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

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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.

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