"Basically, the compiler has a state which can be modified by these directives. Since every .c file is treated independently, every .c file that is being compiled has its own state. The headers that are included modify that file's state. The pre-processor works at a string level and replaces the tags in the source file by the result of basic functions based on the state of the compiler."

I almost sort of get what the author means here but then I don't really. I mean, there is no 'state' for the compiler that is modified by precompiler directives, so this is probably an analogy or simplification he's making here, but I don't really understand how he gets to the mental image of 'compiler state'. Why not just say it like it is: the preprocessor generates long-assed .i (or whatever) 'files' in memory before the actual compiler compiles them, the content of which can be different between compilation units, because preprocessor preconditions might vary between compilation units?