Comment by kazinator
1 day ago
There is no "compiler", "IDE" or "include path" in the wording of the ISO C standard. A set of files is somehow presented to the implementation in a way that is not specified. Needless to say, a file that is included like "globals.h" but is not the base file of a translation unit will not be indicated to the implementation as the base of a translation unit. Nevertheless it has to be somehow present, if it is required.
It doesn't seem as if you're engaging with the standard-based point I've been making, in spite of detailed elaboration.
> Any header files shown there are not treated differently by the compiler/preprocessor to any others.
This is absolutely false. Headers which are part of the implementation, such as standard-defined headers like <stdlib.h> need not be implemented as files. When the implementation processes #include <stdlib.h>, it just has to flip an internal switch which makes certain identifiers appear in their respective scopes as required.
For that reason, if an implementation provides <winkle.h>, there need not be such a file anywhere in its installation.
I only discussed things like include directories and IDEs, which are not part of the standard, because I am trying in good faith to understand how you could have come to your position. There is nothing in the standard like the "set of files is somehow presented to the implementation" (in a sense that includes header files) so I reasoned that maybe you were thinking of something outside the standard.
Instead, the standard says that the include directive:
> searches a sequence of implementation-defined places for a header ... and causes the replacement of that directive by the entire contents of the header.
(Note that it talks about simply substituting in text, not anything more magical, but that's digressing.)
It's careful to say "places" rather than "directories" to avoid the requirement that there's an actual file system, but the idea is the same. You don't pass the implementation every individual file that might need to be included, you pass in the places that hold them and a way to search them with a name.
Maybe you were confused by that part of the standard you quoted in an earlier comment.
One part of that says "The text of the program is kept in units called source files, (or preprocessing files) in this document." But the "source files" aren't really relevant to the include directive – those are the top-level files being compiled (what you've called "base files").
The next sentence you quoted says "A source file together with all the headers and source files included via the preprocessing directive #include is known as a preprocessing translation unit." But "all the headers" here is just referring to files that have been found by the search mechanism referred to above, not some explicit list.
My position doesn't revolve around the mechanics of preprocessing. Say we have somehow given the implementation a translation unit which has #include <winkle.h>. Say we did not give the implementation a file winkle.h; we did not place such a file in any of the places where it searches for include files.
OK, now suppose that the implementation resolves #include <winkle.h> and replaces it with tokens.
The subsequent processing is what my position is concerned with.
My position is that since the standard doesn't define what those tokens are, the behavior is not defined.
In other words, a conforming implementation can respond #include <winkle.h> with any behavior whatsoever.
- It can diagnose it as not being found.
- It can replace it with the token __wipe_current_directory which that same implementation then understands as a compile-time instruction to wipe the current direcctory.
- Or any other possibility at all.
This all has to do with the fact that the header is not required to exist, but may exist, and if it does exit, it may have any contents whatsoever, including non-portable contents which that implementation understands which do weird things.
It is not required to document any of it, but if it does, that constitutes a documented extension.
A conforming implementation can support a program like this:
All that <pascal.h> has to do is regurgitate a token like __pascal_mode. This token is procesed by translation phase 7, which tells the implementation to start parsing Pascal, as an extension.