There are three (relevant) phases (see “translation phases” in section 5 of the standard): • program is lexed into preprocessing tokens; comments turn into whi

There are three (relevant) phases (see “translation phases” in section 5 of the standard): • program is lexed into preprocessing tokens; comments turn into whitespace • preprocessor does its thing • preprocessor tokens are turned into proper tokens; different kinds of number are disambiguated; keywords and identifiers are disambiguated If you put an unclosed comment inside #if 0 then it won’t work as you might expect.