I’ve been using C for a little over a year now, and I was wondering exactly how #define works.
I know you can use it as a macro, eg. #define MUL(x, y) (x*y), but what’s happening when you define something without giving a definition?
e.g.

    #ifndef _SYNNOVESLIB_H
    #define _SYNNOVESLIB_H
    #endif

Does it just get marked as “defined”, or true (as in 1) by GCC or whatever compiler is being used?