Re: [Caml-list] Interfacing with C: bad practice

[Dmitry Bely <dmitry.bely@gmail.com>]

2011/8/16 Török Edwin <edwintorok@gmail.com>:
> 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 = 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.

- Dmitry Bely