clanguage-lawyerc11standards-compliancec17

Can extension cancel the existing standard requirements?

Follow-up question for Why do conforming implementations behave differently w.r.t. incomplete array types with internal linkage?.

Context: in both gcc and clang (conforming implementations) by default the requirement C11,6.9.2p3 [1] is cancelled, which is positioned as an extension.

Question: can an extension cancel the existing standard requirements while keeping the implementation conforming?

[1] C11, 6.9.2 External object definitions, 3:

If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.

UPD. Yes. In other words: the standard says: "we do not support this, the diagnostics is required". The extension says: "we do support this (hence, the standard required diagnostics is irrelevant)".

Solution

  • It's not so much that an implementation "cancels" a requirement with an extension, but that extensions add features that the standard doesn't otherwise support. The main requirement is that extensions doesn't alter any strictly conforming programs.

    The definition of a conforming implementation is as follows from section 4p6 of the C11 standard:

    The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers [ ... omitted for brevity ... ]. A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program

    Where a strictly conforming program is defined in section 4p5:

    A strictly conforming program shall use only those features of the language and library specified in this International Standard. It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit

    And a conforming program is defined in section 4p7:

    A conforming program is one that is acceptable to a conforming implementation.

    So given the case of the program from your prior question:

    static int arr[ ];

int main( void )
{
        return arr[ 0 ];
}

static int arr[ ] = { 0 };

    This is not a strictly conforming program because it violates 6.9.2p3. However some implementations such as gcc allows this as an extension. Supporting such a feature doesn't prevent a similar strictly conforming program such as this

    static int arr[1];

int main( void )
{
        return arr[ 0 ];
}

static int arr[ ] = { 0 };

    From behaving any differently. Therefore an implementation supporting this feature still qualifies as a conforming implementation. This also means that the first program, while not a strictly conforming program, is a conforming program because it will run in a well defined manner on a conforming implementation.