[lwip-devel] [bug #27352] Change ip_addr from struct to typedef (u32_t)

[Simon Goldschmidt]

Follow-up Comment #46, bug #27352 (project lwip):

Well, of course I can create an unaligned u32_t pointer if I want to. In
fact, lwIP does this trick in some places and ensures correct alignment by
adding its own defines. However, if I don't take the detour of using a
void-pointer, the compiler warns me:

u8_t *a = &some_u8t;
u32_t *b = (u32_t*)a; <<-- compiler warns
u32_t *c = (u32_t*)(void*)a; <<-- compiler doesn't warn

If we only used the 1st cast, what could go wrong? How could it give an
unaligned type? An of course, the next lines give warnings, too:

typedef u32_t some_type_t;
some_type_t *d = (some_type_t)a;

So far so good. However, I would have expected to be warned when casting from
a (possibly) unaligned u32_t member of a packed struct to an u32_t pointer
(that requires alignment), but it seems I have been wrong there.

Being like that, we need some other guards here to prevent creating unaligned
pointers.

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