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 <g...@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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