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

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:

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.

>
I wish you had said so in the first place.  Of course func(arr+6) has
undefined behavior.  Did anyone in this thread say or imply otherwise?

>
In my view the same reasoning about the meaning applies to both
cases, so there is no reason to talk about them separately.

Again, of course the behavior of func(arr+6) is undefined.  The behavior
of func(arr+5) is less clear *to me*.

"""
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.)

>
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.

>
To me it seems obvious that 6.7.6.3 p7 is meant to cover the
case of 'func(arr+5)' as satisfying the "shall" requirement,
for the same reasons that it is meant to cover the case of
'func(arr+6)' as being undefined behavior.

It does so only if the argument arr+5 provides access to "the first
element of an array".  You seem to think that it's obvious that it does
so, based on the disinction you make between "array" and "array object".

Note that 6.7.6.3 p7 doesn't say "array object", it says just
"array".  I believe the choice of wording is neither an accident nor
an oversight.

>
Then please explain what you see as the difference.  Wording in the
standard to support the distinction would be welcome.
>
Given `int arr[10];`, do the last 5 elements of arr constitute an
"array"?   Do they constitute an "array object"?  And the same
questions for arr as a whole.

I note that you haven't answered the above questions.  They were not
rhetorical.  I asked them because I thought that answers to those
specific questions would help me to understand the distinction that you
make between "array" and "array object".

The meanings follow from a plain understanding of the English
language.

I disagree.  Perhaps my understanding of certain combinations of English
words differs from yours.

Consider the following example:
>
   typedef struct { int s; float f; } T;
>
   extern void foo( unsigned char blah[ static sizeof(T)-1 ] );
>
   void
   bas( T *it ){
      foo( (unsigned char *)it + 1 );
   }
>
There is no array object.  But surely there is an array (or at
least an array is indicatated, and an array is present if 'it' is
a valid pointer).  This example satisfies the "shall" requirement
in 6.7.6.3 p7, despite there being no array object in sight.

Is there no "region of data storage in the execution environment, the
contents of which can represent values"?  Cannot that region represent
a value of type unsigned char[sizeof(T)-1]?  Is a region of data
storage that can represent values of array type not an array object?
Again, none of these questions are rhetorical.

Can you define, preferably in something approaching standardese, what
you mean by "array" and by "array object", and in particular how they
differ?

I believe that, in my example above, arr+5 *does* "provide access
to an array" with at least 5 elements.  (I also believe that that
"array" is an "array object".)  My difficulty is in demonstrating
this based on the normative wording in the standard.  *Maybe*
if you could explain the distinction you make between "array" and
"array object" it would help.

[...]

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