Re: [ECOS] Network programming for eCos under linux

[Jonathan Larmour <jlarmour at redhat dot com>]

"Trenton D. Adams" wrote:
> 
>   > >
>   > > AFAICT, that should work on an ARM.  If it doesn't, it is a
>   > > compiler bug.
>   >
>   > Agreed. It would be different if you were accessing a more strictly
>   > aligned
>   > type through a less strictly aligned type, e.g. casting a char * to
> a
>   > short
>   > * and dereferencing; as Grant says...
>   >
> 
> So, you're not supposed to use a "char buffer[1024];" for a generic
> buffer then?  If not, how should one create a generic buffer?
> 
> So, the following gcc parameter would only warn when there is a problem
> with that particular architecture that you're compiling for?
> -Wcast-align

Yes, but it would also warn even if you have taken care to ensure it is
aligned.

> The one below would make it an error to cast from a char * to an int *?
> -mstrict-align

Yes.

> So, how would one go about making a buffer word aligned or DWORD aligned
> just to be safe?

something along the lines of:

char buffer[1024];
#define ALIGNMENT 8
#define ALIGNUP(_x_) (((char *)(_x_) + ALIGN-1) & ~(ALIGN-1))
int *foo = ALIGNUP(buffer);

You could also use __alignof(int) to give the alignment but that's
obviously a GNU C-ism.

>   > > > Maybe i don't have the casts correct.
>   > >
>   > > There are other casts that are not going to work on ARM
>   > > architecture (accessing words on non-word boundaries, for
>   > > example).  That's not a compiler error, it's just another way
>   > > to shoot yourself in the foot with C.
>   >
>   > Although it's worth mentioning that not all architectures disallow
>   > unaligned accesses which is why people think they can get away with
> it
>   > :).
>   >
> 
> This goes back to the -Wcast-align.  If -Wall is on, that means all
> warnings right?

Not all. Look at "info gcc" or "man gcc" to list the warnings that are and
aren't included e.g. -Wcast-align is not included. "man gcc" sez:

      The remaining `-W...' options are not implied by `-Wall' because 
they  warn  about
       constructions that we consider reasonable to use, on occasion, in
clean programs.

Jifl
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