___test_and_set_bit(9)

💅 ___test_and_set_bit — Kernel API Manual, bestie edition 🎀

okay so welcome to the Kernel API Manual, the little corner of the docs where we talk about bit-fiddling ✨ and today's main character is ___test_and_set_bit. yes it has three leading underscores. no we don't ask why. iykyk 💀

---

✨ NAME ✨

___test_and_set_bit — set a bit and return its old value 🫶

like, it flips the bit ON and hands you back what the bit used to be. that's the whole personality. iconic tbh 👑

---

🌸 SYNOPSIS 🌸

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

literally copy that exactly, spacing and all, don't @ me 🙈

---

🦋 ARGUMENTS 🦋

• nr — the bit to set. which bit, bestie? this one. 🎯
• addr — the address to count from. your starting point, your home base, your origin story 💖

---

🔥 DESCRIPTION 🔥

okay so here's the tea and you need to hear it 🍵

This operation is non-atomic and can be reordered. that's right, it's giving chaos gremlin energy 😭 it does not respect your locks, it does not respect your ordering, it simply vibes ✨

and because of that: if two examples of this operation race, one can appear to succeed but actually fail. 💀 not the fake success!! so rude.

⚠️ so what do you do? You must protect multiple accesses with a lock. fr fr. no cap. if you're touching this from more than one place, lock it down like it's your group chat 💅🔒

---

🌈 SEE ALSO 🌈

• Kernel file ./tools/include/asm-generic/bitops/non-atomic.h — go read the source, bestie, it's all there 📖
• ___set_bit(9)
• ___change_bit(9)
• ___test_and_clear_bit(9)
• _test_bit(9)

(sibling energy, honestly. the whole bitops family ate 🔥)

---

💜 okay that's the page!! this little masterpiece comes to you from ./tools/include/asm-generic/bi... (yes, truncated with dots, that's just how it's written, don't come for me 😭) and it's dated October 2026 — future icon behavior tbh ⭐ and it signs off as ___test_and_set_bit(9), three underscores and all, slay 💅🎀