Re: open/fork from a callback hang the windows (gtk2::helper)

[Brian Manning <cpan xaoc org>]

On Thu, Jan 15, 2015 at 9:21 AM, Prunk Dump <prunkdump gmail com> wrote:
> But if I fork the command with "open" in a callback, the gtkmain
> thread hangs until the command finish. I'm sure that the open call is
> non-blocking.
>
> sub my_callback {
>
>    my $fd = FileHandle->new()
>    open($fd, "mycommand | ") or quitCallback("can't fork")
>
> }
> --> hang on return
>
> I don't really understand why. Maybe Gtk make the callbacks with new
> threads and wait the result with waitpid ?

No, threads aren't the answer here.

I'm guessing that your script hangs because you never read from the
opened filehandle in your callback, so "mycommand" never exits because
it's waiting for it's data to be read.

> On all gtk2::helper examples I've found, the "open" call is made
> before Gtk2->main. How can I do if the windows is already displayed
> and if I'm inside gtkmain ?

There's an example in the Gtk-Perl FAQ that uses
Gtk2::Helper->add_watch after the GUI has been displayed;

https://wiki.gnome.org/Attic/GTK2-Perl/FrequentlyAskedQuestions#How_do_I_keep_my_GUI_updating_while_doing_a_long_file_read.3F

Instead of calling open() in the callback as in your code snippet, you
make the open() call by itself, assign the filehandle created by the
open() call to a $variable, then add a watch on that filehandle
$variable via Gtk2::Helper->add_watch/Glib::IO->add_watch.
Gtk2::Helper->add_watch is just a wrapper around Glib::IO->add_watch
anyways.

Jeffery also suggested previously using IPC::open3 and
Glib::IO->add_watch to accomplish the same thing.

The FAQ also has an example of using the "threads" module, if you're
really trying to use threads, but using open() and adding a watch to
the filehandle will do what I think you are asking for, with less
pain.

Thanks,

Brian

http://perldoc.perl.org/functions/open.html
http://perldoc.perl.org/perlipc.html#Using-open()-for-IPC