A question about tenative definitions

[Stanley Friesen]

In article <4218 at lupine.NCD.COM> rfg at lupine.ncd.com (Ron Guilmette) writes:
>I'm having a discussion with a C implementor I know about the meaning of
>the following sentence from the ANSI C standard. 
 
>	    "If a translation unit contains one or more tenative definitions
>	    for an identifier, and the translation unit contains no external
>	    definition for that identifier, then the behavior is exactly as
>	    if the translation unit contains a file scope declaration of that
>	    identifier, with a composite type as of the end of the translation
>	    unit, with an initializer equal to zero."
>
>What exactly does this mean? ...

< Example consisting of two translation units (=files) deleted>

>... if you compiled them both and then tried to link them together into
>the same single program, that you should get an error at link time because
>there are two conflicting external *definitions* of the variable xxyyzz.
>
>This implementor I know disagrees with my interpretation.
>
>Who is right? 

In essence you are right.  The example you gave generates what ANSI calls
'undefined behavior'.  This means that it is *not* a conforming C program.

It also, generally, means that *any* behavior on the part of the system is
acceptable.  So, this *may* mean that the implementor's approach is a legal
local *extension* to ANSI C.  Of course any programs that depend on such an
extension will not be portable.  [It is a legal extension as long as the
ANSI standard does not *require* a diagnostic for this error; I do not
believe it does, so the extension is likely to be legal].

The whole matter is discussed in some detail in the rationale document.
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uunet!tdatirv!sarima				(Stanley Friesen)