I don't have reference for that but I always though that CPP directives are preceded by # because the character was previously used as comment delimiter by some

I don't have reference for that but I always though that CPP directives are preceded by # because the character was previously used as comment delimiter by something (probably csh). In the grand scheme of things cpp is somewhat modern invention. IIRC the first book on Unix programming I had read mentioned in passing that cpp exists but the C code examples in there did not #include anything and if some declaration was required it was just written verbatim in the example code.