Preprocessor Directives
& Separate Compilation

#includes

• Only #include the header files at the top of a .c file that are needed by the code in that file.
• Since the header file you have created for your modular functions, (Let's call it foo.h) needs to be seen before the function definitions (found in foo.c) and also any calls to those functions (found in proj?.c), the header file foo.h needs to be #included at the top of both the foo.c and the proj?.c files.
• Don't #include .c files.
  .c files are to be compiled separately from one another and then linked together at compile time.
• Don't have #includes of any files within header files
  Since header files contain prototypes and never have any function calls in them, it isn't necessary to have any #include directives in a header file.

#defines

• If a constant is used in only one .c file, then its #define should be placed at the top of that file.
• If a constant is used by 2 .c files, then you should place the #define for that constant in the header file that gets #included in both of those .c files.