open_classid(9)

✨ open_classid(9) ✨

okay so bestie, we're in the Kernel API Manual today and this page is literally called open_classid — say it with me, it's giving cgroupv1 energy 💅

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🌸 NAME 🌸

open_classid — opens a cgroupv1 net_cls classid, that's the whole vibe 🎀

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🦋 SYNOPSIS 🦋

int open_classid (void );

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💜 ARGUMENTS 💜

• void — no arguments. none. zero. she takes nothing and she still serves 💅

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🔥 DESCRIPTION 🔥

So here's the tea ☕ — this function expects the cgroup work dir to already be created, because that's where she does the opening. Like, she's not gonna make the folder for you, she's just gonna walk in and open the door, yk? iykyk 🙈

On success, it returns the file descriptor. On failure it returns -1. that's it. that's the whole story. no cap 💀

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👑 SEE ALSO 👑

the whole friend group came to support, we love a squad 🫶:

• Kernel file ./tools/testing/selftests/bpf/cgroup_helpers.c
• enable_controllers(9)
• write_cgroup_file(9)
• write_cgroup_file_parent(9)
• setup_cgroup_environment(9)
• join_cgroup(9)
• join_root_cgroup(9)
• join_parent_cgroup(9)
• set_cgroup_xattr(9)
• __cleanup_cgroup_environment(9)
• cleanup_cgroup_environment(9)
• get_root_cgroup(9)
• create_and_get_cgroup(9)
• get_cgroup_id_from_path(9)
• setup_classid_environment(9)
• set_classid(9)
• join_classid(9)
• cleanup_classid_environment(9)
• get_classid_cgroup_id(9)
• get_cgroup1_hierarchy_id(9)

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fr fr go read the source before you poke at this, it lives at ./tools/testing/selftests/bpf/cgroup_helpers.c and she's iconic ✨

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and that's the page bestie, mwah 💖 this one came from ./tools/testing/selftests/bpf, dated October 2026 — periodt, stan, and have a blessed lil cgroupv1 day 🌈⭐🧚