___test_and_change_bit(9)

💅 ___test_and_change_bit(9) 💅

okay so welcome to the Kernel API Manual, bestie — this is the page for ___test_and_change_bit, and yes, those are three underscores up front. she's extra like that. 🎀

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✨ NAME ✨

___test_and_change_bit — change a bit and return its old value 👑

like, she flips the bit and tells you what it used to be. a little gossipy. we love her for it. 💖

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🦋 SYNOPSIS 🦋

bool ___test_and_change_bit (unsigned long nr , volatile unsigned long
*addr );

that's it. that's the whole vibe check. two args, one bool coming back at you. ✨

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🌸 ARGUMENTS 🌸

• nr — the bit you're changing, babes. that's the one. 🎯
• addr — the address to count from. yes it's volatile unsigned long *, no you may not fight me about it. 🙈

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💀 DESCRIPTION 💀

okay this is the important part so like. pay attention. 👀

this operation is non-atomic.

as in — if two instances of this operation race, one can appear to succeed but actually fail. 😭 not "fail loudly." not "throw an error." appear to succeed. fake success. the worst kind. it's giving gaslighting, gatekeeping, girlbossing. 🚩

so if you've got threads doing this on the same bit at the same time, no cap, you have a bug, and it will not be obvious, and you will be sad. atomic variants exist for a reason, bestie. use them. 💅

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🔥 SEE ALSO 🔥

• Kernel file ./include/asm-generic/bitops/instrumented-non-atomic.h — yes the whole filename. she's long. she's iconic. 👑
• ___set_bit(9)
• ___clear_bit(9)
• ___change_bit(9)
• ___test_and_set_bit(9)
• ___test_and_clear_bit(9)
• _test_bit(9)
• _test_bit_acquire(9)

the whole bit-twiddling friend group fr fr. scroll the list, find your girl. 🫶

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🌈 SIGN-OFF 🌈

this page came from the kernel file tree at ./include/asm-generic/bitops, dated October 2026 — so like, future energy, main character energy, ___test_and_change_bit(9) energy. ⭐

stay atomic out there. don't let the race conditions win. bye bestie!! 💜🧚🍓