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Re: gzip use of memcpy

From: Jim Meyering
Subject: Re: gzip use of memcpy
Date: Fri, 08 Jan 2010 18:55:32 +0100

Alain Magloire wrote:
> Bonjour,

Bonjour Alain !  Bonne anneƩ !
It's been too long ;-)

> The behavior of memcpy is well defined:
> "Copying overlapping buffers isn't guaranteed to work; use memmove() to
> copy buffers that overlap"
> In our case for the SH architecture we have a version of memcpy that
> takes advantage at some specific instruction and above a certain size
> the copy will be done in reverse (side effect of the optimization).
> For overlapping copies, the code should be using memmove() as describe
> in POSIX.
> Index: inflate.c
>          if (w - d >= e)
>          {
> -          memcpy(slide + w, slide + d, e);
> +          memmove(slide + w, slide + d, e);

Can you give sample values of w, d and e for which this change
makes a difference?  Doesn't the preceding (w - d >= e) guard
ensure that the source and destination buffers never overlap?

> Index: deflate.c
> -        memcpy((char*)window, (char*)window+WSIZE, (unsigned)WSIZE);
> +        memmove((char*)window, (char*)window+WSIZE, (unsigned)WSIZE);

The source and destination buffers do not overlap.

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