Messages in this thread | | | Date | Wed, 10 Jan 2007 01:20:31 -0500 (EST) | From | "Robert P. J. Day" <> | Subject | Re: macros: "do-while" versus "({ })" and a compile-time error |
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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.
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