a.s

// in asm_atoi.S...
fbld (%rdi)
fstpl -8(%rbp)
movq -8(%rbp), %xmm0

// in main.c...
int main(int argc, const char * argv[]) {
    const char *c = "2384";
    char b[10] = {0};
    for (int i = 0; i < sizeof(c); i++) {
        int shift = (i & 1) ? 0 : 4;
        b[i / 2] |= (c[i] - '0') << shift;
    }
    double z = asm_atoi(b);
    printf("%lf\n", z); // should print 2384
    return 0;
}