What does preprocessing exactly mean in compiler

Question

I am trying to understand the difference between a typedef and define. There are a lot of good posts specially at this previous question on SO, however I can\'t understand the p

Answer 1

The C and C++ preprocessors are a logical phase in the compilation that occurs very early on. The preprocessor converts a source file into a translation unit by including the text of header files that are specified by #include, by conditionally eliminating parts of the files (#if, #elif, #else, #endif, and variants #ifdef and #ifndef), and by doing macro substitutions. The macros are defined via #define; the macros can be detected by the preprocessor scanning the source.

The preprocessor eliminates most lines that start with # (it only leaves #line directives behind and its own abbreviated variants on #line behind to tell the compiler where the source came from). The compiler proper then sees the result of the pre-processing and compiles the source code that is defined.

The preprocessor would not normally modify the word typedef. An insane programmer could define a macro for typedef and the preprocessor might not care; the programmer could not legitimately include any system header while there is a macro defined with the same name as any keyword in the language. Otherwise, though, typedef is a problem for the compiler, not the preprocessor. So, too, is sizeof(); that is not something the preprocessor understands.

There are usually compiler options to allow you to see the pre-processed output. For gcc, the options are -E and -P.