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

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

[Steffen Dettmer]

Hi,

we implement some features either as functions or macros (if
small). One of the used compilers is gcc. With gcc-4.6.0 we get
new warnings.

With macros, we for example have:

  #define fooGetFooPtr(bar) \
     (const uint8*)(((bar) != NULL ? (bar)->foo_ : (const uint8*)bar))

when used as:

  struct bar_s bar;
  ptr *p = fooGetFooPtr(&bar);

leading to:

  error: the comparison will always evaluate as 'true' for the
  address of 'bar' will never be NULL [-Werror=address]

Of course, we considering the preprocessor output this is
obiously a warning and the check can be omitted, but for the
developer it is not because "hidden" inside the macro. The
developer shall not need to care whether the used fooGet*
function is implemented in form of a C function or a CPP macro
(the usage semantics should be equal).

What is recommended to do here?

- Removing the check from the macro changes behavior when called
  with a "free" pointer passed from somewhere else,

- implementing the macro as function would probably help but may
  cause overhead and may have additional effects

- implementing the macro as function for gcc of gcc-4.6.0 only
  seems to be ugly and needlessly conditional

- disabling the warning would (I guess) also affect when checking
  addresses without using macros

How to address this in a good way? What is recommended?

Regards,
Steffen

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

[Jonathan Wakely]

On 9 June 2011 09:15, Steffen Dettmer wrote:
>
> - implementing the macro as function would probably help but may
>  cause overhead and may have additional effects

http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Inline.html

> - implementing the macro as function for gcc of gcc-4.6.0 only
>  seems to be ugly and needlessly conditional
>
> - 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

Re: 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