I recall a prior extensive discussion about 'cpp'. I looked, and found it (March 30, 2017) but it was a private discussion, not on TUHS (although you were part of it :-). Here are clips of what I wrote (I don't want to re-post what others wrote) from what I wrote, which tell most of the story: There were a series of changes to C before V7 came out, resulting in the so-called 'phototypsetter C compiler' (previously discussed on TUHS), and they included the preprocessor. There's that series of short notes describing changes to C (and the compiler), and they include mention of the preprocessor. [Available here: http://gunkies.org/wiki/Typesetter_C for those who want to see them.] The MIT 'V6' Unix (which was, AFAICT, an augmented version of an early version of PWB Unix) had that C compiler; and if you look at the PWB1 tree online, it does have the C with 'cpp': http://minnie.tuhs.org/cgi-bin/utree.pl?file=PWB1/sys/c/c I did a diff of that 'cpp' with the MIT one, and they are basically identical. ---- I went looking for the C manual in the V6 distro, to see if it mentioned the pre-processor. And it does: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/doc/c/c5 (Section 12, "Compiler control lines", about half way down.) So, I'm like, 'WTF? I just looked at cc.c and no mention of cpp!' So I looked a little harder, and if you look at the cc.c in the distro (URL above), you see this: insym(&defloc, "define"); insym(&incloc, "include"); insym(&eifloc, "endif"); insym(&ifdloc, "ifdef"); insym(&ifnloc, "ifndef"); insym(&unxloc, "unix"); The pre-processor is integrated into 'cc' in the initial V6. So we do have a very early version of it, after all... ---- So, 'cc' in V5 also included pre-processor support (just #define and #include, though): http://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s1/cc.c Although we don't have the source to 'cc' to show it, V4 also appears to have had it, per the man page: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V4/man/man1/cc.1 "If the -p flag is used, only the macro prepass is run on all files whose name ends in .c"; and the V4 system source: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V4/nsys also has .h files. No sign of it in the man page for cc.1 in V3, though. This all makes sense. .h files aren't any use with[out] #include, and without #include, you have to have the structure definition, etc in multiple source files. So #include would have gotten added very early on. In V3, the system was apparently still in assembler, so no need. ----- Also, there's an error in: https://ewe2.ninja/computers/cno/ when it says "V6 was a very different beast for programming to V7. No c preprocessor. The practical upshot of this is no #includes." that's clearly incorrect (see above). Also, if you look at Lions (which is pure early V6), in the source section, all the .c files have #include's. Noel