[Caml-list] Questions about the C interface

[Caml-list] Questions about the C interface

[Falk Hueffner]

Hi,

I would like to use C functions to manipulate small binary objects and
Ocaml to manage these objects in data structures. So far, I used
alloc_small(WORDS, Abstract_tag) to allocate them. However, that means
I cannot put them into a Hashtbl, and I cannot compare them. Do I need
"custom blocks" to achieve that? I don't really need any fancy
comparison or hash function, plain memcmp and hash would do. So I
would like to save the overhead of one pointer per object and the more
costly calling.

Also, I'm wondering when exactly I need CAMLparam/local. I would think
that if I don't allocate anything in the C function, I don't need to
protect locals. The manual doesn't mention that, though. Also, I would
think that values that are really ints don't need protection...

Finally, I wonder why the first parameter to alloc_custom (the ops
block) is not a const pointer?

-- 
	Falk
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Re: [Caml-list] Questions about the C interface

[Damien Doligez]

On Thursday, February 13, 2003, at 07:39 AM, Falk Hueffner wrote:

> I would like to use C functions to manipulate small binary objects and
> Ocaml to manage these objects in data structures. So far, I used
> alloc_small(WORDS, Abstract_tag) to allocate them. However, that means
> I cannot put them into a Hashtbl, and I cannot compare them. Do I need
> "custom blocks" to achieve that?

Yes.

>  I don't really need any fancy
> comparison or hash function, plain memcmp and hash would do. So I
> would like to save the overhead of one pointer per object and the more
> costly calling.

Unfortunately, we don't have a tag for uninterpreted binary data.
The closest we have is strings, but you'd still lose one word,
because of the final NUL byte.

> Also, I'm wondering when exactly I need CAMLparam/local.

If you want to know *exactly*, then the answer is incredibly
complex.

>  I would think
> that if I don't allocate anything in the C function, I don't need to
> protect locals.

That's true.

>  Also, I would
> think that values that are really ints don't need protection...

Also true.

>  The manual doesn't mention that, though.

Taking advantage of all these exceptions is very error-prone, and we
don't want to encourage it.

> Finally, I wonder why the first parameter to alloc_custom (the ops
> block) is not a const pointer?

Because we don't believe in the const keyword.  It's more trouble
than it's worth.

-- Damien

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