I’d argue that’s not true. That’s what the extern keyword is for. If you do #include , you don’t get the actual printf function defined by the preprocessor. You just get an extern declaration (though extern is optional for function signatures). The preprocessed source code that is fed to cc is still not complete, and cannot be used until it is linked to an object file that defines printf. So really, the unnamed “C preprocessor output language” can access functions or values from elsewhere.