A compilation unit is essentially a single source file and its header file. That source file may be a complete program or it may be just one among several or many source files that make up a final executable. Each source file is preprocessed and compiled individually in the compilation phase. The result of this is an intermediate object file. An object file knows about external functions and variables via header declarations but defers the resolution of their actual addresses until later.
When all source files have been successfully compiled into object files, the link phase is entered. In the link phase, the addresses of functions in other files or libraries are resolved and the addresses of external global variables are resolved. When all unresolved addresses have been successfully resolved (linked together), the object files are then combined into a single executable.
In the dealer.c program, there were four source files. Each...
