bitmap_fold(9)

📦 version 7.3.1, in nixpkgs unstable

💫 bitmap_fold(9) 💫

okay so bestie, welcome to bitmap_fold — the kernel API manual page that's here to teach you how to fold a bigger bitmap into a smaller one, modulo a size you specify. yes, we're doing math. yes, it's iconic. ✨

🦋 SYNOPSIS 🦋

void bitmap_fold (unsigned long *dst , const unsigned long *orig ,
       unsigned int sz , unsigned int nbits );

like, that's the whole signature. four things go in, nothing comes out. it's giving "void because I don't need to explain myself" energy 💅

🎀 ARGUMENTS 🎀

  • dst — the resulting smaller bitmap. this is the glow-up destination, the itty-bitty baby bitmap where everything ends up. 🍓
  • orig — the original larger bitmap. the big one. the main character, honestly, but it's not the one getting written to. 😭
  • sz — the specified size. this is the modulus, the magic number, the ~vibe check~ that decides where each bit lands. 🔥
  • nbits — number of bits in each of these bitmaps. both of them. they're matching, okay? 🫶

🌸 DESCRIPTION 🌸

okay so here's the tea, and I mean the whole tea ☕

for each bit oldbit in orig, you set bit oldbit mod sz in dst. that's it. that's the fold. the way that a bit at position oldbit literally just slides down into oldbit mod sz — chef's kiss. 🧚

and then? clear all other bits in dst. all of them. not some, not most — all the other bits. no cap. 💀

for the why and the how of using this, the original says to see further the comment and Example [2] for bitmap_onto. go read that, bestie, it's all connected, iykyk. 📖

👑 SEE ALSO 👑

so like, the whole friend group. the kernel file ./lib/bitmap.c, plus:

go read them all, they're literally all bangers, periodt. ⭐

and that's the page, bestie!! 🎀 this little number lives in ./lib and it's dated October 2026, so like — screenshot it, cite it, love it. mwah 💜✨

📦 where this page comes from