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

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?  I don't see how it does either.

"""
A declaration of a parameter as ‘‘array of _type_’’ shall
be adjusted to ‘‘qualified pointer to _type_’’, where the
type qualifiers (if any) are those specified within the [ and ]
of the array type derivation. 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.
"""

The question is whether, for example, the last 5 elements of a
10-element array object can be treated as a 5-element array object.
If someone can cite wording in the standard that answers that
question, I'd appreciate it.  (I'll be happier if the answer is yes.)

Looking into this a bit more, I realize that the question doesn't
matter if there's no "static" keyword between the [ and ].  In that
case, the parameter is of pointer type, and the description of
pointer arithmetic (N1570 6.5.6p8) explicitly allows the pointer
to point to the i-th element of an array object.  The wording for
[static N] is the only place I've seen (so far) that specifically
refers to the *first* element of an array object, raising the
question of whether a subobject of an array object is itself an
array object.  This might just be some slightly sloppy wording that
was introduced in C99 and never corrected.

For example, given this code:

```
void without_static(int arr[]) {
    (void)arr[4];
}

void with_static(int arr[static 5]) {
    (void)arr[4];
}

int main(void) {
    int arr[10] = { 0 };
    without_static(arr+5);
    with_static(arr+5);
}
```

there's no problem with the call `without_static(arr+5)`, but the
call `with_static(arr+5)` has defined behavior if and only if the
last 5 elements of a 10-element array object can be treated as a
5-element array object.

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