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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.std.c
Subject: Re: Interaction between conditional inclusion and source file inclusion.
Date: Sat, 25 Dec 2021 08:52:04 -0800
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:

> This discussion requires familiarity with the standard's specifications
> and terminology from sections 6.10, 6.10.1 and 6.10.2.  Unless you've got
> those sections memorized, you might need to cross-reference them to
> understand what I'm saying.
>
> To simplify the following discussion, I'm going to write it as if the
> only conditional inclusion preprocessing directives were #if, #else, and
> #endif.  Code using the other conditional inclusion directives can always
> be rewritten to use only those three, with essentially the same
> behavior, with a minor exception in the case of #elsif, where the
> subsequent occurrences of __LINE__ would have increased values.  Those
> other directives don't change the issue I'm discussing, they only
> complicate the discussion.
>
> I've long understood that, during translation phase 4, as soon as a
> compiler reaches the new-line at the end of a #if directive, it knows
> whether the #if group will be included.  It not, and there's a
> corresponding #else, it knows that the #else group will be included.
> Either way, as soon as it starts reading a group that will be included,
> it can immediately start preprocessing that group (and this is the
> important part:) while searching for the #else or #endif directive that
> terminates the group.
>
> I've also long understood that the #if, #else (if any) and #endif
> directives that make up an if-section must all occur in the same file.
> I'm not sure how I reached that conclusion - it's not anything that the
> standard says explicitly.  [...]

The first rule of grammar in 6.10 paragraph 1 says (with \sub()
to mean subscript)

    preprocessing-file:
        group \sub(opt)

Thus each preprocessing file must consist of an integral number
of group-part, and so cannot contain any unbalanced #if/#endif
directives, or any #else directive outside an #if/#endif section.

Note to all:  Merry Christmas!