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". Of course, the
stack copy of "list" is not registered as a global root so wrp_ml_cons
gets an invalid value.
Maybe Ocaml docs should be updated to warn about pitfalls like that?
- Dmitry Bely
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