Re: [TUHS] origin of C header files

Warning: off-topic info The suggestion that I invented macros can also be found on-line, but it's not true. I learned of macros in 1957 or before. GE had added a basic macro capability to an assembler; I don't know whether they invented the idea or borrowed it. In 1959 George Mealy suggested that Bell Labs install a similar facility in SAP (SHARE assembly program). Doug Eastwood wrote the definition part and I handled expansions. Vic Vyssotsky later asked whether a macro could define a macro--a neat idea that was obviously useful. When we went to demonstrate it, we were chagrinned that it didn't work: definition happening during expansion resulted in colliding calls to a low-level string-handling subroutine that was not re-entrant. Once that was fixed, Steve Johnson (as a high-school intern!) observed that it allowed the macro table to serve as an addressable memory, for which the store and load operations were MOP (macro define) and MAC (macro call). Probably before Steve's bright insight, Eastwood had folded the separate macro table into the opcode table, and I had implemented conditional assembly, iteration over a list, and automatic generation of symbols. These features yielded a clean Turing-complete language-extension mechanism. I believe we were the first to achieve this power via macros. However, with GPM, Christopher Strachey showed you don't need conditionals; the ability to generate new macro names is enough. It's conceivable, but unlikely, that this trick could be done with earlier macro mechanisms. As for publication, our macroprocessor inspired my CACM paper, "Macro nstruction extension of compiler languages", but the note that Steve remembers circulated only in the Labs. A nontrivial example of our original macros in action--a Turing machine simulator that ran completely within the assembler--was reproduced in Peter Wegner's programming book, so confusingly described that I am glad not to have been acknowledged as the original author. Doug