What pre-processer will do in c language?

Answer 1

The preprocessor (or precompiler) processes all the preprocessor directives and macro definitions within your source files. In other words it creates modified source files where all directives and macros are completely stripped out and replaced by their respective definitions. For instance, each #include statement in your source inserts the named header just as if you'd copy/pasted that entire header into your source. However, prior to insertion, the header itself must be preprocessed, thus it is the modified code that is physically inserted. Once inserted, the #include directive is removed from the modified source.

Since the compiler works with modified source files (intermediate sources), the compiler never sees your macro definitions and therefore cannot help you debug them. A macro is a primitive text replacement system so if this creates invalid code, a compiler error occurs but the compiler cannot tell you where that error originated because it is processing code that does not exist in your source.