How does the following "assert_disabled()" macro work?

Question

I see this macro appearing in many places in a code base to find if a particular field is disabled or not (0 or 1).

#define assert_disabled(e)      ((void)sizeof(e))

How does sizeof help here in finding if a field is 0 or 1?

Can someone explain this with a working example?

Answer 1

I'm pretty sure this macro is just being used when assertions are turned off. The trick of using ((void)sizeof(e)) instead of just (void)0 or similar is clever: it avoids evaluating e (mostly), but still has the compiler check that e is a valid expression, so you won't get surprise compile errors when you change the definition to turn assertions on.

Answer 2

This is kind of blackmagic I saw in Linux Kernel codes.

Its used to check expression 'e' at compile time.