On 28/12/2023 16:05, René Scharfe wrote:
Am 28.12.23 um 16:10 schrieb Phillip Wood:
The diff at the end of
this email shows a possible implementation of a check_ptr() macro for
the unit test library. I'm wary of adding it though because I'm not sure
printing the pointer values is actually very useful most of the
time. I'm also concerned that the rules around pointer arithmetic and
comparisons mean that many pointer tests such as
check_ptr(pool->mp_block->next_free, <=, pool->mp_block->end);
will be undefined if they fail.
True, the compiler could legally emit mush when it finds out that the
pointers are for different objects. And the error being fixed produces
such unrelated pointer pairs -- oops.
This check is not important here, we can just drop it.
mem_pool_contains() has the same problem, by the way.
Restricting ourselves to only equality comparisons for pointers prevents
some interesting sanity checks, though. Casting to intptr_t or
uintptr_t would allow arbitrary comparisons without risk of undefined
behavior, though. Perhaps that would make a check_ptr() macro viable
and useful.
That certainly helps and the check_ptr() macro in my previous email
casts the pointers to uintptr_t before comparing them. Maybe I'm
worrying too much, but my concern is that in a failing comparison it is
likely one of the pointers is invalid (for example it is the result of
some undefined pointer arithmetic) and the program is undefined from the
point the invalid pointer is created. The documentation for check_ptr()
in my previous mail contains the following example
For example if `start` and `end` are pointers to the beginning and
end of an allocation and `offset` is an integer then
check_ptr(start + offset, <=, end)
is undefined when `offset` is larger than `end - start`. Rewriting
the comparison as
check_uint(offset, <=, end - start)
avoids undefined behavior when offset is too large, but is still
undefined if there is a bug that means `start` and `end` do not
point to the same allocation.
I agree it would be nice to allow arbitrary pointer comparisons but it
would be good to do it in a way that does not expose us to undefined
behavior. I'm not sure what the right balance is here.
Best Wishes
Phillip
