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In general terms the compilation of Objective-C programs from source code to executable binary is a three phase process. In the first phase, a tool called the ''preprocessor'' scans the human written source code and converts it to compiler friendly content and format. In the second phase the compiler generates object code (usually in the form of file with a .o filename extension) from the preprocessed source code. Finally, the linker brings all the object code modules and libraries together, resolves symbol references and creates the executable binary.

As you develop larger and more complex applications you will, of course, be writing and importing many of your own header files. Many of these will contain the preprocessor directives we have covered in this chapter.

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