Why / how does this C code compile? and What does it do?

Asked Apr 12 '17 at 11:50

Active Apr 12 '17 at 11:51

Viewed 69 times

I have a C code, in which a do loop is started in the first branch of a switch statement. What is weird, is that the rest of the branches of the switch statement are inside the body of the do loop. First, how is it possible that this code compiles? Second, what does it possibly do?

void f(char *x, char *y, int z) {
    int z2 = (z + 7) / 8;
    switch(z % 8) {
        case 0: do { *x++ = *y++;
        case 7: *x++ = *y++;
        case 6: *x++ = *y++;
        case 5: *x++ = *y++;
        case 4: *x++ = *y++;
        case 3: *x++ = *y++;
        case 2: *x++ = *y++;
        case 1: *x++ = *y++;
                } while(--z2 > 0);
    }
}

edited Apr 12 '17 at 11:51

BLUEPIXY

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asked Apr 12 '17 at 11:50

zardosht

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It compile because it's well written, and correct. – kaldoran Apr 12 '17 at 11:52
4

Duffs device. See wikipedia. – Joe Apr 12 '17 at 11:52
its a memcpy in blocks of 8, taking care of the fact not all "copy lengths" are divisible by 8. – Nish Apr 12 '17 at 11:55
that said it is not worth using this nowadays. memcpy should that just fine and loopunroling is a common feature in optimizing compilers afaik. – Kami Kaze Apr 12 '17 at 12:06
1

Yeah - whenever you see something like Duff, or similar, think hard about how difficult it will be to debug... – ThingyWotsit Apr 12 '17 at 13:01

Why / how does this C code compile? and What does it do?

0 Answers0