Re: [Libguestfs] [nbdkit PATCH 2/2] ocaml: Implement .list_exports and friends
On 9/1/20 9:13 AM, Richard W.M. Jones wrote:
On Tue, Sep 01, 2020 at 08:41:40AM -0500, Eric Blake wrote:
> On 9/1/20 8:25 AM, Eric Blake wrote:
>> Fairly straightforward. I'd love for type export to be a bit more
>> flexible to make description optional, but could not figure out how to
>> decode that from the C side of things, so for now this just requires
>> the caller to supply a description for all exports during
>> .list_exports.
>>
>
OCaml blocks are lists of values allocated on the OCaml heap:
https://rwmj.wordpress.com/2009/08/05/ocaml-internals-part-2-strings-and-...
and those values may be ints or pointers.
Therefore the easiest way to tell if something is None from C is
to compare it with Val_int(0), ie:
value v = /* something which has type ‘string option’ */;
if (v == Val_int(0)) /* It's None */
printf ("None\n");
else {
/* It's Some string, set sv to the string value */
value sv = Field (v, 0);
printf ("Some %s\n", String_val (sv));
}
The weird casting macros have the form: To_from()
Also that String_val is only valid for a short period of time,
basically until the next OCaml allocation in the current thread.
That's helpful, as well.
> +++ w/plugins/ocaml/ocaml.c
> @@ -332,11 +332,12 @@ list_exports_wrapper (int readonly, int
> is_tls, struct nbdkit_exports *exports)
>
> /* Convert exports list into calls to nbdkit_add_export. */
> while (rv != Val_int (0)) {
> - const char *name, *desc;
> + const char *name, *desc = NULL;
>
> v = Field (rv, 0); /* export struct */
> name = String_val (Field (v, 0));
> - desc = String_val (Field (v, 1));
> + if (Is_block (Field (v, 1)))
> + desc = String_val (Field (Field (v, 1), 0));
> if (nbdkit_add_export (exports, name, desc) == -1) {
> caml_enter_blocking_section ();
> CAMLreturnT (int, -1);
Seems OK. I'm guessing this crashes?
Nope, it worked. So compared to your explanation above, my test for
Some was Is_block, rather than Int_val(0). And I repeated Field(v,1)
for getting to the 'Some "string"' rather than an intermediate variable,
but matched your code in the Field(x, 0) for accessing the string
portion of it.
I guess my remaining questions are whether there is a better approach than:
func : export list =
[ { "name1"; None }; { "name2"; Some "desc" } ]
In python, I was able to use:
return [ "name1", ( "name2", "desc" ) ]
where the alternation was on a string vs. a 2-tuple of strings, rather
than all list members being a record but where the record had an
optional member. But I'm not sure how to represent that in the ocaml
interface. Maybe I'm thinking of something like:
type export =
| Name of string
| NameDesc of string * string
but I'm not quite sure how to write a list that initializes that. On
the other hand, I think I understand the C binding for such a type:
either val is Val_int(0) (C 1) for the Name branch (Field(v, 0) is a
string), or val is Val_int(1) (C 2) for the NameDesc branch (Field(v, 0)
is name, Field(v, 1) is desc).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
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