How to implement memmove in standard C without an intermediate copy?

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I think you’re right, it’s not possible to implement memmove efficiently in standard C.

The only truly portable way to test whether the regions overlap, I think, is something like this:

for (size_t l = 0; l < len; ++l) {
    if (src + l == dst) || (src + l == dst + len - 1) {
      // they overlap, so now we can use comparison,
      // and copy forwards or backwards as appropriate.
      ...
      return dst;
    }
}
// No overlap, doesn't matter which direction we copy
return memcpy(dst, src, len);

You can’t implement either memcpy or memmove all that efficiently in portable code, because the platform-specific implementation is likely to kick your butt whatever you do. But a portable memcpy at least looks plausible.

C++ introduced a pointer specialization of std::less, which is defined to work for any two pointers of the same type. It might in theory be slower than <, but obviously on a non-segmented architecture it isn’t.

C has no such thing, so in a sense, the C++ standard agrees with you that C doesn’t have enough defined behaviour. But then, C++ needs it for std::map and so on. It’s much more likely that you’d want to implement std::map (or something like it) without knowledge of the implementation than that you’d want to implement memmove (or something like it) without knowledge of the implementation.

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