> HI David,
>
> Macro is fine. For example, gcc FP lib does the same trick.
> (actually '#define ToChar(x) (*(char*)(&(x)))' is enough :)
>
> Alternatively, you can do:
>
> typedef union {
> char reg;
> struct {char a : 1; char b : 1; char c : 1;} bits;
> } tFlags;
>
> tFlags f, g, h;
>
> then call
>
> f.reg = g.reg | h.reg;
> f.reg = g.reg & h.reg;
>
That's neat - I had thought about using unions to do the convertion, but
only as a type conversion afterwards (i.e., keeping the original tFlags
definition and then making a new union with a tFlags and a char element).
Instead of giving the "bits" struct a name, I have now left it unnamed,
creating an anonymous union which works very neatly - thanks !
> You'll get exactly the same result.
>
> ~d
>
> P.S. it is better to declare bitfields as 'unsigned' cause zero extension
is
> much simpler if required. Also, attribute 'packed' might make sense.
>
I simplified the code a bit for posting - I invariably use typedefs defining
"byte" and "word", and use "unsigned" for almost everything. I also
have -funsigned-bitfields and -funsigned-char on.
Many thanks,
David
>
>
> On Monday 16 December 2002 16:15, David Brown wrote:
> > I know this is really a general C question rather than a mspgcc-specific
> > one, but it is fairly relevant to anyone working with the new bitfield
> > structures for I/O pins.
> >
> > If I have a structure defined:
> > typedef { char a : 1; char b : 1; char c : 1;} tFlags;
> >
> > and variables:
> > tFlags f, g, h;
> >
> > what is the best way to get their union or intersection? If tFlags had
> > been a plain char, I would write "f = g | h;" or "f = g & h". But that
is
> > not allowed when they are structures, nor is it possible to cast them to
> > "chars" directly. My best solution at the moment is to use a macro:
> >
> > #define ToChar(x) (*(char*)(void*)(&(x)))
> >
> > ToChar(f) = ToChar(g) | ToChar(h);
> > ToChar(f) = ToChar(g) & ToChar(h);
> >
> > This works fine, and produces optimal code (as far as I have seen,
with -O2
> > active) - there seems to be no problem keeping everything in registers
even
> > though the macro messes around with pointers, which is very impressive.
> >
> > However, if anyone has any ideas of how to do this without such casts,
I'd
> > be very interested to hear.
> >
> > mvh.
> >
> > David
> >
> >
> >
> >
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