memcpy copying partly over itself

Is this ok?

char buf[] = { 0, 1, 2 };
memcpy(b开发者_Python百科uf, buf + 1, 2);

Does having a bigger datatype make any difference? I know I could use memmove(), but I'm just curious.

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The effects of memcpy are not defined when the input and output overlap. You should use memmove.

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Not okay. You must use memmove when the source and destination overlap.

It might work with memcpy, depending on your compiler's implementation, but that is not something you should rely on!

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A typical naive implementation might be a byte-by-byte copy (uint8) like this:

typedef unsigned char uint8;

void * memcpy ( void * destination, const void * source, size_t num )
{
    const uint8* pSrc = (const uint8*)source;
    const uint8* const pSrcEnd = pSrc + num;
    uint8* pDst = (uint8*)destination;

    while (pSrc != pSrcEnd)
        *(pDst++) = *(pSrc++);

    return destination;
}

which would work okay with your memcpy(buf, buf + 1, 2), but less well with memcpy(buf + 1, buf, 2)

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The behavior is undefined if the memory regions overlap, so this is not ok.

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It is undefined behaviour, but I have used it like so,

char buf[] = {0, 1, 2, 3);
memcpy(buf, buf+2, 2);

where the end point of the destination is less than the start point of the source. You should still use memmove to be sure...