What is the fundamental difference between source and header files in C?

There is no technical difference. The compiler will happily let you include a .c file, or compile a .h file directly, if you want to.

There is, however, a huge cultural difference:

• Declarations (prototypes) go in .h files. The .h file is the interface to whatever is implemented in the corresponding .c file.

• Definitions go in .c files. They implement the interface specified in the .h file.

The difference is that a .h file can (and usually will) be #included into multiple compilation units (.c files). If you define a function in a .h file, it will end up in multiple .o files, and the linker will complain about a multiply defined symbol. That's why definitions should not go in .h files. (Inline functions are the exception.)

If a function is defined in a .c file, and you want to use it from other .c files, a declaration of that function needs to be available in each of those other .c files. That's why you put the declaration in a .h, and #include that in each of them. You could also repeat the declaration in each .c file, but that leads to lots of code duplication and an unmantainable mess.

If a function is defined in a .c file, but you don't want to use it from other .c files, there's no need to declare it in the header. It's essentially an implementation detail of that .c file. In that case, make the function static as well, so it doesn't conflict with identically-named functions in other files.

What should be in headers and what should be in the source files?

Typically headers contain one or more of the following:

• Function declaration (except statics)
• Variable declaration (typically global)
• User defined type declaration (read struct, union etc.)
• Macro definition

Source files on the other hand have:

• Function/variable definition
• Static function declaration and definition (you don't want to expose these to your clients)
• Variable definition
• Some prefer to define inline functions (C99) in a header

How do I implement this separation?

The One Definition Rule is your friend.

Remember, if you are writing a library, this is what your client gets to see. So, be helpful and provide all the information you can for them to use your library. The source files are typically compiled and supplied in binary form.

And by the way, C does not have the concept of classes.