANDARDS 👑

Linux. that's it, that's the whole standard, it's a Linux thing bestie 🐧

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🕰️ HISTORY 🕰️

Linux 2.6.0. she's been here since 2.6.0, a true icon, an OG ⭐

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🌟 SEE ALSO 🌟

• futex(2)

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and that's the whole page 🫶 the Linux man-pages 6.20 girlies shipped this cutie on 2025-05-30, so if you're reading it, know that the FUTEX_WAKE(2const) page came straight from that release. mwah 💋 go wake somebody up (politely, at most val of them) 🎀✨