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    <p>I think you're right, it's not possible to implement <code>memmove</code> efficiently in standard C.</p> <p>The only truly portable way to test whether the regions overlap, I think, is something like this:</p> <p>[Edit: looking at this much later, I think it should be <code>dst+len-1</code> at the end of the second line. But I can't be bothered to test it so I'll leave as it is for now, it's possible I knew what I was talking about the first time.]</p> <pre><code>for (size_t l = 0; l &lt; len; ++l) { if (src + l == dst) || (src + l == dst + len) { // 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); </code></pre> <p>You can't implement either <code>memcpy</code> or <code>memmove</code> all <em>that</em> efficiently in portable code, because the platform-specific implementation is likely to kick your butt whatever you do. But a portable <code>memcpy</code> at least looks plausible.</p> <p>C++ introduced a pointer specialization of <code>std::less</code>, which is defined to work for any two pointers of the same type. It might in theory be slower than <code>&lt;</code>, but obviously on a non-segmented architecture it isn't. </p> <p>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 <code>std::map</code> and so on. It's much more likely that you'd want to implement <code>std::map</code> (or something like it) without knowledge of the implementation than that you'd want to implement <code>memmove</code> (or something like it) without knowledge of the implementation.</p>
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    1. POHow to implement memmove in standard C without an intermediate copy?
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    1. USSteve Jessop
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    1. COOh, beautiful (comparison for equality of only valid pointers).
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      1. USPascal Cuoq
    2. COI hope that by "beautiful", you mean "My eyes! The goggles do nothing!" ;-)
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    3. CONo, I mean "beautiful" because the pseudo-code is technically, as far as I can see, an answer to the question, rather than a discussion about why the C standards are the way they are.
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