Re: [Caml-list] Interfacing C with OCaml and file descriptors

[Thierry Martinez <thierry.martinez@inria.fr>]

Laurent:
> So, if possible, how to convert the ‘value channel’ to a ‘FILE*’?

As the other answers already said, there does not seem to exist a
portable way to do that.
FWIW, here is what I do for Pyml, which appears to works both on
Unix and Windows:
the function file_of_file_descr takes two arguments:
- an OCaml value which can be either in_channel or out_channel,
- and a corresponding mode ("r" or "w", typically),
and returns a FILE *.
Of course, since the API for channels is not documented, this
solution can break on future OCaml versions.

#ifdef _WIN32
#include <windows.h>

extern int win_CRT_fd_of_filedescr(value handle);

static FILE *
file_of_file_descr(value file_descr, const char *mode)
{
    CAMLparam1(file_descr);
    int fd = win_CRT_fd_of_filedescr(file_descr);
    FILE *result = _fdopen(dup(fd), mode);
    CAMLreturnT(FILE *, result);
}
#else 
static FILE *
file_of_file_descr(value file_descr, const char *mode)
{
    CAMLparam1(file_descr);
    int fd = Int_val(file_descr);
    FILE *result = fdopen(dup(fd), mode);
    CAMLreturnT(FILE *, result);
}
#endif

Cheers.
-- 
Thierry.

----- Original Message -----
> From: "Laurent Thévenoux" <laurent.thevenoux@inria.fr>
> To: caml-list@inria.fr
> Sent: Friday, October 13, 2017 12:57:33 PM
> Subject: [Caml-list] Interfacing C with OCaml and file descriptors
> 
> 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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