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

linux-os (Dick Johnson)
2007-01-09T19:23:06Z

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.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_
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