do {...} while (0)

[Jerry Feldman]

On 02/06/2009 04:53 PM, Michael ODonnell wrote:
>   
>> What I see confufsing is:
>>    do { ... } while(0);
>> What this means is to go through the loop once.  You need a leading
>> curly so you can set up counter as a local variable as variable
>> names are block scope.  { ...  } would be equivalent to above.
>>     
>
> They're definitely not equivalent - that's why you see do-while(0) so
> often.  Try this (admittedly stupid) little program to demonstrate:
>
>    #define b0rken(x) {return x;}
>    #define works(x)  do {return x;} while(0)
>
>    int main( int argc, char **argv )
>    {
>        if( argc > 1 )
>            b0rken( argc ); /* Try works() here, instead */
>        else
>            return( 42 );
>    }
>   
This is a totally different thing. In your line:

 b0rken( argc ); 
The expansion is 

{return arc;};
Note the closing ';'.
The reason the works macro compiles is that while(0) requires a 
semicolon. In other words, in b0rken, is syntactically incorrect as a 
macro.
But if you change the macro to:

#define b0rken(x) ({return x;}) 
you will find that the compiler likes it again. it has nothing to do with { ... } vs. do { ... ] while(0); In your example it is about the correct compiler syntax. Basically { .. } defines a block.
Do statement while(expression)
is a statement, so do { ... } while(0) must be terminated by a semicolon because the semicolon terminates a statement. As I stated before, do { ... while(0) generates  code to handle the loop. Because the while(0) contains a constant, most compilers can figure that out at parse time. 


-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
PGP key id: 537C5846
PGP Key fingerprint: 3D1B 8377 A3C0 A5F2 ECBB  CA3B 4607 4319 537C 5846


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