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SubjectRe: [Infiniband-general] Getting an Infiniband access layer in theLinux kernel
That's one of the many reasons of using do-while macros..
Other than compilation error, it may generate wrong code too!
this is a FAQ in kernel newbies , look at:
http://www.kernelnewbies.org/faq/index.php3#dowhile

thanks, som.

Roland Dreier wrote:

> Richard> If some major changes are being considered, I think it's
> Richard> time to get rid of the:
>
> Richard> do { } while(0) stuff that permiates a lot of MACROS and
> Richard> just use the { } as they were designed.
>
> Richard> Before everybody screams, think. It's perfectly correct
> Richard> to start a new "program unit" without a conditional
> Richard> expression. You just add a curley-brace, then close the
> Richard> brace when you are though.
>
>This is totally, totally wrong. If you get rid of do { } while (0),
>then you can't use the macro in an if statement. Read any C FAQ for
>details, or try the following:
>
> #define MAC(x) { x = x + 1; }
>
> int main() {
> int x = 0;
>
> if (1)
> MAC(x);
> else
> x = x - 1;
> }
>
>I get the following (correct) error:
>
> $ gcc a.c
> a.c: In function `main':
> a.c:8: syntax error before "else"
> $ gcc --version
> gcc (GCC) 3.2.3 20030502 (Red Hat Linux 3.2.3-20)
>
>because
>
> if (1)
> { x = x + 1 } ; /* <-- note semicolon
> else
> x = x - 1;
>
>is not correct C.
>
>By the way, it is possible to use parentheses and commas for some
>simple macros, so for example the following is OK:
>
> #define MAC(x) ( x = x + 1, x = x * 2 )
>
> int main() {
> int x = 0;
>
> if (1)
> MAC(x);
> else
> x = x - 1;
> }
>
>However I don't see anything wrong with the perfectly standard "do { }
>while (0)" idiom. Certainly if some compiler generates worse code for
>that construct that just a plain { }, _that_ is a compiler bug that we
>shouldn't have to work around.
>
> - Roland
>
>
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>



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