ip_fast_csum(9)

💖 ip_fast_csum(9) — Kernel API Manual, bestie 💖

Okay so this is the ip_fast_csum(9) page from the Kernel API Manual, and it is short but iconic. No cap.

✨ NAME ✨

ip_fast_csum — Compute the IPv4 header checksum efficiently.

The NAME line also drops two little clues: iph: ipv4 header and ihl: length of header / 4. That’s the whole lore drop, bestie 🫶.

🌸 SYNOPSIS 🌸

__sum16 ip_fast_csum (const void *iph , unsigned int ihl );

It’s giving “one function, two args, zero extra commentary” 💅. Return type is __sum16; the parameters are const void *iph and unsigned int ihl. Keep the spacing exactly like that if you’re copying it — yes, even the space before the comma, iykyk.

🦋 ARGUMENTS 🦋

The manpage is literally leaving us on read for both of these — it says -- undescribed -- and nothing else 😭.

• iph — -- undescribed -- (the audacity, honestly 🙈)
• ihl — -- undescribed -- (same vibe, same mystery 💀)

🔥 SEE ALSO 🔥

• Kernel file ./arch/x86/um/asm/checksum.h — and peep csum_fold(9) while you’re there ✨

Source/version: ./arch/x86/um/asm. Date: October 2026. Page: ip_fast_csum(9). That’s the cute lil sign-off, bestie — slay and stay checksumming 💖🎀🦋