Re: [avr-gcc-list] mass stupidity with registers and longs

[Richard Urwin]

On Friday 14 Jan 2005 6:18 am, Russell Shaw wrote:
> Curtis Maloney wrote:
> > I give up.  I've been trying to shift 4 chars into an unsigned
> > long, but can't stop GCC 3.4.1 (what's shipped with the latest
> > WinAVR) from sign extending values, let alone producing sane code.
> >
> > I've tried -Os, -O2, -O3, with and without -fnew-ra... the asm
> > varies, but is generaly crap.
> >
> > now, if I have:
> >     extern char a[4];
> >     unsigned long ul;
> >
> >     ul = ( (unsigned)a[0] << 8 ) | (unsigned)a[1];
> >
> > why would it be sign extending a[0] and a[1] when they're cast to
> > unsigneds?
>
> a[] is sign extended and the resulting integer interpreted as
> unsigned.

So what happens if you catch the type cast before it has to dereference 
a byte into an int? :-

      extern char a[4];
      unsigned long ul;

      ul = ( ((unsigned char*)a)[0] << 8) | ((unsigned char*)a)[1];

-- 
Richard Urwin