Re: macros: "do-while" versus "({ })" and a compile-time error

Robert P. J. Day
2007-01-10T06:25:47Z

On Tue, 9 Jan 2007, linux-os (Dick Johnson) wrote:

>
> On Tue, 9 Jan 2007, Stefan Richter wrote:
>
> > Robert P. J. Day wrote:
> >> just to stir the pot a bit regarding the discussion of the two
> >> different ways to define macros,
> >
> > You mean function-like macros, right?
> >
> >> i've just noticed that the "({ })"
> >> notation is not universally acceptable. i've seen examples where
> >> using that notation causes gcc to produce:
> >>
> >> error: braced-group within expression allowed only inside a function
> >
> > And function calls and macros which expand to "do { expr; } while (0)"
> > won't work anywhere outside of functions either.
> >
> >> i wasn't aware that there were limits on this notation. can someone
> >> clarify this? under what circumstances *can't* you use that notation?
> >> thanks.
> >
> > The limitations are certainly highly compiler-specific.
>
> I don't think so. You certainly couldn't write working 'C' code like
> this:
>
> do { a = 1; } while(0);
>
> This _needs_ to be inside a function. In fact any runtime operations
> need to be inside functions. It's only in assembly that you could
> 'roll your own' code like:
>
> main:
> ret 0
>
>
> Most of these errors come about as a result of changes where a macro
> used to define a constant. Later on, it was no longer a constant in
> code that didn't actually get compiled during the testing.

just FYI, the reason i brought this up in the first place is that i
noticed that the ALIGN() macro in kernel.h didn't verify that the
alignment value was a power of 2, so i thought -- hmmm, i wonder if
there are any invocations where that's not true, so i (temporarily)
rewrote ALIGN to incorporate that check, and the build blew up
including include/net/neighbour.h, which contains the out-of-function
declaration:

struct neighbour
{
...
unsigned char ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))];
...

so it's not a big deal, it was just me goofing around and breaking
things.

rday