This small example shows how to use cmpxchg in C using inline assembly

locker.c

#include <stdio.h>
#include <stdint.h>

uint32_t lockin = 0;

uint32_t cmpxchg(uint32_t new, uint32_t old, volatile uint32_t* lock) {
        uint32_t result = 0;
        asm volatile(
            "lock cmpxchg %2,%1\n"
            "sete (%3)\n"
            : : "a"(old),"m"(*lock),"r"(new),"r"(&result)
            : "memory","cc"
        );
        return result;
    }

int main() {
    uint32_t result = cmpxchg(1,0,&lockin);
    printf("Erg: %d\n", result);
    printf("Lock: %d\n", lockin);
    uint32_t result2 = cmpxchg(1,0,&lockin);
    printf("Erg: %d\n", result2);
    printf("Lock: %d\n", lockin);
    uint32_t result3 = cmpxchg(2,1,&lockin);
    printf("Erg: %d\n", result3);
    printf("Lock: %d\n", lockin);
}