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

["Laurent Thévenoux" <laurent.thevenoux@inria.fr>]

Hello,

Thank you, everyone, for your very instructive answers!

Laurent

> Le 14 oct. 2017 à 17:54, Thierry Martinez <thierry.martinez@inria.fr> a écrit :
> 
> 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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