[Ffmpeg-devel] [PATCH] avoid useless memcpy with src==dst (valgrind complains)

[Rich Felker]

On Sun, Jul 23, 2006 at 05:35:59PM +0200, Reimar D?ffinger wrote:
> Hello,
> On Sun, Jul 23, 2006 at 02:24:17PM -0000, Wolfram Gloger wrote:
> > > Though for src == dst I doubt any memcpy function will break
> > > stuff. And for the overlapping case when src != dst does the C standard
> > > specify the behaviour?
> > 
> > Using memcpy() on overlapping objects is undefined behaviour
> > (7.21.2.1#2).
> 
> Which part were you replying to? If to the first: memcpy may strictly
> speaking have undefined behaviour for src == dst, but I very much doubt
> it has in reality.
> If to the second part: I meant if you have two "struct something"
> pointers a and b, is the behaviour of *a = *b defined when the areas for
> *a and *b overlap?

That's an interesting question. *a = *b may be undefined when a and b
both point to the same object; I'm not sure. Anyone care to dig up
this info?

> objdump -S on utils.o gives me:
> [...]
>     temp= *buf;
>      18a:       ba 58 00 00 00          mov    $0x58,%edx
>      18f:       48 89 ee                mov    %rbp,%rsi
>      192:       48 89 e7                mov    %rsp,%rdi
>      195:       e8 00 00 00 00          callq  19a <avcodec_default_release_buffer+0x7a>
>     *buf= *last;
>      19a:       ba 58 00 00 00          mov    $0x58,%edx
>      19f:       4c 89 e6                mov    %r12,%rsi
> [...]
> 
> so there definitly is some placeholder for some call...

Use the -r or -R option to see the relocation information; then you
should see where the call goes.

Rich