Re: [Caml-list] Unix file descriptors vs. in/out channels

[Thomas Braibant <thomas.braibant@gmail.com>]

Hi Adrien,

Thanks a lot for clarifying that one should not use accesses through
the file descriptors and the in/out_channels.

I am pretty sure that using in/out_channel_of_descr more than once
will be an issue too, looking at the code in io.c [1] (at least
because the close function would be called twice on the same fd, which
might refer to something else at that point in time, as Goswin pointed
out recently).

The only part that I still do not fully understand is why the
following code, that does not mix accesses through the file_descr and
the channels has a behavior that depends on the fact that I compute
the length of the input channel. I would expect the out_channel to be
flushed by the call to printf with "%!", isn't it?

let test =
  let open Unix in
  let fd = openfile "foo.bar" [O_RDWR; O_TRUNC; O_CREAT] 0o640 in
  Printf.printf "openfile\n%!";
  let o = out_channel_of_descr fd in
  Printf.printf "out_channel_of_descr\n%!";
  let i = in_channel_of_descr fd in
  Printf.printf "in_channel_of_descr\n%!";
  debug "1" i o;
  let _ = Printf.fprintf o "test1\n%!" in
  debug "2" i o;
  let _ = input_char i  in
  close_in i;
  Printf.printf "Ok\n%!"



[1] https://github.com/ocaml/ocaml/blob/774e30e138dc22a5acd6cfac03ae25194ae8cd6e/byterun/io.c

On Mon, Aug 18, 2014 at 6:10 PM, Adrien Nader <adrien@notk.org> wrote:
> Hi,
>
> You cannot safely mix buffered (in/out_channel) and un-buffered
> (file_descr) uses of the same underlying resource.
>
> IIRC an in_channel or out_channel has a buffer in OCaml memory.
> If you close the underlying file_descr of an out_channel with
> functions operating on file desriptors directly, it is possible that
> some data will still be buffered.
> If you read alternatively through file_descr and in_channel, you might
> skip some data if reading with the in_channel reads more than just "n
> chars" (it could read 4K for instance, I'm not completely sure).
>
> As for using in/out_channel_of_descr more than once, I don't know
> offhand: if it creates new buffers each time (likely), it will be an
> issue.
>
> --
> Adrien Nader