Re: [Xen-devel] debugging frustration

[Luciano Miguel Ferreira Rocha <strange@xxxxxxxxxxxxx>]

On Mon, Feb 14, 2005 at 09:03:05AM -0600, Anthony Liguori wrote:
> Mark Williamson wrote:
> 
> >>Also, is there a reason that not having DEBUG in turns dprintf into do
> >>{} while(0) instead of just turning into ""
> >>   
> >>
> The do {} while (0) idiom is commonly used in macros to protect against 
> the following.  Say you had a macro that looked like this:
> 
> #define DEBUG(a, b)  if (a > debug_level) printf(b);
> 
> If you called it like this:
> 
> if (ptr == NULL)
>   DEBUG(10, "Bad pointer");
> else
>   *ptr = 2;
> 
> What would actually happen is that the else clause would get attached to 
> the if from the DEBUG() statement and you would get very odd behavior.
> 
> Using do {} while (0) is just the common solution to this problem.  It's 
> not the only solution, but it's what most people use.

Yes, but an empty macro results in the same thing as a do {} while (0)

if (ptr == null)
    ;
else
    *ptr = 2

The only problem is when the macro has some code (AFAIK).

Regards,
Luciano Rocha


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