ATOMIC_CAS_UINT(9)

💅 ATOMIC_CAS_UINT(9) 💅

okay so bestie, welcome to the Kernel Developer's Manual ✨ — yes that's the manual section we're in, 9, kernel-land, the cool section, no cap 🎀

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🌸 NAME 🌸

atomic_cas_uint, atomic_cas_ulong, atomic_cas_ptr — atomic compare-and-swap operations 🦋

like literally the three of them, one little squad, all doing the swap-and-compare thing 💖

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✨ SYNOPSIS ✨

#include <sys/atomic.h>

unsigned int
atomic_cas_uint(volatile unsigned int *p, unsigned int expected,
    unsigned int new);

unsigned long
atomic_cas_ulong(volatile unsigned long *p, unsigned long expected,
    unsigned long new);

void *
atomic_cas_ptr(volatile void *p, void *expected, void *new);

that's it, that's the whole vibe check 👑 include the header, call the function, do the atomic thing 💫

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🫶 DESCRIPTION 🫶

okay so the atomic_cas set of functions gives you an interface for atomically performing compare-and-swap operations — and by atomically we mean with respect to interrupts and multiple processors in the system, which is the whole point, this is the main character energy right here 🔥

so here's the tea ☕: the value referenced by the pointer p gets compared against expected. if those two values are equal? then new replaces the value stored at p. that's the swap. that's the whole bit. slay 💅

if they're not equal, nothing gets swapped (that's the compare part doing its job, iykyk) 🧚

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🌈 CONTEXT 🌈

atomic_cas_uint(), atomic_cas_ulong(), and atomic_cas_ptr() can all be called during autoconf, from process context, or from interrupt context 💜

like all three of them, all three contexts, no restrictions, go off bestie ⭐

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🎀 RETURN VALUES 🎀

these functions return the value at p as it was before the attempt to swap it 🙈

so you always get to see what was there first — even if the swap didn't happen, you get the original value back, which is genuinely so useful fr fr 💀

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🦋 SEE ALSO 🦋

• atomic_swap_uint(9)

go read that one too, it's the cousin 🍓

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💖 HISTORY 💖

the atomic_cas functions first appeared in NetBSD 5.0 and OpenBSD 5.5 — iconic from day one, honestly 🌸

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and that's the page bestie!! this one's from Debian, dated February 6, 2019, and the page title is ATOMIC_CAS_UINT(9) — now go forth and swap atomically, mwah 💅✨🎀