cpumask_first_and(9)

🌸 NAME 🌸

cpumask_first_and — bestie, this is the one that returns the first cpu from *srcp1 & *srcp2, okay?? Like, you hand it two cpumasks, it finds the CPUs they share, and it gives you the lowest-numbered one of that overlap. Iconic behaviour, honestly. 💅

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✨ SYNOPSIS ✨

unsigned int cpumask_first_and (const struct cpumask *srcp1 , const struct cpumask *srcp2 );

Lowkey the whole personality of this function is right there in one line. 🫶

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🦋 ARGUMENTS 🦋

• srcp1 — the first input. that's it, that's the descriptor, she's your first mask 💖
• srcp2 — the second input. the other half of the &, she's the second mask 💖

ngl the manpage keeps these two descriptions so short, but the vibe check is clear: both are const struct cpumask *, both get AND-ed together, both matter. no cap. ✨

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🎀 RETURN 🎀

>= nr_cpu_ids if no cpus set in both. 😭

Read that again, gently: if the two masks share zero CPUs, you don't get a real CPU back — you get a value >= nr_cpu_ids (i.e. not a valid CPU id at all), so go check for it, bestie, don't just index an array with it. fr fr.

See also cpumask_next_and. 👑

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🌈 SEE ALSO 🌈

Okay so the friend group is huge, and honestly they all ate. Kernel file ./include/linux/cpumask.h:

• The first-cpu crew: cpumask_pr_args(9), cpumask_first(9), cpumask_first_zero(9), cpumask_first_andnot(9), cpumask_first_and_and(9), cpumask_last(9)
• The next-cpu crew: cpumask_next(9), cpumask_next_zero(9), cpumask_next_and(9), cpumask_next_andnot(9), cpumask_next_and_wrap(9), cpumask_next_wrap(9), cpumask_random(9)
• The loops: for_each_cpu(9), for_each_cpu_wrap(9), for_each_cpu_and(9), for_each_cpu_andnot(9), for_each_cpu_or(9), for_each_cpu_from(9)
• The "any but" / nth squad: cpumask_any_but(9), cpumask_any_and_but(9), cpumask_any_andnot_but(9), cpumask_nth(9), cpumask_nth_and(9), cpumask_nth_and_andnot(9)
• The setters and testers: cpumask_set_cpu(9), cpumask_clear_cpus(9), cpumask_clear_cpu(9), cpumask_test_cpu(9), cpumask_test_and_set_cpu(9), cpumask_test_and_clear_cpu(9), cpumask_setall(9), cpumask_clear(9)
• The bitwise besties: cpumask_and(9), cpumask_or(9), cpumask_weighted_or(9), cpumask_xor(9), cpumask_andnot(9), cpumask_equal(9), cpumask_or_equal(9), cpumask_intersects(9), cpumask_subset(9), cpumask_empty(9), cpumask_full(9), cpumask_weight(9), cpumask_weight_and(9), cpumask_weight_andnot(9), cpumask_shift_right(9), cpumask_shift_left(9), cpumask_copy(9)
• The any / of crew: cpumask_any(9), cpumask_any_and(9), cpumask_of(9)
• The parsing girlies: cpumask_parse_user(9), cpumask_parselist_user(9), cpumask_parse(9), cpulist_parse(9)
• The allocation and misc crew: cpumask_size(9), alloc_cpumask_var(9), to_cpumask(9), num_online_cpus(9)
• The printing divas: cpumap_print_bitmask_to_buf(9), cpumap_print_list_to_buf(9)

Like… the whole cpumask family showed up to the group chat. obsessed. 🧚💜

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💀 reminder bestie: >= nr_cpu_ids means no shared CPUs, not "cpu number thirty-something". do not pass go, do not index anything.

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and that's the page!! 🍓 source is ./include/linux, dated October 2026, and the manual is cpumask_first_and(9) — mwah, go read cpumask_next_and(9) next, ily, bye!! 💖🎀✨