The C language is too complicated and too flexible to allow that. If you are starting from scratch and creating a new language, this could be a design goal from the beginning.
> The C language is too complicated and too flexible to allow that.
I disagree. In fact, I would expect the following could be a pretty reasonable exercise in a book like "Software Tools"[1]: "Write a program to extract all the function declarations from a C header file that does not contain any macro-preprocessor directives." This requires writing a full C lexer; a parser for function declarations (but for function and struct bodies you can do simple brace-matching); and nothing else. To make this tool useful in production, you must either write a full C preprocessor, or else use a pipeline to compose your tool with `cpp` or `gcc -E`. Which is the better choice?
However, I do see that the actual "Software Tools" book doesn't get as advanced as lexing/parsing; it goes only as far as the tools we today call `grep` and `sed`.
I certainly agree that doing the same for C++ would require a full-blown compiler, because of context-dependent constructs like `decltype(arbitrary-expression)::x < y > (z)`; but there's nothing like that in K&R-era C, or even in C89.
No, I think the only reason such a declaration-extracting tool wasn't disseminated widely at the time (say, the mid-to-late 1970s) is that the cost-benefit ratio wouldn't have been seen as very rewarding. It would automate only half the task of writing a header file: the other and more difficult half is writing the accompanying code comments, which cannot be automated. Also, programmers of that era might be more likely to start with the header file (the interface and documentation), and proceed to the implementation only afterward.
> Write a program to extract all the function declarations from a C header file that does not contain any macro-preprocessor directives
There you go. You just threw away the most difficult part of the problem: the macros. Even a medium-sized C library can have maybe 500 lines of dense macros with ifdef/endif/define which depends on the platform, the CPU architecture, as well as user-configurable options at ./configure time. Should you evaluate the macro ifdefs or preserve them when you extract the header? It depends on each macro!
And your tool would still be highly incomplete because it only handles function declarations not struct definitions, typedefs you expect the users to use.
> the other and more difficult half is writing the accompanying code comments, which cannot be automated
Again disagree. Newer languages have taught us that it is valuable to have two syntaxes for comments, one intended for implementation and one intended for the interface. It’s more popularly known as docstrings but you can just reuse the comment syntax and differentiate between // and /// comments for example. The hypothetical extractor tool will work no differently from a documentation extractor tool.
I interpreted OP's post to say that you take a C file after the preprocessor has translated it. How you perform that preprocessing can simply be by passing the file to an existing C preprocessor, or you can implement it as well.
Implementing a C preprocessor is tedious work, but it's nothing remotely complex in terms of challenging data structures, algorithms, or requiring sophisticated architecture. It's basically just ensuring your preprocessor implements all of the rules, each of which is pretty simple.
And you had the same misunderstanding as OP. Because you have eliminated all macros during the preprocessor step, you can no longer have macro-based APIs, including function-like macros you expect library users to use, #ifdef blocks where you expect user code to either #define or #undef, and a primitive form of maintaining API compatibility but not ABI compatibility for many things.
It’s a cute learning project for a student of computer science for sure. It’s not remotely a useful software engineering tool.
Our points of view are probably not too far off, really. Remember this whole thought-experiment is counterfactual: we're imagining what "automatic extraction of function declarations from a .c file" would have looked like in the K&R era, in response to claims (from 50 years later) that "No sane programming language should require a duplication in order to export something" and "The .h could have been a compiler output." So we're both having to imagine the motivations and design requirements of a hypothetical programmer from the 1970s or 1980s.
I agree that the tool I sketched wouldn't let your .h file contain macros, nor C99 inline functions, nor is it clear how it would distinguish between structs whose definition must be "exported" (like sockaddr_t) and structs where a declaration suffices (like FILE). But:
- Does our hypothetical programmer care about those limitations? Maybe he doesn't write libraries that depend on exporting macros. He (counterfactually) wants this tool; maybe that indicates that his preferences and priorities are different from his contemporaries'.
- C++20 Modules also do not let you export macros. The "toy" tool we can build with 1970s technology happens to be the same in this respect as the C++20 tool we're emulating! A modern programmer might indeed say "That's not a useful software engineering tool, because macros" — but I presume they'd say the exact same thing about C++20 Modules. (And I wouldn't even disagree! I'm just saying that that particular objection does not distinguish this hypothetical 1970s .h-file-generator from the modern C++20 Modules facility.)
[EDIT: Or to put it better, maybe: Someone-not-you might say, "I love Modules! Why couldn't we have had it in the 1970s, by auto-generating .h files?" And my answer is, we could have. (Yes it couldn't have handled macros, but then neither can C++20 Modules.) So why didn't we get it in the 1970s? Not because it would have been physically difficult at all, but rather — I speculate — because for cultural reasons it wasn't wanted.]
I disagree. In fact, I would expect the following could be a pretty reasonable exercise in a book like "Software Tools"[1]: "Write a program to extract all the function declarations from a C header file that does not contain any macro-preprocessor directives." This requires writing a full C lexer; a parser for function declarations (but for function and struct bodies you can do simple brace-matching); and nothing else. To make this tool useful in production, you must either write a full C preprocessor, or else use a pipeline to compose your tool with `cpp` or `gcc -E`. Which is the better choice?
However, I do see that the actual "Software Tools" book doesn't get as advanced as lexing/parsing; it goes only as far as the tools we today call `grep` and `sed`.
I certainly agree that doing the same for C++ would require a full-blown compiler, because of context-dependent constructs like `decltype(arbitrary-expression)::x < y > (z)`; but there's nothing like that in K&R-era C, or even in C89.
No, I think the only reason such a declaration-extracting tool wasn't disseminated widely at the time (say, the mid-to-late 1970s) is that the cost-benefit ratio wouldn't have been seen as very rewarding. It would automate only half the task of writing a header file: the other and more difficult half is writing the accompanying code comments, which cannot be automated. Also, programmers of that era might be more likely to start with the header file (the interface and documentation), and proceed to the implementation only afterward.
[1] - K&P's "Software Tools" was originally published in 1976, with exercises in Ratfor. "Software Tools in Pascal" (1981) is here: https://archive.org/details/softwaretoolsinp00kern/