Re: "extern" inside a block

From: G Patel (gaya.patel_at_gmail.com)
Date: 02/23/05 

Date: 23 Feb 2005 12:35:33 -0800

Eric Sosman wrote:
> G Patel wrote:
> >
> > I've seen things like this too:
> >
> > {
> > extern int foo = 10;
> >
> > /* rest of block */
> > }
>
> If you've seen them and the compiler didn't complain,
> the compiler was being operated in a non-conforming mode:
>
> 6.7.8 Initialization
> /5/ If the declaration of an identifier has block
> scope, and the identifier has external or internal
> linkage, the declaration shall have no initializer
> for the identifier.
>
> If you've seen `extern int foo = 10;' at file scope,
> outside a block, that's fine: it's a definition of `foo'
> with an initializer, specifying (unnecessarily) external
> linkage. But it's not permitted to do this inside a block.
>

You're right, I've only seen extern with initializer at the top of a
file. This is weird, so with an intializer it's a definition, without
an initializer it's a declaraction.

I have one more question. In multi source file programs, I put extern
declaractions for variables I want to share amongst many source files
in a headder, and include that in all those source files. I never used
to include that in the file I defined the file-scope variable. I
tried it after a suggestion and it works. But is this 100% proper (not
undefined)?

Technically, after preprocessing that source file has this at the top:

extern T var; /* came in from #include */

T var; /* was in the .c file */

I know the behaviour when extern declaration follows a non-extern
declaraction is okay, because the extern takes the
linkage/characteristics of the non-extern declaration (either static or
implicity external). But how does this work when extern declaration
PRECEDES a non-extern declaration?

Am I safe to include the header like this?

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