__fls(9)

✨ __fls ✨

okay so this is a page from the Kernel API Manual (yes, the kernel has lore and we're reading it 💅) and it's about __fls, bestie. let's get into it.

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🌸 NAME 🌸

__fls — find the last (most-significant) set bit in a long word 🎀

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🦋 SYNOPSIS 🦋

unsigned int __fls (unsigned long word );

like, that's the whole signature. that's her. no cap. ✨

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💖 ARGUMENTS 💖

• word — the word you're searching 🔍 that's it, that's the argument, she's serving one single parameter and she doesn't need more 👑

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🌈 DESCRIPTION 🌈

okay so listen closely because this part actually matters 🫶 — if there's no set bit in the word at all, the result is undefined. literally undefined. not "zero", not "whatever", undefined 😭 so the docs are like: your code should check against 0 first.

💀 bestie do NOT skip the zero check. this is the "look both ways before crossing" of bitops. undefined behaviour is not a vibe, it's a jumpscare 🙈

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🔥 SEE ALSO 🔥

• Kernel file ./include/asm-generic/bitops/builtin-__fls.h — go read the source, iykyk 📖✨

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and that's the whole page, gorgeous 🌸 this one came from ./include/asm-generic/bitops and it's dated October 2026 — so yeah, the kernel docs said "see you in the future, babe" and honestly? iconic. signed, sealed, __fls(9) 💜🎀