By using pre-processor directives you can exclude parts of your code from being seen by the compiler. Excluded from the assembly they are never seen at run-time. This is different from a regular if (x) {} block where code is actually compiled in, evaluated at runtime and added to the assembly.

To understand why you would want to do this, a little history: One of the good things about C is that it works on every imaginable platform, the bad thing was of course that it works on every imaginable platform. There was always a little tweaking required to get your code to compile. Your program might have needed to compile on Amiga, DOS , OS/2, Windows or Linux. The invention of the preprocessor made this much easier. As a separate step prior to compilation it combs through the source code and modifies it according to the pre-processing instructions found in the source code. The C compiler never gets to see the things that don’t apply to it.

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