[ocaml-ctypes] Static function callback.

[Romain Beauxis]

2017-02-01 9:22 GMT-06:00 Jeremy Yallop <yallop at gmail.com>:
> On 31 January 2017 at 22:31, Romain Beauxis <romain.beauxis at gmail.com> wrote:
>> I am playing with a binding for OSX's SecureTransport API. The API
>> abstracts away the underlying socket implementation and, thus,
>> requires to pass (static) callbacks, which is where I am having
>> trouble..
>>
>> Here are the relevant part of the code:
>>
>> let fd_read = (...)
>>
>> let fd_write = (...)
>>
>> let _set_io_funcs = foreign ~from:lib "SSLSetIOFuncs"
>>   (ptr void @-> fd_io_typ @-> fd_io_typ @-> returning int)
>
> Assuming that fd_io_typ is defined something like this
>
>    let fd_io_typ =
>      funptr (connection_ref @-> ptr void @-> ptr size_t @-> returning
> oss_status)
>
> and that fd_read and fd_write are actually top-level (i.e. "static")
> functions then you shouldn't need to do anything special.  Top-level
> functions stay alive for the duration of the program, so there's no
> danger of their being prematurely collected.
>
>>   let read = Root.create fd_read in
>
> I'm not sure that the Root module is really what you need here.  Root
> is useful for passing pointers to OCaml values to C when the OCaml
> part of the program doesn't keep a reference to the values.  But in
> this case the OCaml program should always have a reference to fd_read
> and fd_write.  So I suspect the problem may lie elsewhere.
>
>> I've tried my best to make sure that the passed callback would be
>> immune to Gc issues. Unfortunately, I am still experiencing issues
>> with them which, I think, relate to the Gc messing with those.
>
>> Besides using C stubs (which I am trying to avoid as an exercise
>> here), is there anything else that I could do to make this work?
>
> One useful trick for tracking down GC issues is to add calls to
> Gc.compact to your program, so that objects that are collected too
> early are collected even earlier.

Thanks for the feedback! Turned out I did not understand how to use the API..

The code has been pushed and it works fine:
https://github.com/toots/ocaml-osx-secure-transport

Romain