A builtin is definitely better for the efficiency alone. Vanilla chibicc needs 5GB to preprocess the example.c from map-macro, in comparison to compiling an amalgamation of itself only takes 50 MB. It of course wasn't optimized for memory use but it's a peak into how much token expansions are involved in the wizardry.
But that repo says it's effortful for the preprocessor and you can see in the code it's a recursion limited but what's in the file. If you had, maybe, a few thousand register definitions in an embedded system, that won't do it.
Doing non-trivial preprocessing is a mistake. It's probably a good idea to pretend that the only preprocessing directive is #include. With constexpr, even #define is obsolescent.
Doing non-trivial anything you slow down compilation, make debugging harder and increase fragility and complexity. Sometimes that's a mistake, sometimes not.
Getting rid of something because someone sometimes abuses it, is short-sighted.
I am not sure. I agree one should minimize macros in C, but for some things a small macro layer still beats the complexity of C++ and is not inferior in my opinion.
C++ templates are far cleaner, easier to debug, and produce more comprehensible errors than C macros. And C++ templates are rightly famous for being terrible at all of the above.
Hey, I agree about your second sentence but I do not really see the first. Alone that I can directly inspect the result of the preprocessor makes macros easier to debug than templates in my experience.
> I can directly inspect the result of the preprocessor makes macros easier to debug
Tangential note but: why does this require a separate re-invocation of the compiler with the '-E' flag? Say I have this silly example with a comma instead of the semicolon inside W() macro:
#define W(a, b) b, int a
#define X(a, b) W(b, a)
int kekw(int x) {
int z = X(23, k) = x;
return z + k;
}
When I shove it into GCC without -E, it says
<source>: In function 'int kekw(int)':
<source>:1:21: error: expected unqualified-id before 'int'
1 | #define W(a, b) b, int a
| ^~~
<source>:2:18: note: in expansion of macro 'W'
2 | #define X(a, b) W(b, a)
| ^
<source>:5:13: note: in expansion of macro 'X'
5 | int z = X(23, k) = x;
| ^
<source>:6:16: error: 'k' was not declared in this scope
6 | return z + k;
|
instead of, you know, showing me the expanded text so that I could see what's actually wrong with the generated variable declarations? Something like
<source>: In function 'int kekw(int)':
<source>:1:21: error: expected unqualified-id before 'int'
1 | #define W(a, b) b, int a
| ^~~
<source>:2:18: note: in expansion of macro 'W'
2 | #define X(a, b) W(b, a)
| ^
<source>:5:13: note: in expansion of macro 'X'
5 | int z = X(23, k) = x;
| ^
<source>:5:-: note: the expanded source text is
| int z = 23, int k = x;
| ^~~
<source>:6:16: error: 'k' was not declared in this scope
6 | return z + k;
|
perhaps? I understand that tracking the lines/columns inside the expanded text is highly non-trivial (although collapsing them all onto the position of the top-level macro is already just as non-trivial), but still. Show me what the source that you actually can't compile, not the three-steps removed precursor to it — I sure as hell can't expand macros in my head.
Also, what is this first error message about in the first place? Why the hell does it expect unqualified-id before "int"? If you put "int" after the unqualified-id, it still would be illegal; and an unqualified-id is not the only thing that can follow the comma in this context anyway.
During translation of source text to a C program, the source text goes through several phases. It turns raw bytes into preprocessor tokens, then preprocessor tokens get expanded into more preprocessor tokens and then preprocessor tokens get translated into C tokens, which are fed into the C parser. There is no “expanded text”, only expanded tokens. Generally this is implemented as a state machine that produces tokens on demand, so there is no expanded text. When GCC shows you that error message, it uses the recorded source location and looks it up in the original source file. This is trivial to do. Expanding just one line of the source file would either require re-invoking the preprocessor from the beginning or storing already consumed preprocessor tokens.
