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

[Alain M.]

Michael Williamson escreveu:
> Actually, I happen to be a developer that uses a compiler that doesn't
> even have structure packing (TI's Code Composer Studio for their DSP
> products),

that is an important platform... :(

> It would be really handy, if, somehow, there could be a sys_arch style
> hook or macro for protocol fragment encoding/decoding that would default
> to structure overlaying, but let you override it with byte-by-byte
> copying to a compiler friendly data structure if you need to.

I have seen a comm packet long time ago that used macros with pointers, 
and I used that for some small things. It is 100% portable and 
performance would be hw dependent. Probably in chips like ARM7/9 it 
would be bad, but on PCs and Cortex that have byte access it would be 
better, no idea about 16 bits DSP. Also remember that 99% of protocol 
header variables are contained in 32 bit words so it will not be too bad.

the example would be:
struct dns_answer {
  /* DNS answer record starts with either a domain name or a pointer
     to a name already present somewhere in the packet. */
  u16_t type;
  u16_t class;
  u32_t ttl;
  u16_t len;
} PACK_STRUCT_STRUCT;

char dns_answer[10];
#define dns_answer_type  (*((u16_t*)dns_answer))
#define dns_answer_class (*((u16_t*)&dns_answer[2]))
#define dns_answer_ttl   (*((u32_t*)&dns_answer[4]))
#define dns_answer_len   (*((u16_t*)&dns_answer[8]))

or with pointers:
#define dns_answer_type(p)  (*((u16_t*)p))
#define dns_answer_class(p) (*((u16_t*)(p+2)))

or some variations... (or fixes, I may have it wrong)

This could be a

Alain