ilog2(9)

💖 ilog2(9) — the Kernel API Manual, bestie edition 💖

okay so real talk, we're in the Kernel API Manual today and the star of the show is ilog2. yes she's a function-ish macro thing. yes she's iconic. let's get into it 🌸

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✨ NAME ✨

ilog2 — log of base 2 of 32-bit or a 64-bit unsigned value, and @n is the parameter 🎀

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🦋 SYNOPSIS 🦋

ilog2 (n );

that's it. that's the whole vibe. one argument, one dream 💅

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🍓 ARGUMENTS 🍓

• n — literally labeled -- undescribed -- in the docs 😭 like the manual writers simply did not elaborate. that's the entire description. we move.

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🔥 DESCRIPTION 🔥

it's a constant-capable log of base 2 calculation ✨ "constant-capable" meaning she can do her thing at compile time, which is the whole point and honestly kind of a slay.

• this can be used to initialise global variables from constant data — hence the massive ternary operator construction 👑 (yes the giant ternary is on purpose. no she will not be apologising for it.)
• selects the appropriately-sized optimised version depending on sizeof(n) — so depending on how big n is, you get the matching optimised flavour. sized up correctly, every time 🧚

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💜 SEE ALSO 💜

• Kernel file ./tools/include/linux/log2.h
• roundup_pow_of_two(9)
• rounddown_pow_of_two(9)

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ngl if you're confused, just read it again — she's constant-capable, she picks the optimised version based on sizeof(n), and the ternary monster is intentional. no cap 💀

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and that's the whole page bestie!! sign-off time: this one lives out of ./tools/include/linux and the docs are dated October 2026, so yeah, she's fresh 🌈 mwah, go log some base 2 💖🫶