Re: [boost] [GIL] 64-bit alignment issue

From: John Maddock (john_at_[hidden])
Date: 2007-11-14 04:52:13

Lubomir Bourdev wrote:
> John Maddock wrote:
>> So... it's a data alignment error: I wonder are you trying to write
>> to
> an
>> integer on a char boundary or something?
>
> John, thanks for your help.
> I believe this is indeed the problem. GIL needs to read/write a type
> bigger than a byte that starts at a char* address, so it is possibly
> not aligned.
>
> One approach is to special-case platforms which cannot dereference at
> a non-byte boundary (I couldn't find a config flag that detects this,
> anyone?) and provide a special solution for such platforms. The
> solution
> I imagine will involve static recursion that reads/writes each byte
> and shifts it in place.
> Is there such a code in Boost? My search resulted only in references
> to
> a "Boost.Endian" proposal by Darin Adler / Beman Dawes but I couldn't
> find the code and I don't know if it has a solution to the above
> problem.

No I don't believe we have any such code or a config macro: one of the 
problems I believe is that whether or not the code will trigger a hardware 
signal may depend upon whether certain hardware flags are set or not: for 
example but I seem to remember that even Intel hardware can signal on 
unaligned access if the right flag is set :-(

You could use memcpy, or std::copy to copy the bytes I guess, but that 
doesn't seem all that efficient?  Or it shouldn't be too hard to roll your 
own "byte copier" for some type T?

Regards, John.