Re: [PATCH] describe: fix --no-exact-match

From: René Scharfe <l.s.r@web.de>
Date: Aug 12, 2023, 05:11 UTC
Message-ID: <6e05309d-c2be-c949-ae83-c4dd3247e790@web.de>
In-Reply-To: <20230811182415.GC2816191@coredump.intra.peff.net>

Am 11.08.23 um 20:24 schrieb Jeff King:

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> On Fri, Aug 11, 2023 at 07:59:12PM +0200, René Scharfe wrote:
>
>>> we are defining an inline function with the explicit goal of passing it
>>> as a function pointer. I don't remember all of the standard's rules
>>> here. Are we guaranteed that it will create a linkable version if
>>> necessary?
>>
>> I don't see on which basis the compiler could refuse.  We can't expect
>> the function address to be the same across compilation units, but we
>> don't need that.  If there's a compiler that won't do it or a standards
>> verse that makes this dubious then I'd like to know very much.
>
> I seem to recall some quirks where an inline function that is not called
> directly is not required to be compiled at all, and the compiler can
> assume that there is a definition available in another translation unit.

C99 says in 6.7.4 Function specifiers: "It is unspecified whether a call to the function uses the inline definition or the external definition.", referring to functions with both types of definition. So a compiler could ignore the inline version for those.

> But I think that only applies when no storage-class specifier is
> provided. In this case, you said "static", so I think it's OK?

That's how I understand it as well -- there is no external version to choose and the compiler is not free to ignore the inline one.

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> It's possible I'm mis-remembering the issues, too. One problem is that
> pre-C99, you might end up with the opposite problem (a compiled function
> with visible linkage that conflicts with other translation units at link
> time). E.g. here:
>
>   https://stackoverflow.com/questions/51533082/clarification-over-internal-linkage-of-inline-functions-in-c/51533367#51533367
>
> But I think with "static" we should always be OK.

*nod*

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> Don't get me wrong, I like type checking. It's just that doing weird
> things with the language and pre-processor also carries a cost,
> especially in an open source project where new folks may show up and say
> "what the hell is this macro doing?". That's a friction for new
> developers, even if they're comfortable with idiomatic C.

Sure, but that ship has sailed in this specific case. Standard option parsing would use getopt or getopt_long, neither of which has void pointers. We already carry the cost of our OPT_ macros. Let's ease it.

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> That said...
>
>> A good example in parseopt: The patch below adds type checking to the
>> int options and yields 79 warning about incompatible pointers, because
>> enum pointers were used in integer option definitions.  The storage size
>> of enums depends on the member values and the compiler; an enum could be
>> char-sized.  When we access such a thing with an int pointer we write up
>> to seven bytes of garbage ... somewhere.  We better fix that.
>
> ...I do find this evidence compelling.

It's 3 instead of 7 bytes of garbage, but the point still stands..

René