Re: [Infiniband-general] Getting an Infiniband access layer in theLinux kernel

[somenath]

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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