CASU(9)

💖 CASU(9) — the kernel dev manual, but make it slay 💖

okay bestie, so this is the page about casueword, casueword32, casuword, and casuword32 — aka the little guys who go fetch, compare and store data from user-space. it's giving kernel-mode, it's giving atomic, it's giving "please align your pointers or perish". let's get into it 🦋

✨ NAME ✨

casueword, casueword32, casuword, casuword32 — fetch, compare and store data from user-space

🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <sys/systm.h>

int
casueword(volatile u_long *base, u_long oldval, u_long *oldvalp,
    u_long newval);

int
casueword32(volatile uint32_t *base, uint32_t oldval, uint32_t *oldvalp,
    uint32_t newval);

u_long
casuword(volatile u_long *base, u_long oldval, u_long newval);

uint32_t
casuword32(volatile uint32_t *base, uint32_t oldval, uint32_t newval);

🎀 DESCRIPTION 🎀

so the casueword girlies are literally designed to do an atomic compare-and-swap operation on the value living in the usermode memory of the current process. main character energy fr fr 💅

here's the tea: the casueword routines read the value from user memory at address base, and then compare the value they read with oldval. if those two are equal — slay, match — then newval gets written to *base. and for casueword32() and casueword(), the old value is stored into the (kernel-mode) variable pointed to by *oldvalp. no cap, that's just how it works ✨

okay but the way that the userspace value must be naturally aligned — like, non-negotiable, bestie. it's giving strict. it's giving boundaries. respect them 🫶

and here's the sad part 😭 the callers of casuword() and casuword32() cannot distinguish between -1 read from userspace and function failure. like... it's the same number either way. tragic. iconic. anyway 💀

⭐ RETURN VALUES ⭐

• the casuword() and casuword32() functions return the data fetched — or -1 on failure. that's it, that's the whole vibe.
• the casueword() and casueword32() functions are a lil more chatty: they return 0 on success, -1 on failure to access memory, and 1 when comparison or store failed. three whole moods 🌈
• also!! the store can fail on load-linked/store-conditional architectures. so if it fails, it's not you, it's the architecture 💜

👑 SEE ALSO 👑

atomic(9), fetch(9), store(9)

and that's the page, bestie!! this one came out of the Debian crew, dated April 19, 2019 — so like, vintage but still serving. go forth and compare-and-swap responsibly ✨💖