Obviously this is possible, but no one has gotten around to implementing this feature and it’s not clear you would want either solution: buffering preprocessor tokens in case you need to expand a line for an error message greatly increases memory usage and the vast majority of compilations are successful ones with no errors, slowing down the normal case. And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.
> There is no “expanded text”, only expanded tokens.
If the compiler supports "-E", there absolutely is some equivalent of the expanded text, with (somewhat limited) whitespace info available: if you insert whitespace into between "b" and the comma in "b,", then "-E" invocation will dutifully show it. Yes, multiple spaces are collapsed into one, but their presence/absence is tracked already.
> Expanding just one line of the source file would either require re-invoking the preprocessor from the beginning or storing already consumed preprocessor tokens.
Yeah, keep the whole expansion of the latest top-level (not nested) macro. I believe some bytecode-like compression scheme (a replay of that state machine you mentioned) à la LZ would work fairly well, with minor additional memory necessary. Hell, scratch that: since #define/#undef are impossible inside a macro, you don't even need to remember the expansion of the macro, you simply need to keep the current macro definitions and remember what was the last macro you tried to expand — if your error happened inside its expansion, just re-expand again. Zero additional memory needed, since this information is already stored for the current way of error reporting — you can see in the current output that it remembers what macros it expanded and where, the whole path down to the offending token: at 1:21 "in expansion of macro 'W'" which was at 2:18 "in expansion of macro 'X'" which was at 5:13.
> And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.
Eh, it's on the whole faster than making the programmer re-run with "-E" option manually.
> If the compiler supports "-E", there absolutely is some equivalent of the expanded text, with (somewhat limited) whitespace info available: if you insert whitespace into between "b" and the comma in "b,", then "-E" invocation will dutifully show it. Yes, multiple spaces are collapsed into one, but their presence/absence is tracked already.
Whitespace is significant in the preprocessor so of course it tracks whitespace. Whitespace isn’t dropped until phase 7 when pp-tokens are converted to C tokens.
As I said it’s not impossible, write the patch and see if GCC accepts it.
In fact, this should be doable. The answer why something is not done in a C compiler is usually: Nobody did substantial work on on it in the last two decades.
What you're saying is like saying "C can be expanded to assembly, and that makes it easier to debug". If you need to do that, the entire process has failed.
Yes and no. If you want to make something like a generic dynamic array, you can do it with type erasure (`void*`), but then your code is no longer typesafe. I would find that more fragile. The preprocessor allows you to add constructs that make it behave like any other modern language.
As an aside, clangd can dereference a macro in place to make debugging easy.
Has anyone tried to extend the C Preprocessor to be a fully turing complete language (perhaps something like Snobol) to allow for Terra like meta-programming?
(I am not talking about metalang99 or Cello like stuff, which exploits the Turing tarpit of the currently specified preprocessor but rather tasteful additions to the preprocessor itself).
It's always bothered me that programming languages aren't programmable.
Well, I keep hearing that Lisp is, but I haven't put much effort into it. (I'd need a Lisp that runs in the browser, for starters!) Maybe I should take another look...
Or maybe I wish the VM was programmable. "Warn me when this type of operation takes place", or "Print the name of every function before it is invoked", etc. I end up just copy-pasting my own temporary statements all over the program.
You can play with it in the proposal author's ellipsis[2] and my slimcc[3], it's simple enough to implement (only about 30 lines of C) so I kept it in despite not seeing signs of it getting accepted.
Right, you can work around it with things like this, but it means you’re forcing the C compiler to parse a compound literal just to get its length, 5 times! when the preprocessor already knows the length of `__VA_ARGS__`. Such contortions when you want a little bit of info that the compiler already has are what cause your compiler times to blow up if you try to do anything sophisticated with the cpp.
41 comments
[ 0.29 ms ] story [ 35.2 ms ] threadOne of those is the two-stage CAT, another one is the NARG the article talks about.
