On Mon, 10 Mar 2008, William "Chops" Westfield wrote:

> 
> On Mar 10, 2008, at 7:03 PM, sergio masci wrote:
> 
> > Could you please explain what you meen by "making a multi-line macro
> > safe". I have never seen this contruct used to protect code.
> 
> A macro that contains multiple lines of code is not "safe" to use in
> assorted places, for example, to avoid compiler warnings, I convert  
> all my old
> macro definitions:
> 	#define CHECK_ERR if (errcode = get_error()) {print_error(errcode)}
> to	#define CHECK_ERR errcode = get_error(); if (errcode) {print_error 
> (errcode)}
> 
> that's all very nice, but the macro now fails in cases like:
> 	if (suspiciousstuff)
> 	    CHECK_ERR;
> 	else
> 	   /* everything peachy keen. */
> 
> (It's VERY similar to the problem you get using assembly macros that can
> expand to more than one instruction after SKIP instructions.)
> 
> A common way to fix this in C is to make your macro a block statement  
> that
> executes exactly once:
> 	#define CHECK_ERR do { \
>                  errcode = get_error(); \
>                   if (errcode) {  \
>                      print_error(errcode) \
> 		} while (0)
> Now, not matter how complex the code inside the do ... while  
> statement, it
> it lexes/parses the same as a single C statement.
> 
> BillW
> 


I must be dense. Why not just use a compound statement

e.g.
 	#define CHECK_ERR { \
                  errcode = get_error(); \
                   if (errcode) {  \
                      print_error(errcode) \
 		   } \
		}

Regards
Sergio
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