How to avoid "the address of 'foo' will never be NULL" warning inside macros?

[Steffen Dettmer]

On Thu, Jun 9, 2011 at 11:23 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> - disabling the warning would (I guess) also affect when checking
>>  addresses without using macros
>
> Not if you do it locally in the macro only, using _Pragma
> http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Diagnostic-Pragmas.html

Thank you for the pointers.

Today I tried again to use it, but I'm unable to get it right.
I understood that I should use _Pragma inside my `function-style
macro', but I get errors. I assume because I'm trying to use
_Pragma `inside' an expression.

I'm sorry to bother you again but could you please give an
concrete example or implement/fix my example?

The example macro is getLen (implemented as macro because some
compilers may not `inline' correctly automatically and do not
offer a way to explicitely do so).
The usage is:
  len = getLen(&stack_var);
Here some code I tried:

/* with "len = getLen(&bb);":
 *   (68 of 85): 4: warning: the comparison will always evaluate as 'true'
 *               for the address of 'ref' will never be NULL [-Waddress]
 */
struct bb_s {
   void *m;
   size_t l;
};
/* The macro that I would like to use without warnings: */
#define getLen(bb) (size_t)(((bb) != NULL ? (bb)->l : 0))

#if 0
/* (11 of 17): 1: error: expected expression before '#pragma' */
#define getLen(bb) \
   _Pragma ("GCC diagnostic push") \
   _Pragma ("GCC diagnostic ignored \"-Waddress\"") \
   (size_t)(((bb) != NULL ? (bb)->l : 0)) \
   _Pragma ("GCC diagnostic pop")
#endif

void func(void);
void func(void)
{
   struct bb_s bb = { 0 };
   size_t len;

   /* This is the usage I would like to have without warnings */
   len = getLen(&bb);

   assert(len == 0);
}



Have a nice weekend everyone,
Steffen