build_udp_v4_csum(9)

💅 build_udp_v4_csum(9) — the Kernel API Manual, bestie edition ✨

okay so picture this: you're deep in the kernel API manual (section 9, iykyk) and you need a UDP checksum for UDP over IPv4. That's literally this function. She's small, she's focused, she's iconic.

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🌸 SYNOPSIS 🌸

__sum16 build_udp_v4_csum (const struct iphdr *iph , const struct udphdr *udph );

that's it. that's the whole vibe. two pointers in, one __sum16 out 🫶

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🎀 ARGUMENTS 🎀

• iph — -- undescribed -- 👀 yes bestie, the manpage really says that. No description. We move.
• udph — -- undescribed -- 💀 same energy. Also undescribed. Not me reading the docs and the docs are on read.

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🦋 DESCRIPTION 🦋

Okay so here's what she does: compute the checksum to embed in the UDP header. 🧚

That checksum is composed of the sum of three things:

• the IP pseudo-header checksum,
• the UDP header checksum,
• and the UDP data checksum.

And the cast, for context: iph is the IP header, udph is the UDP header — and udph must be immediately followed by UDP data. Like, right after. No gaps, no other stuff wedged in between. Keep them together, periodt. 🔥

⚠️ fr fr pay attention to that adjacency rule — udph has to be immediately followed by the UDP data, no cap.

And the return? Returns the total checksum. ✨ That's the answer. Slay.

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👑 SEE ALSO 👑

Kernel file ./tools/testing/selftests/bpf/network_helpers.h — go look, it's giving source material 💖

• open_netns(9)
• append_tid(9)
• csum_tcpudp_magic(9)
• csum_ipv6_magic(9)
• build_udp_v6_csum(9)
• tc_prog_attach(9)

the whole friend group honestly 🌈

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anyway that's her. short page, big main character energy. now go checksum something 💜

signing off with the receipts: this page lives in ./tools/testing/selftests/bpf, dated October 2026, and yes it's still build_udp_v4_csum(9) at the bottom — copyright and version line accounted for, we don't gatekeep 💅