Re: [Caml-list] C interop: Return values in parameters

["Török Edwin" <edwin+ml-ocaml@etorok.net>]

On 12/30/2012 03:08 PM, Marek Kubica wrote:
> Hi,
> 
> I am trying to wrap a C library in OCaml but I don't know how to do
> this particular thing:
> 
> I have a library that looks roughly like this:
> 
> int function(void** ptr);
> 
> So I get an int as return value to show whether the function succeeded
> and it *sets* the ptr.
> 
> void* ptr;
> function(&ptr);
> // ptr is different now
> 
> How can I wrap such a function in OCaml?
> When I call my OCaml wrapper
> 
> let retval = function ptr in
>     ...
> 
> the pointer does not get updated. Is there a function in the C API to
> force OCaml to update the values?

Use a '<yourtype> ref' for the parameter (or a record with a mutable field) on the OCaml side,
and you can update the field on the C side then.

Or if your C type is not actually void*, and your C function doesn't have side-effects (besides updating ptr)
you can also make the OCaml function return the actual value, and raise an exception if the function failed.

Best regards,
--Edwin