C99 Macro Expansion for Struct Member Access

Is it possible to do a nullity check and an access in a macro?

Eg:

#define LOG(mystruct, severity, format, ...) ({ \
  severity_t current = ERROR; \
  if (mystruct) { \
    current = mystruct->error_level; \
  } \
  if (severity >= current) { \
   ... //handle logging 
  } \
})

If I call this with LOG(NULL, DEBUG, "test %s", "one"); I get an error as such:

error: member reference base type 'void' is not a structure or union note: expanded from macro 'LOG' current = mystruct->error_level;

mystruct is defined as:

typedef struct mystruct_t {
  severity_t error_level;
}

I want to allow the possibility of working with a NULL mystruct. Eg: case of when there is an error creating the structure itself.

Solution

Your problem is that although the first branch will never been taken, NULL hasn't the correct type to do a ->error_level.

You can avoid that by giving it the right type. I would do that with a local variable, not a cast so you'd capture wrong use cases of your macro. Just add

yourType* myStr = mystruct;
current = myStr->error_level;

and you should be fine.