parent(9)

✨ parent(9) ✨

okay so bestie, welcome to a lil corner of the Kernel API Manual that is genuinely so cute once you get it 🎀 this is parent(9), and no it's not about your mom — it's about heap math. slay.

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💖 NAME 💖

parent — ok so given the offset of the child, find the offset of the parent. that's it. that's the whole vibe. 🦋

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🌸 SYNOPSIS 🌸

size_t parent (size_t i , unsigned int lsbit , size_t size );

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🎀 ARGUMENTS 🎀

• i — the offset of the heap element whose parent is sought. must be non-zero, no cap. like literally don't pass zero, it's not that kind of party 🙈
• lsbit — a precomputed 1-bit mask, equal to "size & -size". yes bestie, precompute it, it's giving prepared ✨
• size — size of each element. the unit. the whole point. 💅

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🔥 DESCRIPTION 🔥

so in terms of array indexes, the parent of element j = i/size is simply (j-1)/2. easy, iconic, no notes. 👑

but — and here's the plot twist — when you're working in byte offsets, we can't use implicit truncation of integer divides. nope. not allowed. the math just doesn't math. 😭

fortunately (say it with me: fortunately 💜), we only need one bit of the quotient, not the full divide. size has a least significant bit. that bit will be clear if i is an even multiple of size, and set if it's an odd multiple. one bit carrying the whole team fr fr.

logically, we're doing "if (i & lsbit) i -= size;" — but since the branch is unpredictable, it's done with a bit of clever branch-free code instead. not me writing branch-free code 💅🔥 main character optimization energy.

lowkey the whole trick is: unpredictable branches are a vibe check the CPU fails, so we skip the branch entirely. no branch, no problem 🧚

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🌈 SEE ALSO 🌈

• Kernel file ./lib/sort.c
• is_aligned(9)
• swap_words_32(9)
• swap_words_64(9)
• swap_bytes(9)
• sort_r(9)
• sort_r_nonatomic(9)

go read those too, they're all in the same friend group 🫶

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⭐ that's the whole page bestie, ily, go find some parents 💖

source: ./lib · dated: October 2026 🎀✨