> If you really want to use packing, keep it.
>
> But please remove this:
> """
> +    /* ensure that the frame_t structure is packed */
> +    assert(sizeof(frame_t) == (sizeof(PyObject*) + sizeof(int)));
> """

I added this assertion to ensure that I used correct GCC __attribute__((packed)) and Visual Studio #pragma pack(4). It can now be removed, I checked at least one per compiler that the structure is packed :-)

I will add a comment explaining that packing the structure is an optimization to reduce the memory footprint, it can be disabled if tracemalloc does crash because of it.