Re: [Xen-devel] debugging frustration

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

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.
Regards,
Anthony Liguori

> > It makes the macro behave more like a proper statement.  e.g.  If we try to 
> > do:
> > if ( 1 == 2 debug_print("foo!") )
> > {
> >
> > }
> >
> > It won't compile, even if debugging is switched off.  If we just did "#define 
> > debug_print(...)" then this would compile and we'd only notice the error when 
> > debugging was switched on.
> > That's a quite contrived example but basically it keeps the developers 
> > honest ;-)
> > Cheers,
> > Mark
> >
> >
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> >    

--
Anthony Liguori
anthony@xxxxxxxxxxxxx



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