Re: [PATCH] Various pages: SYNOPSIS: Use VLA syntax in function parameters

Alejandro Colomar
2022-11-13T15:15:45Z

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Hi Martin,

On 11/13/22 15:58, Martin Uecker wrote:
> Am Sonntag, den 13.11.2022, 15:02 +0100 schrieb Alejandro Colomar:
>>
>> On 11/13/22 14:33, Alejandro Colomar wrote:
>>>
>>> On 11/13/22 14:19, Alejandro Colomar wrote:
>>>>> But there are not only syntactical problems, because
>>>>> also the type of the parameter might become relevant
>>>>> and then you can get circular dependencies:
>>>>>
>>>>> void foo(char (*a)[sizeof *.b], char (*b)[sizeof *.a]);
>>>>
>>>> This seems to be a difficult stone in the road.
>
> But note that GNU forward declarations solve this nicely.

How would that above be solved with GNU fwd decl? I'm guessing that it can't.
How do you forward declare incomplete VMTs?.

>
>>>>
>>>>> I am not sure what would the best way to fix it. One
>>>>> could specifiy that parameters referred to by
>>>>> the .identifer syntax must of some integer type and
>>>>> that the sub-expression .identifer is always
>>>>> converted to a 'size_t'.
>>>>
>>>> That makes sense, but then overnight some quite useful thing came to my mind
>>>> that would not be possible with this limitation:
>>>>
>>>>
>>>> <https://software.codidact.com/posts/285946>
>>>>
>>>> char *
>>>> stpecpy(char dst[.end - .dst], char *src, char end[1])
>>
>> Heh, I got an off-by-one error. It should be dst[.end - .dst + 1], of course,
>> and then the result of the whole expression would be 0, which is fine as size_t.
>>
>> So, never mind.
>
> .end and .dst would have pointer size though.
>
>>>> {
>>>> for (/* void */; dst <= end; dst++) {
>>>> *dst = *src++;
>>>> if (*dst == '\0')
>>>> return dst;
>>>> }
>>>> /* Truncation detected */
>>>> *end = '\0';
>>>>
>>>> #if !defined(NDEBUG)
>>>> /* Consume the rest of the input string. */
>>>> while (*src++) {};
>>>> #endif
>>>>
>>>> return end + 1;
>>>> }
>>> And I forgot to say it: Default promotions rank high (probably the highest) in
>>> my list of most hated features^Wbugs in C.
>
> If you replaced them with explicit conversion you then have
> to add by hand all the time, I am pretty sure most people
> would hate this more. (and it could also hide bugs)

Yeah, casts are also in my top 3 list of things to avoid (although in this case
there's no bug); maybe a bit over default promotions :)

I didn't mean that all promotions are bad. Just the gratuitous ones, like
promoting everything to int before even needing it. That makes uint16_t a
theoretical type, because whenever you try to use it, you end up with a signed
32-bit type; fun heh? :P _BitInt() solves that for me.

But sure, in (1u + 1l), promotions are fine to get a common type.

>
>>> I wouldn't convert it to size_t, but
>>> rather follow normal promotion rules.
>
> The point of making it size_t is that you then
> do need to know the type of the parameter to make
> sense of the expression. If the type matters, then you get
> mutual dependencies as in the example above.

Except if you treat incomplete types as... incomplete types (for which sizeof()
is disallowed by the standard). And the issue we're having is that the types
are not yet complete at the time we're using them, aren't they?

Kind of like the initialization order fiasco, but since we're in a limited
scope, we can detect it.

Cheers,

Alex