Messages in this thread | | | Date | Wed, 10 Jan 2007 22:49:22 +0100 | From | "J.A. Magallón" <> | Subject | Re: macros: "do-while" versus "({ })" and a compile-time error |
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On Wed, 10 Jan 2007 07:16:55 -0500, "linux-os \(Dick Johnson\)" <linux-os@analogic.com> wrote:
> > On Wed, 10 Jan 2007, Robert P. J. Day wrote: > > > 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 > > > Hmmm, in that case you would be trying to put code inside a structure! > Neat --if you could do it! >
The ({ }) is a block expression, ie, it allows declaring variables and executing code. Its a gcc extension trying to resemble what other languages like ML have:
ML: f = let y = x*x in 2*y + sin(y) end
GNU C: f = ({ int y = x*x; 2*y + sin(y); })
So you can put it on every place you could also put a { } block or declare a variable. {} is a compund command and ({ }) is a compund expression (or block expression, do not know which is the good name in engelish).
-- J.A. Magallon <jamagallon()ono!com> \ Software is like sex: \ It's better when it's free Mandriva Linux release 2007.1 (Cooker) for i586 Linux 2.6.19-jam03 (gcc 4.1.2 20061110 (prerelease) (4.1.2-0.20061110.1mdv2007.1)) #1 SMP PREEMPT - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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