memmove in osdev meaty-skeleton: why does it have these if/else statements?

Asked Jul 23 '19 at 19:59

Active Jul 23 '19 at 19:59

Viewed 74 times

In the os-dev meaty skeleton tutorial this is the implementation of memmove:

#include <string.h>


void* memmove(void* dstptr, const void* srcptr, size_t size) {
    unsigned char* dst = (unsigned char*) dstptr;
    const unsigned char* src = (const unsigned char*) srcptr;
    if (dst < src) {
        for (size_t i = 0; i < size; i++)
            dst[i] = src[i];
    } else {
        for (size_t i = size; i != 0; i--)
            dst[i-1] = src[i-1];
    }
    return dstptr;
}

They check whether the dst pointer is less than src, if so they just move the memory. If not they move the memory in reverse order. But why?

Does that handle some edge case?

asked Jul 23 '19 at 19:59

Adham Zahran

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Do you know what is the difference between `memcpy` and `memmove` ? – Eugene Sh. Jul 23 '19 at 19:59
No I do not know the difference. But I'm reading about it now that you've pointed that out. – Adham Zahran Jul 23 '19 at 20:03
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So it is handling that difference :) – Eugene Sh. Jul 23 '19 at 20:03
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Imagine what would happen if the source and destination regions overlap... – Lee Daniel Crocker Jul 23 '19 at 20:03
Understood. Thank you very much :D – Adham Zahran Jul 23 '19 at 20:14

memmove in osdev meaty-skeleton: why does it have these if/else statements?

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