Re: Regarding assignment to struct

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

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

Andrey Tarasevich <noone@noone.net> writes:
[...]
>

  #include <stdio.h>
>
  struct S { int a[10]; };
>
  int main()
  {
    struct S a, b = { 0 };
    int *pa, *pb, *pc;
>
    pa = &a.a[5];
    pb = &b.a[5];
    pc = &(a = b).a[5];
>
    printf("%p %p %p\n", pa, pb, pc);
  }

>
[...]
>
I think that code has undefined behavior.

>
Right.  [*]
>

(a = b) is an rvalue that refers to an object of type struct S with
temporary lifetime.  pc holds the address of a subobject of that
temporary object.  The object reaches the end of its lifetime at the end
of the evaluation of the full expression.  You then print its value.

>
Even if the printf() statement were replaced by
>
   (void)pc;
>
the behavior would be undefined, because the pointer held in pc
becomes indeterminate as soon as the statement containing the
assignment to pc completes.

Agreed.

[*] Assuming C11 semantics.  At best inadvisable under C99
semantics, and a constraint violation under C90 semantics.

What C90 constraint does it violate?  Both gcc and clang reject it
with "-std=c90 -pedantic-errors", with an error message "ISO C90
forbids subscripting non-lvalue array", but I don't see a relevant
constraint in the C90 standard.

I know that C11 introduced "temporary lifetime" to cover cases
like this.  In C99, the wording for the indexing operator implicitly
assumes that there's an array object; if there isn't, I'd argue the
behavior is undefined by omission.  I'm not aware of any relevant
change from C90 to C99.

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