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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 11 May 2022 11:47:56 -0700
Message-ID: <CAKwvOdmK4oH0t8Q6F19sWKX1fT=AgS=kfvn05FT01HffLJwgMQ@mail.gmail.com>
Subject: Re: [PATCH v4 03/14] modpost: split the section mismatch checks into section-check.c
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nicolas Schier <nicolas@fjasle.eu>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-modules <linux-modules@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        clang-built-linux <llvm@lists.linux.dev>
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On Mon, May 9, 2022 at 11:57 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> > > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > > index a78b75f0eeb0..e7e2c70a98f5 100644
> > > --- a/scripts/mod/modpost.c
> > > +++ b/scripts/mod/modpost.c
> > > @@ -31,7 +31,7 @@ static bool external_module;
> > >  /* Only warn about unresolved symbols */
> > >  static bool warn_unresolved;
> > >
> > > -static int sec_mismatch_count;
> > > +int sec_mismatch_count;
> >
> > ^ this should go in modpost.h if it is to be used in two translation
> > units, rather than forward declaring it in section-check.c.  You did
> > this for the functions.
>
>
> Sorry, I do not understand.
>
>
> In modpost.h, I put the declaration:
>
>   extern int sec_mismatch_count;
>
> If I moved it to the header without 'extern'
> I would get multiple definitions.

Yeah, you need to _declare_ it w/ extern in the header, then _define_
it in one source file.

That way, if the type ever changes, the sources will agree on type in
all source files. You will get a redefinition error if the definition
changes the type of the variable since the last declaration.

What you're doing is forward declaring, which works, and is a common
pattern for (bloated) C++, but is less type safe than sharing a single
common declaration between multiple source files via a single common
shared header. (Sorry I didn't respond before you sent v5)
-- 
Thanks,
~Nick Desaulniers