Rapidly wandering off the topic...

I agree with Franc: have a header file with your extern declarations and #include it where needed and in the same file as the definitions:

main.h:

extern char MainChar;

main.c:

#include "main.h" // Declaration

char MainChar; // Definition - Compiler will warn if it doesn't match the Declaration!

This is all perfectly legal, and has caught several errors in my current Client's Project where the Definition & Declaration have got out of step!

I think I've mentioned it here before - search for "tentative definition" (or see K&R)
Or it might've been on the Keil site...

Anyway, back at the Original Question, do you have any reason to believe that the use of #define in preference to typedef produces any better-optimised results,