Re: [Caml-list] Channels not closed on gc?

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am Montag, den 21.05.2012, 14:53 +0200 schrieb Philippe Wang:
> On Mon, May 21, 2012 at 1:57 PM, Gerd Stolpmann <info@gerd-stolpmann.de> wrote:
> > Am 21.05.2012 13:23:36 schrieb(en) Lauri Alanko:
> >
> >> I only recently noticed that ocaml does not close open channels when they
> >> are garbage collected. This is evidently intentional behavior, but it was
> >> quite unexpected.
> >>
> >> To be clear, I do think it's bad style to rely on GC for releasing OS
> >> resources, but that doesn't explain why GC shouldn't do this if the
> >> programmer has failed to explicitly close the channel. And if the intention
> >> were to _enforce_ good style, the channel finaliser would spout out an error
> >> or warning upon detecting that the channel hasn't yet been closed, instead
> >> of just silently leaking file handles like it does currently.
> >>
> >> It is of course trivial to "fix" this by attaching a simple finaliser, but
> >> the fact that this is not done by default makes me suspect that there would
> >> be something fishy with this approach. So, what's the rationale for the
> >> current behavior?
> >
> >
> > It's predictable.
> >
> > Closing a channel is not only about releasing OS resources. Imagine the
> > channel is actually a pipe - closing it means to signal EOF, i.e. it's a way
> > of notifying another program about an event. This should not happen behind
> > one's back.
> >
> > Closing a regular file would in deed be harmless, but there is no generic
> > way to identify such channels (in the OS).
> >
> > Gerd
> >
> >
> >> Thanks,
> >>
> >>
> >> Lauri
> 
> Wouldn't it be a good idea to provide both ways?
> i.e., one that collects automatically, in addition to the current one
> that doesn't.

You can easily do this yourself:

Gc.finalise close_in ch

(if ch is an in_channel; for out_channels use close_out_noerr). We
exploit here that channels track whether they are already closed. If
there is the danger of getting an exception, catch it:

let close_out_catch ch =
  try close_out ch
  with error ->
    prerr_endline("Exception from attempt to close channel: " ^ Printexc.to_string error)

This points, btw, to another problem: Exceptions can no longer be
reasonably handled, but only be printed. You cannot do much in a
finaliser.

Gerd

> -- 
> Philippe Wang
>    mail@philippewang.info
> 

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