[cfe-dev] How clang handle user's include

mats petersson via cfe-dev cfe-dev at lists.llvm.org
Tue Mar 14 05:01:56 PDT 2017

It's not entirely clear what you are asking - the full answer to your
question would probably take a few days to write down with sufficient
precision to explain "everything that happens due to #include "foo.h"
inside clang. So if my explanation below isn't "enough", please provide a
more specific follow-up question - I'm not sure I can give the answer,
because what I've explained is nearly the full extent of my knowledge...

Clang doesn't use a traditional pre-processor (as per the "old school C
compiler", where you'd get a "foo.i" file of the preprocessed input),
instead it parses and understand preprocessor directives in the main
processing of the file. It has code that understands the file-structure of
the overall compilation unit. [Whether it's a "user" or "system" include is
not really important, except for a few cases of "if it's a system include
don't issue warnings for <something that is considered harmless but
sometimes 'wrong' in system headers)],

This comment in include/clang/Lex/Preprocessor.h explains the same thing I
just said:
/// Lexers know only about tokens within a single source file, and don't
/// know anything about preprocessor-level issues like the \#include stack,
/// token expansion, etc.
class Preprocessor : public RefCountedBase<Preprocessor>

The Preprocessor class is used to "help" with the task of dealing with
this. To the "compiler proper", it will seem like all the input came from a
single file.


On 14 March 2017 at 09:17, via cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Hi,
> I'm looking at the clang source code and I'm wandering where clang handle
> the include from users.
> If you have a simple C file foo.c:
> #include <stdio.h>
> #include "foo.h"
> void foo(void) {
>   printf("foo\n");
> }
> int main(void) {
>   foo();
>   return 0;
> }
> and the corresponding foo.h header:
> void foo(void);
> Which part of clang "merge" (or inline) foo.c and foo.h together? If I
> understand well that is what is happening, right?
> Thank you for your answers!
> Cheers,
> Julien
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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