Re: [boost] offset_cast proposal

From: Pavol Droba (droba_at_[hidden])
Date: 2002-11-25 10:24:08

On Mon, Nov 25, 2002 at 02:47:54PM +0100, Terje Sletteb? wrote:

[snip]
> As you say, and as was pointed out at the start of the thread, it may work
> on a specific platform (like x86). Perhaps it could be possible to have code
> that detects the platform, and which could then report an error on
> misalignment, if that is not tolerated on the given platform. It appears
> Boost.Config isn't enough for this, though, as it only detects the OS, not
> the underlying hardware.
> 
> How did you do it on the ARM? As I understand, it requires word-alignment
> for words (and half-word alignment for half-words, if that is supported on
> the given ARM version).
>
Well it is probably a conjuntion of luck and the kind of data we were working
with. As I have mentioned before, offset_cast was used mostly for network
packet analysis. And fortunately almost all network protocol structures
have correct aligmnent. So we were casting only to correct offsets :) 

> > I have mentioned before, that binary analysis could be done also using a
> > more
> > complicated framework. Would it make sense to create one?
> > 
> What does it do?

The framework I'm proposing should allow user to specify a structure layout 
( like what field starts where and what type if have ). Then there would
be a kind of possibly hidden management which would do nescesary copy-in and
copy-out operations.

I can imagine it should be something like this:

void modify_binary_header( void* pData )
{
        binary_struct b;
        b.add_field<int>( 0, "field1" );
        b.add_field<char>( 4, "field2" );
        b.add_field<int>( 5, "field3" );

        b.map_to_binary( pData, offset );

        b["field1"]=b["field2"]*2;
}

Please consider this as very preliminary example, just to show a desired functionality.
Construction of  tructure can be probably done in some better way using static templates.

Idea is that binary_struct would be responsible to some sort of safe copying ( possibly byte-by-byte )
of binary data into/out from the internal field representation. It can provide also some
other functionality like byte order corretion or handling of sub-byte fields ( something what
is not aligned to byte boundary )

Well, this is my idea how to resolve such a problem where simple mapping using offset_cast
is not sufficient.

Regards,

Pavol