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Re: [RFC] Do not call memmove when memcpy suffices.

On Tue, 13 Aug 2013, Ondrej Bilka wrote:

> #define memmove(_dest, _src, _n)                                              \
>   ({                                                                          \
>      char *dest = (_dest);                                                    \
>      char *src = (_src);                                                      \
>      size_t n = (_n);                                                         \
>      if (__glibc_likely ((size_t)((src - n) - dest) >= 2 * n))                \
>        return memcpy (dest, src, n);                                          \
>      else                                                                     \
>        return memmove (dest, src, n);                                         \
>    })

I don't see why this is safe.  Say the arguments overlap with src < dest, 
((src - n) - dest) negative and becoming large and positive when cast to 
size_t, so the test passes and memcpy is called - but the glibc ABI 
guarantee for memcpy requires that there is no overlap in either direction 
(except maybe in the case where src == dest).  Do you mean ((src + n) - 

Joseph S. Myers

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