Architectures with strict alignment?

[Bernd Walter]

On Sun, Dec 30, 2007 at 11:18:08PM -0500, Mike Meyer wrote:
> On Sun, 30 Dec 2007 20:37:18 +0100 Bernd Walter <ticso at cicely12.cicely.de> wrote:
> > On Sun, Dec 30, 2007 at 01:55:06PM -0500, Mike Meyer wrote:
> > > Ok, I'm a bit confused. Since you're talking about moving code from
> > > the x86 to the alpha, I'm assuming you're talking about C code. Isn't
> > > it the *compilers* job to enforce alignment issues, unless the
> > > programmer specifically asks for byte-specific control of the layout
> > > of a set of variables?
> > Not in every case.
> > It is not unusual to read raw bytes from e.g. network and use
> > a struct pointer to it.
> > But in case of network data it may be missaligned and if the struct
> > contains anything else then bytes you may be doing missaligned accesses.
> 
> Actually, network data is the most common case where the programmer
> has to ask for byte-specific control of the layout. As far as I know,
> the proper way to do this is to declare the struct properly - with a
> compiler-dependent request for packing - then read the data into
> that. If you could be reading more than one type of struct into it,
> then set up the appropriate unions for all of them. In either case,
> the compiler should DTRT.

reading into a struct is one way.
The other way is pointing into a read buffer.

> But it sounds like this people just read raw bytes, and then
> disassembled them by hand, which I would say qualifies as "crappy
> software."

It has it's reasons, but you have to be carefull if doing this and
that's what people fail with - or better said other people fail with
their software.
In the same class of bugs we now have problems with arm from time to
time, since arm pads single bytes to full 32bit aligment unless the
struct is declared as being packed.

> > > Or are these the issue, and the problem is that people do that and
> > > then don't use the appropriate APIs to pull data from them, thus
> > > causing you headaches?
> > No - they just don't think about alignment when parsing data from random
> > source.
> 
> That seems to be common: if something doesn't matter on their
> platform, programmers tend not to worry about it. I saw plenty of
> evidence of that as Unix moved from 16 bit systems to 32 bit ones, and
> again as it went to 64 bits (though not quite so bad).

Yes - only for the last few years with all those amd64 systems out
there those kind of problems are rare.
But unlike missalignment bugs the compiler catches many of them as
long as they are not reading random source data into structs...

-- 
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