From: already5chosen@yahoo.com
On Mon, 12 Jan 2026 15:58:15 +0000
bart wrote:
> On 12/01/2026 14:28, Michael S wrote:
> > On Thu, 1 Jan 2026 22:54:05 +0100
>
> > On related note.
> >
> >
> > struct bar1 {
> > int table[4];
> > int other_table[4];
> > };
> >
> > struct bar2 {
> > int other_table[4];
> > int table[4];
> > };
> >
> > int foo1(struct bar1* p, int v)
> > {
> > for (int i = 0; i <= 4; ++i)
> > if (p->table[i] == v)
> > return 1;
> > return 0;
> > }
> >
> >
> > int foo2(struct bar2* p, int v)
> > {
> > for (int i = 0; i <= 4; ++i)
> > if (p->table[i] == v)
> > return 1;
> > return 0;
> > }
> >
> > According to C Standard, access to p->table[4] in foo1() is UB.
> > [O.T.]
> > I want to use language (or, better, standardize dialect of C) in
> > which behavior in this case is defined, but I am bad at influencing
> > other people. So can not get what I want.
> > [/O.T.]
>
>
> So you want to deliberately read one element past the end because you
> know it will be the first element of other_table?
>
Yes. I primarily want it for multi-dimensional arrays. Making the same
pattern defined in 'struct' is less important in practice, but desirable
for consistency between arrays and structures.
> I think then it would be better writing it like this:
>
> struct bar1 {
> union {
> struct {
> int table[4];
> int other_table[4];
> };
> int xtable[8];
> };
> };
>
> int foo1(struct bar1* p, int v)
> {
> for (int i = 0; i <= 4; ++i)
> if (p->xtable[i] == v)
> return 1;
> return 0;
> }
>
> At least your intent is signaled to whomever is reading your code.
>
If were use language or dialect in which the behavior is defined, why
would you consider the second variant better?
I don't mean in this particular very simplified example, but generally,
where layout is more complicated.
> But I don't know if UB goes away, if you intend writing to .table and
> .other_table, and reading those values via .xtable (I can't remember
> the rules).
>
> I'm not even sure about there being no padding between .table and
> .other_table.
Considering that they both 'int' I don't think that it could happen,
even in standard C. In "my" dialect, padding in such situation can be
explicitly disallowed by the standard.
>
> (In my systems language, the behaviour of your original foo1, in an
> equivalent program, is well-defined. But not of foo2, given that you
> may read some garbage value beyond the struct, which may or may not
> be within valid memory.)
>
>
> > Now the question.
> > What The Standard says about foo2() ? Is there UB in foo2() as
> > well?
>
> Given that you may be reading garbage as I said, whether it is UB or
> not is irrelevant; your program has a bug.
Whether there is bug or not depends on what caller passed to foo2().
There are great many programs around that do similar things and contain
no bugs. Most typically, caller creates argument p by casting of char
array that is long enough for table member to hold more than 4
elements.
Without seeing code on the caller's site we could only guess, due to
suspect way the code is written, that there is bug. But we can't be
sure.
>
> Unless you can add extra context which would make that reasonable.
> For example, the struct is within an array, it's not the last
> element, so it will read the first element of .other_table, and you
> are doing this knowingly rather than through oversight.
>
> It might well be UB, but that is a separate problem.
>
>
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