__COUNTER__ is going to be added to C29, the next version of the standard, maybe there's hope for __VA_COUNT__ and __CAT__ too!
Getting rid of something because someone sometimes abuses it, is short-sighted.
Most people describe C++ in those terms.
Tangential note but: why does this require a separate re-invocation of the compiler with the '-E' flag? Say I have this silly example with a comma instead of the semicolon inside W() macro:
When I shove it into GCC without -E, it says instead of, you know, showing me the expanded text so that I could see what's actually wrong with the generated variable declarations? Something like perhaps? I understand that tracking the lines/columns inside the expanded text is highly non-trivial (although collapsing them all onto the position of the top-level macro is already just as non-trivial), but still. Show me what the source that you actually can't compile, not the three-steps removed precursor to it — I sure as hell can't expand macros in my head.Also, what is this first error message about in the first place? Why the hell does it expect unqualified-id before "int"? If you put "int" after the unqualified-id, it still would be illegal; and an unqualified-id is not the only thing that can follow the comma in this context anyway.
Obviously this is possible, but no one has gotten around to implementing this feature and it’s not clear you would want either solution: buffering preprocessor tokens in case you need to expand a line for an error message greatly increases memory usage and the vast majority of compilations are successful ones with no errors, slowing down the normal case. And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.
If the compiler supports "-E", there absolutely is some equivalent of the expanded text, with (somewhat limited) whitespace info available: if you insert whitespace into between "b" and the comma in "b,", then "-E" invocation will dutifully show it. Yes, multiple spaces are collapsed into one, but their presence/absence is tracked already.
> Expanding just one line of the source file would either require re-invoking the preprocessor from the beginning or storing already consumed preprocessor tokens.
Yeah, keep the whole expansion of the latest top-level (not nested) macro. I believe some bytecode-like compression scheme (a replay of that state machine you mentioned) à la LZ would work fairly well, with minor additional memory necessary. Hell, scratch that: since #define/#undef are impossible inside a macro, you don't even need to remember the expansion of the macro, you simply need to keep the current macro definitions and remember what was the last macro you tried to expand — if your error happened inside its expansion, just re-expand again. Zero additional memory needed, since this information is already stored for the current way of error reporting — you can see in the current output that it remembers what macros it expanded and where, the whole path down to the offending token: at 1:21 "in expansion of macro 'W'" which was at 2:18 "in expansion of macro 'X'" which was at 5:13.
> And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.
Eh, it's on the whole faster than making the programmer re-run with "-E" option manually.
Whitespace is significant in the preprocessor so of course it tracks whitespace. Whitespace isn’t dropped until phase 7 when pp-tokens are converted to C tokens.
As I said it’s not impossible, write the patch and see if GCC accepts it.
As an aside, clangd can dereference a macro in place to make debugging easy.
(I am not talking about metalang99 or Cello like stuff, which exploits the Turing tarpit of the currently specified preprocessor but rather tasteful additions to the preprocessor itself).
Well, I keep hearing that Lisp is, but I haven't put much effort into it. (I'd need a Lisp that runs in the browser, for starters!) Maybe I should take another look...
Or maybe I wish the VM was programmable. "Warn me when this type of operation takes place", or "Print the name of every function before it is invoked", etc. I end up just copy-pasting my own temporary statements all over the program.
We would be in a very different world if Netscape’s exec team had let Brendan Eich just use scheme as he had planned to.
> #define T(x, ...) printf(#x " = %d\n", x); __VA_TAIL__()
> T(1, 2, 3, 4, x, y, z)
You can play with it in the proposal author's ellipsis[2] and my slimcc[3], it's simple enough to implement (only about 30 lines of C) so I kept it in despite not seeing signs of it getting accepted.
[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3307.htm
[2] https://codeberg.org/gustedt/ellipsis
[3] https://github.com/fuhsnn/slimcc
Your version expands `sum(1, 2, 3, 4, 5, 6)` to:
whereas with __VA_COUNT__ it is just: