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out of range array subscript

 
 
Richard Delorme
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      05-03-2004
The n869 draft says:

J.2 Undefined behavior

[#1] The behavior is undefined in the following
circumstances:

-- An array subscript is out of range, even if an object
is apparently accessible with the given subscript (as
in the lvalue expression a[1][7] given the declaration
int a[4][5]) (6.5.6).

I am wondering if a cast can change this behaviour to something well
defined, ie if the las following line is ok:

int a[4][5];

a[1][7] = 3; /* <- undefined behaviour */

((int*)a[1])[7] = 42; /* defined or not? */

--
Richard
 
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Hallvard B Furuseth
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      05-03-2004
Richard Delorme wrote:
> The n869 draft says:
>
> J.2 Undefined behavior
>
> [#1] The behavior is undefined in the following
> circumstances:
>
> -- An array subscript is out of range, even if an object
> is apparently accessible with the given subscript (as
> in the lvalue expression a[1][7] given the declaration
> int a[4][5]) (6.5.6).
>
> I am wondering if a cast can change this behaviour to something well
> defined, ie if the las following line is ok:
>
> int a[4][5];
>
> a[1][7] = 3; /* <- undefined behaviour */
>
> ((int*)a[1])[7] = 42; /* defined or not? */


Still undefined. The cast does not change the type of the object
itself, only the type of the expression you are using to access the
object.

For example, imagine an array bounds checking implementation which
represents a[1] as the triple (pointer to a[1][], offset 0, size 5).
The cast may preserve this representation, just as it would with a
pointer to a malloced area. The lvalue (a[1][], offset 0, size 5)[7]
translates into *(a[1][], offset 7, size 5), and fails on the array
bounds check that offset < size. (Or the check offset <= size, if you
had not dereferenced the pointer.)

--
Hallvard
 
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Jarno A Wuolijoki
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      05-03-2004
On 3 May 2004, Hallvard B Furuseth wrote:

> Richard Delorme wrote:
> > The n869 draft says:
> >
> > J.2 Undefined behavior
> >
> > [#1] The behavior is undefined in the following
> > circumstances:
> >
> > -- An array subscript is out of range, even if an object
> > is apparently accessible with the given subscript (as
> > in the lvalue expression a[1][7] given the declaration
> > int a[4][5]) (6.5.6).
> >
> > I am wondering if a cast can change this behaviour to something well
> > defined, ie if the las following line is ok:
> >
> > int a[4][5];
> >
> > a[1][7] = 3; /* <- undefined behaviour */
> >
> > ((int*)a[1])[7] = 42; /* defined or not? */

>
> Still undefined. The cast does not change the type of the object
> itself, only the type of the expression you are using to access the
> object.


So let's alter it slightly just to make things unclear:

((int*)(a+1))[7] = 42; /* still UB? */

 
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Hallvard B Furuseth
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      05-03-2004
Jarno A Wuolijoki wrote:
> On 3 May 2004, Hallvard B Furuseth wrote:
>>> int a[4][5];

>
>>> ((int*)a[1])[7] = 42; /* defined or not? */

>>
>> Still undefined. The cast does not change the type of the object
>> itself, only the type of the expression you are using to access the
>> object.

>
> So let's alter it slightly just to make things unclear:
>
> ((int*)(a+1))[7] = 42; /* still UB? */


Sure, still UB. `a+1' degenerates to `&a[0] + 1' = `&a[1]', so it still
refers to an object where the [7] index goes above the array bound.

OTOH, this is OK:

((int*)&a)[1*5 + 7] = 42;

because `&a' gives the address of the entire object `a', which consists
of 20 `int's, and 1*5 + 7 < 20.

--
Hallvard
 
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Jarno A Wuolijoki
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      05-03-2004
On 3 May 2004, Hallvard B Furuseth wrote:

> Jarno A Wuolijoki wrote:
> > On 3 May 2004, Hallvard B Furuseth wrote:
> >>> int a[4][5];
> >>> ((int*)a[1])[7] = 42; /* defined or not? */
> >>
> >> Still undefined. The cast does not change the type of the object
> >> itself, only the type of the expression you are using to access the
> >> object.

> >
> > So let's alter it slightly just to make things unclear:
> > ((int*)(a+1))[7] = 42; /* still UB? */

>
> Sure, still UB. `a+1' degenerates to `&a[0] + 1' = `&a[1]', so it still
> refers to an object where the [7] index goes above the array bound.


Wouldn't, say, a+1+2 be one as well by that logic?

a+1 becomes &a[1] and &a[1]+2 would be an out of bounds access of a[1].

Or does (int*)&a[1] somehow descend to the "subobject" in a way
mere &a[1] doesn't?

 
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Chris Torek
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      05-15-2004
[I *think* I have all the attributions correct...]

Someone wrote:
>>>>> int a[4][5];
>>>>> ((int*)a[1])[7] = 42; /* defined or not? */


>>> On 3 May 2004, Hallvard B Furuseth wrote:
>>>> Still undefined. The cast does not change the type of the object
>>>> itself, only the type of the expression you are using to access the
>>>> object.


>> Jarno A Wuolijoki wrote:
>>> So let's alter it slightly just to make things unclear:
>>> ((int*)(a+1))[7] = 42; /* still UB? */


>On 3 May 2004, Hallvard B Furuseth wrote:
>>Sure, still UB. `a+1' degenerates to `&a[0] + 1' = `&a[1]', so it still
>>refers to an object where the [7] index goes above the array bound.


In article
<news(E-Mail Removed)>
Jarno A Wuolijoki <(E-Mail Removed)> writes:
>Wouldn't, say, a+1+2 be one as well by that logic?


Assuming "be one" means "be an instance of undefined behavior": no.
Let me give the intermediate expressions names here:

>a+1 becomes &a[1] and &a[1]+2 would be an out of bounds access of a[1].


a+1 and &a[1] denote the same thing, a value of type "pointer to
array 5 of int" pointing to the row of 5 "int"s in a[1]:

int (*p)[5] = &a[1];
/*
* Now we have, e.g.:
*
* a[0]: { 0, 1, 2, 3, 4}
* a[1]: { 5, 6, 7, 8, 9}
* a[2]: {10,11,12,13,14}
* a[3]: {15,16,17,18,19}
*
* and p points to all of a[1].
*/

Adding 2 to this value steps forward by two of the objects to
which this points:

int (*q)[5] = p + 2;

Since "p" points to one complete row of 5 "int"s, q steps forward
by two complete rows of 5 "int"s, and now points to all of a[3].

Note that *p is an array -- it names all 5 elements of a[1] --
and *q is also an array (all 5 elements of a[3]). Because *p
and *q are arrays, they are subject to The Rule about arrays
and pointers in C, namely:

In a value context, an object of type "array N of T"
becomes a value of type "pointer to T", pointing to the
first element of that array, i.e., the one with subscript 0.

So if we write (*p)[2], that puts *p -- an array object --
into a value context to subscript it with "[2]", and thus converts
from "the entire row {5,6,7,8,9} as an object" into "a pointer
to the array's first element, i.e., a pointer to the int 5".
Subscripting by 2 is really pointer arithmetic, where to add
2 we move forward by 2 of whatever it is that this new pointer
points to -- in this case, 2 "int"s. This takes us from pointing
to the 5 to pointing to the 7, and then the last step is to
indirect again, giving the "int" 7 as an object.

Hence:

(*p)[2] += 70;

makes a[1][2] change from 7 to 77. We can write (*p) as p[0]
of course:

p[0][2] += 70; /* same thing */

and moreover, we can use pointer arithmetic on p -- as we did
to get q -- as part of the subscripting operation:

p[1][2] -= 20;

Since p[1] "means" *(p + 1), we move forward by one of the things
that "p" points to, i.e., one "array 5 of int" row in the array
named "a". Now we point to the entire row {10,11,12,13,14}. The
indirection gets us the entire array object, which "decays" to a
pointer per The Rule for the [2] step. Subscriping this pointer
works just as before, and p[1][2] names the same single "int" as
a[2][2], which in the example above is "12" initially. Subtracting
20 from it changes a[2][2] from 12 to -8.

>Or does (int*)&a[1] somehow descend to the "subobject" in a way
>mere &a[1] doesn't?


Here we have &a[1] -- a pointer to the entire row {5,6,7,8,9} in
the example above -- and convert the pointer through pointer casting
into some other pointer. What exactly does this do? The rules
here are at least to some extent up to the implementation. The
"input" type (before the cast) is "int (*)[5]" or "pointer to array
5 of int". The "output" type is "int *", or "pointer to int".
The implementor gets to decide how to achieve the conversion and
what the result should be.

Suppose a hypothetical Evil Implementor decides that the rule is
"conversion of `pointer to array N of T' to `pointer to T' finds
the N-1th element of the array and gives you that pointer". (This
would be quite *surprising* but I do not believe the C standards
forbid it. It might even be the "natural" implementation on certain
ancient IBM mainframes, the ones where Fortran array subscripting
worked from the top down, as it were.) In this case, the result
of (int *)p -- where p points to the entire row {5,6,7,8,9} --
would be a pointer to the element a[1][4], currently holding 9.

Of course, the "usual" implementation is to take the *lowest*
machine byte or word address, so that (int *)p is just a pointer
to the element a[1][0], currently holding 5.

This pointer cast, like all pointer casts, should be viewed with
suspicion. ("Cast a jaundiced eye upon the pointer"? See
<http://www.phrases.org.uk/bulletin_board/19/messages/133.html>)
C's history, particularly with "const", makes some pointer
casts inevitable in some code, but the more you "evit" them
the better, in general. (Aside: www.m-w.com claims "evitable"
*is* a word. I thought it was a "lost positive" myself.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (4039.22'N, 11150.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
 
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