What is a Module anyway?

A C module normally consists of three separate files:

• A header file, normally having the extension .h . This provides a specification of the external interface to the module. That is, it specifies exactly how other modules can invoke or use the facilities of this module.

• A source file, normally having the extension .c . This is the actual C source code, or definition, of the module. It specifies the internal implementation of the module.

• An object file, normally having the extension .o . This is the file produced by using the C compiler to translate the C source code in the source file into the specific, binary, instruction codes (so-called "machine code") which can be directly processed by the CPU in the computer on which programs will be executed.

A user of a module, who wishes to use its facilities in her programs, needs access to the header file and the object file. The header file is necessary so that, while the compiler is translating your source code, it can check that it is interfacing in a correct and valid manner with the external module. The object file is necessary so that a complete executable file can be created by combining or linking together the object code from your source file, with the object code of the external module.

The user of a module does not require access to the source code of that module. The source code is only required if someone wants to modify the module in some way - correct some defect, or add new facilities etc.