Re: [boost] Access to unaligned (packed) structs

From: Zara (yozara_at_[hidden])
Date: 2007-10-11 08:49:21

On Thu, 11 Oct 2007 14:19:34 +0200, Roland Schwarz
<roland.schwarz_at_[hidden]> wrote:

>
>First there was a typo:
>it should read
>     operator T() const
>not
>     operator T&() const
>
>Zara wrote:
>> I may not work for PODs. It depends on the data alignment of the
>> compiler, and the data alignment capabilities of the processor.
>> 
>> You are invoking the dreaded devils of Undefined Behaviour!
>
>Of course I want to avoid this, but could you please be more specific?
>
>Is it the
>static_cast<T>(*data)
>
>that will cause troubles?
>
>If so why?

Because not all processors may access words not aligned with the
natural word boundary. This could result in some type of exception
(depends on processor, OS, compiler...).

 OTOH, if you compile with 4-byte alignment, the cast *may* strip the
lower bits of the pointer and give you rubbish instead of your value.
This is UB, and should never be invoked.

>
>Would it be better to use something like
>
>operator T() const
>{
>     T t = 0;
>     for (unsigned n=0; n<sizeof(T); ++n)
>     {
>         t |= ((T)data[n])<<(8*n);
>     }
>     return t;
>}

yes, certainly. Just check if the endiannes is the right one, or if
you must caculate the toal the oother way round.

Best regrads,

Zara