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

[Jonathan Larmour]

Bill Auerbach wrote:
> > I'm not sure which ARM processor you're referring to, but that can't
> > actually be true. 

On the contrary, the LDRB instruction only appeared in the ARMV4T 
architecture. There are still simple small ARM7 variants with cores like 
ARM7DI, ARM720, etc. floating around, which do no support LDRB.

> > By definition in the C standard, every compiler is
> > guaranteed to have byte access anywhere it wants (IIRC, a byte is
> > defined as the minimal unit addressable on the architecture).

As Bill noted "byte"->"char" really. But even then, a compiler may say 
that chars are 8-bits, however the minimum addressable unit for an 
architecture can be something else, e.g. a 32-bit word. There's a 
difference between what the compiler does and what the processor accepts. 
It is then up to the compiler to load a word and mask off the relevant 
bits to get a byte.

> Are there really any processors in the last 10-20 years that didn't have
> bytes that are 8-bits?

Yes. Someone I know is using one right now, and it was released only a few 
years ago.

> To be semantically correct, C defines char (not byte).  Sizeof char is
> always 1, and if bytes have 8-bits, then the compiler is guaranteed to load
> chars as you stated.  Then it follows that macros are possible that do 2
> char accesses.  The macros I posted that do this work - they do 2 separate
> byte reads and form the final int.  This should work everywhere.

(Apart from the pointer aliasing issue I mentioned).

Despite all the chatter, I note that no-one seems bothered enough to 
confirm whether or not the patch I suggested for the specific issue with 
dns.c works!

Jifl
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