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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