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

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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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