Re: [lwip-users] Problem With dns.c Using 32-Bit Compilers

[Jonathan Larmour]

Alain M. wrote:
> Jonathan Larmour escreveu:
>
> > > char dns_answer[10];
> > > #define dns_answer_type  (*((u16_t*)dns_answer))
> >
> >
> > Unfortunately, the above is still not portable. That may well cause a 
> > mis-aligned access if dns_answer is not already aligned, and being 
> > char that isn't guaranteed.
>
> I believe that for byte access it should never get an error, do you know 
> of an exception?

Suppose an architecture requires 16-bit values to be aligned to 16-bits - 
this is common. dns_answer, being a char array, is only required to be 
8-bit aligned, and thus can be at an odd address. Therefore you can get a 
misalignment processor exception on that architecture if trying to load it 
through a u16_t*.

Some people may never see a problem as it can depend on what came before 
dns_answer in memory - if it was preceded by e.g. a u16_t, then as it 
happens it would work out because dns_answer would end up always starting 
at an at least 16-bit aligned address. e.g.

u16_t x;
char dns_answer[10];

On many systems, the above would essentially mean there is never a problem.

But if someone put a single char before it, then dns_answer might be at an 
odd address e.g.:
char y;
char dns_answer[10];

Although even then it depends what came before y, etc.etc.

In case you may be thinking this could be a way of guaranteeing alignment, 
you can't make that assumption more generally - e.g. targets with small 
data/bss sections (.sdata/.sbss) may allocate variables of different sizes 
in different locations. Order of definition doesn't guarantee order in memory.

> As for alignment, as it is independand of packing, as far as I know (not 
> much in this case) arrays start aligned if that is the compiler setting.

They are only aligned according to their type. For char, that means 8-bit 
alignment[1].

Jifl
[1] I will conveniently ignore the bonkers systems out there with 16-bit 
chars (yes, they do exist and yes, this is legal in the C standard. No, 
lwIP would not work with them without quite a bit of work).
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