fls64(9)

💅 fls64(9) — the Kernel API Manual, but make it make sense

okay so bestie, quick vibe check: this is fls64, from the Kernel API Manual section 9 (iykyk, that's the kernel-internals girlies 🧚).

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🌸 NAME 🌸

fls64 — find last set bit in a 64-bit word. that's the whole job. that's her main character energy. ✨

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🎀 SYNOPSIS 🎀

int fls64 (__u64 x );

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💖 ARGUMENTS 💖

• x — the word to search. like, the 64-bit word. the one you're hunting through 🕵️‍♀️

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✨ DESCRIPTION ✨

okay so this is defined in a similar way as the libc and compiler builtin ffsll — same family, same bloodline 💅 🫶 — but she returns the position of the most significant set bit instead. so not the first one, bestie, the last one. highkey a completely different girl.

fls64(value) returns 0 if value is 0 or the position of the last set bit if value is nonzero. yes, that's real, and yes, that's the whole contract. 📝

the last (most significant) bit is at position 64. sixty-four. the very top. the it-girl of the word 🔥👑

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🦋 SEE ALSO 🦋

• Kernel file ./tools/include/asm-generic/bitops/fls64.h — go read the source, she ate 💅

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so yeah, that's fls64. no cap, she's iconic. 💜

and a little sign-off to close the group chat: this was the Kernel API Manual for fls64(9), dropped in October 2026, straight from the source path ./tools/include/asm-generic/bi... (they truncated her name and honestly she deserved better 😭). mwah, bye bestie 🍓🌈⭐