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Date: Wed, 02 Jan 2002 23:19:29 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: Momchil Velikov <velco@fadata.bg>
CC: Oliver Xymoron <oxymoron@waste.org>,
        vda <vda@port.imtp.ilyichevsk.odessa.ua>, linux-kernel@vger.kernel.org,
        "H . J . Lu" <hjl@lucon.org>
Subject: Re: Extern variables in *.c files
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Momchil Velikov wrote:
> 
> >>>>> "Andrew" == Andrew Morton <akpm@zip.com.au> writes:
> 
> Andrew> Oliver Xymoron wrote:
> >>
> >> On Wed, 2 Jan 2002, vda wrote:
> >>
> >> > I grepped kernel *.c (not *.h!) files for extern variable definitions.
> >> > Much to my surprize, I found ~1500 such defs.
> >> >
> >> > Isn't that bad C code style? What will happen if/when type of variable gets
> >> > changed? (int->long).
> >>
> >> Yes; Int->long won't change anything on 32-bit machines and will break
> >> silently on 64-bit ones. The trick is finding appropriate places to put
> >> such definitions so that all the things that need them can include them
> >> without circular dependencies.
> >>
> 
> Andrew> Isn't there some way to get the linker to detect the differing
> Andrew> sizes?
> `--warn-common'
>      Warn when a common symbol is combined with another common symbol
>      or with a symbol definition.  Unix linkers allow this somewhat
>      sloppy practice, but linkers on some other operating systems do
>      not.  This option allows you to find potential problems from
>      combining global symbols.  Unfortunately, some C libraries use
>      this practice, so you may get some warnings about symbols in the
>      libraries as well as in your programs.
> 

Alas, it doesn't quite work as we'd like:

akpm-1:/home/akpm> cat a.c
int a;

main()
{}
akpm-1:/home/akpm> cat b.c
extern char a;

int b()
{return a;}
akpm-1:/home/akpm> gcc -fno-common -Wl,--warn-common a.c b.c
akpm-1:/home/akpm> 

No warnings emitted.  If b.c doesn't use `extern' it will fail
to link because of -fno-common.

What we'd *like* to happen is for the linker to determine that
the `extern char' and the `int' declarations are a mismatch.
Maybe the object file doesn't have the size info for `extern'
variables.   The compiler emits it though.

We can actually get what we want by disabling `-fno-common' and enabling
`--warn-common', but that's rather awkward. 

Perhaps HJ can suggest a solution?

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