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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Loops (was Re: do { quit; } else { })
Date: Tue, 13 May 2025 14:35:06 -0700
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Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
> On 12.05.2025 17:08, David Brown wrote:
>> On 11/05/2025 23:43, James Kuyper wrote:
> [...]
>>> More precisely, it makes it undefined behavior for values to point to an
>>> array of less than 10 doubles.
>> 
>> The wording of the C standard (C11, as that's what I have open at the
>> moment) is :
>> 
>> """
>> If the keyword static also appears within the [ and ] of the array type
>> derivation, then for each call to the function, the value of the
>> corresponding actual argument shall provide access to the first element
>> of an array with at least as many elements as specified by the size
>> expression.
>> """
>
> Oh! - This is somewhat surprising to me. - When I first read about
> the "arr[static N]" I assumed that it would be possible to pass any
> sub-array, say "&arr[8]" (or "arr+8") as long as it's large enough
> to still have N elements from the starting point, but the quote says
> explicitly "access to the _first_ element of an array" (which I'd
> interpret as "&arr[0]" (or "arr"). - Unless I misinterpreted that;
> what would be the reason for that?

My personal interpretation is that this:

void func(int arr[static 5]) {
}

int main(void) {
    int arr[10];
    func(arr+5);    // OK
    // func(arr+6); // UB
}

is valid, because, for example, the last 5 elements of a 10-element
array object can be treated as a 5-element array object.  gcc seems
to agree, based on the fact that it warns about func(arr+6) but
not about func(arr+5).

This is a fundamental part of my mental model of C, but in a few
minutes of searching I wasn't able to find explicit wording in the
standard that supports it.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */