[Rd] sending signals to embedded R

Prof Brian Ripley ripley at stats.ox.ac.uk
Sat May 5 19:09:39 CEST 2007

On Sat, 5 May 2007, Luke Tierney wrote:

> On Sat, 5 May 2007, Prof Brian Ripley wrote:
>> On Fri, 4 May 2007, Luke Tierney wrote:
>>> On Fri, 4 May 2007, Deepayan Sarkar wrote:
>>>> On 5/4/07, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
>>>>> On Fri, 4 May 2007, Deepayan Sarkar wrote:
>>>>>> one thing I haven't been able to figure out from R-exts is how to
>>>>>> interrupt a calculation running inside an embedded R. C code inside R
>>>>>> calls R_CheckUserInterrupt() intermittently to check for interrupts,
>>>>>> but how does my GUI tell R that the user wants it interrupted?
>>>>> Well, the intention is that you send an interrupt, which hardly needs 
>>>>> to
>>>>> be in the manual.
>>>> I didn't mean to imply that it does. I'm just new to signals and
>>>> things that should be obvious aren't.
>>>> Basically kill(2) seems to be the right thing to use, but I wasn't
>>>> sure what the PID needs to be. Turns out sending SIGINT to my GUI from
>>>> a shell interrupts R, so raise(SIGINT) should be enough.
>>> The tricky bit here is figuring out who does the sending.  It you have
>>> a separate thread/process for the GUI and R then that is fine (though
>>> may raise other issues).  If it is a single thread then you need your
>>> event processing to get an occasional look in to recognise the user
>>> action that triggers an interrupt. The Windows version handles this by
>>> having R_CheckUserInterrupt() do a limited amount of event processing
>>> (you need to be careful in GUI events have R actions associated with
>>> them).  I believe the Mac version is similar though it has been a
>> I was assuming that Deepayan's GUI (which seems to need Qt4, BTW, so I was 
>> unable to compile it) worked via the R-Unix eventloop, in which case it 
>> gets some CPU time from time to time.
> I was assuming that as well.  But my recollection is that on unix the
> event loop is only run from within the console reader.  On Windows
> (and Mac OS X I believe) some event processing also happens in
> R_CheckUserInterrupt(); on Windows there is also some more in some
> blocking library calls, like socket reads as I recall.  But unless
> things have changed since I last looked none of that happens on unix.
>> gnomeGUI has an interrupt menu item with action 'onintr', which may well 
>> be what Deepayan is looking for: the only reason that package still exists 
>> is to provide example code.  (Not that it was ever properly integrated 
>> with the R event loop.)
> It does have some sort of interrupt device (I can't recall if it is a
> menu item or a butto and I can't seem to build a working gnomeGUI to
> check). And I believe if you try to use that item (or button?) during
> a long-running computation you can't because the events won't be
> looked at until R gets back to a console read, at which point the
> events will be processed and you jump to the top level (where you
> already are).

That belief is correct (it has a menu item and a button), but my final 
parenthetical remark was that gnomeGUI was not wedged into the event loop.

>> If the issue is what happens when the user Ctrl-C's in the GUI console, 
>> that depends on what the GUI toolkit does with keyboard input: if it 
>> generates a SIGINT this should just work, but otherwise the keyboard 
>> handler needs to be told to call onintr() one way or another.
> Again only if the GUI gets a chance to look at the keyboard input,
> which I don't think we currently give it.

We builtin the ability for a front-end to register handlers with the R 
event loop, including a polling handler (and that is how we can have a 
Tcl/Tk front end).  That postdates gnomeGUI, which runs the Gtk 
event-loop, not R's.

So my assumption 'worked via the R-Unix eventloop' was that a handler 
(probably a polling handler) had been wedged in the eventloop.
That was in contrast to running under a separate thread.

> The UI provided by a shell running in a separate process may not have
> a 'G' but it does have its advantages :-)

Or a separate thread, as Rterm.exe does.  Really RGui should also run in a 
separate thread, but when Guido did so, it did not work under Windows 95: 
if we ever give up support for pre-NT Windows I will take a look again at 

I guess my underlying point is that rather than run the GUI from 
R_ProcessEvents (as RGui is), on Unix you can run it from an eventloop 


> Best,
> luke
>>> while since I looked at that. I don't believe the unix version of
>>> R_CheckUserInterrupt() does not provide hooks for installing such
>>> checking (we have talked about this off an on but I don't believe
>>> anything happened -- could be wrong there though).
>>> If Qt allows this one option may be to have events on your nterrupt
>>> widget managed by a small thread that does nothing other than send a
>>> signal to the main thread if the widget is clicked.
>>> Best,
>>> luke

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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