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Date: Thu, 10 Dec 2020 21:45:52 +0200
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From: "Eli Zaretskii (eliz AT gnu DOT org) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
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Subject: Re: Difficulties compiling libc from repository using gcc 10.N.0
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> From: "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
> Date: Thu, 10 Dec 2020 19:55:11 +0100
> 
> >> -void (*__stdio_cleanup_hook)(void);
> >> +extern void (*__stdio_cleanup_hook)(void); /* stdio/stdiohk.c */
> > 
> > This part I don't understand: are you saying that it is no longer the
> > default that a function not explicitly declared 'static' has the
> > external linkage?  Why do we need an explicit 'extern' qualifier?
> > 
> > Thanks.
> 
> This only applies to global variables (and __stdio_cleanup_hook here is a
> function *pointer*), functions themselves still have implicit external linkage.

And global variables don't?

> I'm not a C language lawyer but this page suggests that it was always an error
> to omit 'extern' on global variable declarations.  It just wasn't reported as
> such, until now:
> 
> https://gcc.gnu.org/gcc-10/porting_to.html

That's got to break a lot of code, but OK.