Well, not spcifically an 8052 problem.

The basic concept is, I think, pretty universal:

You need a "Public" definition for each symbol to "create" the "object";

You need "External" references to gain access to the "Public" symbols from other files.


Of course, the detail of precisely how to do this in a particular language and/or tool will vary...


In other words, it's time to open up the language text book, and/or tool Manuals...