Re: debugging frustration

[Anthony Liguori <anthony@codemonkey.ws>]

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@codemonkey.ws



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