Dear Laurent,

Underlying OCaml I/O channels are Unix file descriptors, not FILE *'s. But
you can use the POSIX function fdopen to get a FILE* form the file
descriptor of the channel, which you can access by Channel(channel)->fd
(see <caml/io.h> for the precise definition of the channel struct).

That said, it is always tricky when different buffering mechanisms (OCaml's
and libc's) interact, so caveat emptor!

Best wishes,
Nicolás


--
Nicolás Ojeda Bär
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On Fri, Oct 13, 2017 at 12:57 PM, Laurent Thévenoux <
laurent.thevenoux@inria.fr> wrote:

> Hi,
>
> I have a naive question about file descriptors and C interface (when
> interfacing C with OCaml).
>
> For instance, a file.ml file which looks like:
>
>
> external my_c_interface : out_channel -> int = c_function
>
>
> where c_function writes something to a file and return the number of
> written char.
> The c_function takes a FILE* file descriptor, as for example:
>
>
> CAMLprim value c_function (value channel)
> {
>         CAMLparam1 (channel);
>         FILE* fd = ???;
>         CAMLreturn (Val_int (call_to_c_func (fd)));
> }
>
>
> So, if possible, how to convert the ‘value channel’ to a ‘FILE*’?
>
> Thanks for your help,
> Laurent
>
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