On 08/16/2011 11:25 AM, Dmitry Bely wrote:
> 2011/8/16 Török Edwin <[email protected]>:
>> On 08/16/2011 10:37 AM, Dmitry Bely wrote:
>>> I would like to share my experience of writing bad C bindings. The
>>> following code is wrong, although no "living in harmony with the
>>> garbage collector" rule seems to be violated:
>>>
>>> value wrp_ml_cons (value v, value l)
>>> {
>>> CAMLparam2(v, l);
>>> CAMLlocal1(cell);
>>> cell = caml_alloc_small(2, Tag_cons);
>>> Field(cell, 0) = v;
>>> Field(cell, 1) = l;
>>> CAMLreturn(cell);
>>> }
>>>
>>> value string_list(const char ** s)
>>> {
>>> CAMLparam0();
>>> CAMLlocal1(list);
list is registered there, is that not enough? (the macro registers address of
list AFAIK,
so even if you change its value, it'll stay registered as a local root)
>>> list = Val_emptylist;
>>> while (*s != NULL) {
>>> list = wrp_ml_cons(caml_copy_string(*s), list); /* bug! */
>>> }
>>> CAMLreturn(list);
>>> }
>>>
>>> In the line
>>>
>>> list = wrp_ml_cons(caml_copy_string(*s), list); /* bug! */
>>>
>>> C compiler first puts "list" pointer on stack and then calls
>>> caml_copy_string(*s), potentially invalidating "list".
>>
>> list is a local root though, shouldn't that prevent the invalidation?
>
> Unfortunately not because wrp_ml_cons() second parameter is not
> registered.
> So wrp_ml_cons() gets an invalid value.
'list' should be reachable via caml_local_roots, so if it really gets an
invalid value
it sounds like a bug to me.
Best regards,
--Edwin
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