Sorry, I was referring to the point raised by Peter; eg,

main.h:

#ifdef MAIN 
#define U8 unsigned char 
: 
#else 
#define U8 extern unsigned char 
:
#endif
:

main.c:

#define MAIN 
#include "main.h"

U8 main_function( U8 param ) // No problem - "U8" is "unsigned char"
{ 
   U8 local_temp = 1; 
   return param + local_temp; 
} 

other.c:

#include "main.h"

U8 main_function( U8 param ) // Problem!  "U8" is "extern unsigned char" 
{ 
   U8 local_temp = 1; 
   return param + local_temp; 
}