hmmm...

void my_function(); //whatever this does

class __call_before_main { 
public:__call_before_main ( void (*func)()) {func();}
};
#define CALL_BEFORE_MAIN(a) static __call_before_main __call_ ## a ( a ) ;
CALL_BEFORE_MAIN(my_function);

is not equivelent to:

void __attribute__((constructor)) my_function() { }

thats 'GCC C' but Im sure thiers methods for other compilers, that I don't use...

And with some nice defines like yours above it can be just as compiler independent. (unless of course your compiler uses a #pragma directive to set a constructor)