Re: Loops (was Re: do { quit; } else { })

On 5/13/25 22:37, Keith Thompson wrote:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

[...]
>

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.

>
In N1570, 6.7.6.3 p7.

>
Did you mean to imply that that paragraph supports (or refutes) my
statement?  [...]

>
No.  I posted the reference to say that the cited paragraph supports
the conclusion that 'func(arr+6)' is undefined behavior.

...

To me it seems obvious that 6.7.6.3 p7 is meant to cover the
case of 'func(arr+6)' as being undefined behavior.

 
But that's not the question I was addressing.  My question is whether
func(arr+5) has defined behavior, based on whether or not a+5 points to
the *first element* of an array.

Tim - you very commonly post messages leaving out lots of relevant
information, seemingly expecting people to figure out the details that
you don't bother to provide. This can be a valid teaching technique, but
using that technique relies on the assumption that what you've left out
can be determined by careful examination of what you put in.  However,
Keith had no reasonable way of guessing what you had left out, because
he did not anticipate that you would address the wrong question. As a
general rule, if there's any possibility that you've made a mistake like
you did in this case, leaving out too many details as "an exorcise for
the student" runs the risk of not not providing enough information for
others to identify and correct the mistake.