topology_unit_count(9)

📦 version 7.3.1, in nixpkgs unstable

💅 topology_unit_count(9) — like, the whole Kernel API Manual vibe ✨

okay bestie, so this one lives in section 9 of the kernel API manual and it's called topology_unit_count — say that three times fast, i dare you 🫶

🌸 NAME 🌸

topology_unit_count — retrieves the count of specified units at a given topology domain level. that's it, that's the whole job, and she does it slayfully 💖

✨ SYNOPSIS ✨

unsigned int topology_unit_count (u32 apicid , enum x86_topology_domains which_units , enum x86_topology_domains at_level );

look at that signature. three args, one unsigned int coming back at ya. iconic behavior honestly 👑

🎀 ARGUMENTS 🎀

  • apicid — the APIC ID, aka the little guy that specifies the search range 🔍
  • which_units — the domain level that specifies which units we're counting, like, what are we even tallying here
  • at_level — the domain level at which which_units have to be counted. so… where the counting happens 🧮

🦋 DESCRIPTION 🦋

so what does she actually do? she returns the number of possible units according to the enumerated information. no cap, that's the whole thing 💅

like, for example — and this is the moment, bestie — topology_count_units(apicid, TOPO_CORE_DOMAIN, TOPO_PKG_DOMAIN) counts the number of possible cores in the package that apicid belongs to. cute right? cores in a package, counted, done 🌸

now the important part, pay attention:

at_level must obviously be greater than which_level to produce useful results. like, obviously. this is not a drill, this is the useful-results prerequisite 🔥
  • if at_level is equal to which_units, the result is — unsurprisingly — 1. yeah. one. that's it. one unit. don't act shocked 💀
  • if at_level is less than which_units, the result is, by definition, undefined, and the function returns 0. undefined behavior bestie, we do NOT stan, be careful out there 🙈

🌈 SEE ALSO 🌈

the kernel file ./arch/x86/kernel/cpu/topology.c and its whole friend group:

go read them, they're all main character energy in their own way, iykyk ⭐

okay that's the whole page, mwah 💜 this gem is brought to you straight from ./arch/x86/kernel/cpu, dated October 2026, and yes she's still serving topology_unit_count(9) realness. periodt. 🎀🦋✨

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