csum_fold(9)

📦 version 7.3.1, in nixpkgs unstable

okay bestie, welcome to csum_fold(9) in the Kernel API Manual — it’s giving “fold the checksum, invert it, serve” energy 💖✨

✨ NAME ✨

csum_fold — fold and invert a 32bit checksum. Also, the page says sum: 32bit unfolded sum, so that’s the vibe for the argument 🫶

🎀 SYNOPSIS 🎀

The signature, exactly:

__sum16 csum_fold (__wsum sum );

💅 ARGUMENTS 💅

  • sum — the manpage is so real for just saying -- undescribed -- 😭 but the NAME line tells us it’s the 32bit unfolded sum.

🌸 DESCRIPTION 🌸

Okay so csum_fold folds a 32bit running checksum down to 16bit and inverts it. That’s the whole move ✨ This is usually the last step before putting a checksum into a packet.

Make sure not to mix with 64bit checksums. No mixing, periodt 💀

🦋 SEE ALSO 🦋

cute little sign-off: this page comes from ./arch/x86/um/asm (that’s the source/version) and is dated October 2026. csum_fold(9) said what it said, and we ate 💅🫶

📦 where this page comes from