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Subject: Re: Difficulties compiling libc from repository using gcc 10.N.0
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From: Richard Damon <Richard AT Damon-Family DOT org>
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Date: Thu, 10 Dec 2020 15:13:15 -0500
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On 12/10/20 1:55 PM, J.W. Jagersma (jwjagersma AT gmail DOT com) [via
djgpp AT delorie DOT com] wrote:
> This only applies to global variables (and __stdio_cleanup_hook here is a
> function *pointer*), functions themselves still have implicit external linkage.
>
> 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

Yes, The C Standard explicitly states that if two Translation Units
define the same variable, it is undefined behavior.

A Translation Unit is considered to define (vs declare) a variable if it
is mentioned with the extern storage specifier.

Many programs have broken this rule, but gcc (and others) have covered
this issue with the -fcommon being assumed.

-- 
Richard Damon