On 01/02/10 13:36, John Lask wrote:
I understand these are internals of ghc and subject to change. The reason for their use: to support asynchronous interrupts safe with respect to the Haskell code that is being interrupted. To my knowledge (please correct me if I am wrong) there is no way to do this other than the following alternatives and the already mentioned functions.
As an example, suppose I want to provide a call back to a win32 OS hook which takes a c-call-back routine. My understanding is that I cannot use a wrapped Haskell call-back routine as there are no guarantees what state the Haskell rts will be in when the routine is called.
It's not clear to me that this wouldn't work. I believe it would be perfectly safe for the Win32 console handler callback to invoke Haskell functions, because the handler is executed in a separate thread, unlike Unix signals which happen in the context of one of the existing threads (which is why you can't use any inter-thread communication or synchronisation in a Unix signal handler).
At least initially I have used the above mentioned functions to support win32 signal handling, as the ghc rts just catches (and dispatches) console events, which do not encompass all the (rather limited) c-rts signals.
The obvious solution is to provide a c call-back routine, use an WIN32 event object, use a Haskell bound thread to wait on that event.
another alternative would be to poll.
The first alternative requires threaded rts which for various reasons I don't wish to use under all circumstances, the other alternative is inefficient or unresponsive.
Discussion of either of these alternatives distract from the question "shouldn't there be a method for asynchronous call-back that is safe with respect to the Haskell rts state"?
But there already exists such a method, that of the backdoor already mentioned, really, all that is required is for this to become more formalised and a single api adopted that is usable from c and consistent across threaded and un-threaded rts, but in the mean time the existing structure is quite usable for this purpose aside from the cumbersome libraries issue.
And the reason for this libraries issue is that the methods exposed by the ghc-runtime to collect and post events into the ghc runtime system differ between the threaded and non-threaded runtimes, which is why short of changing ghc rts myself I can't avoid it (or adopting either of the above alternatives)
As the facility (to capture arbitrary asynchronous interrupts) is generally useful I believe it to be advantageous to address it rather than side-stepping it.
You might want to look at the work that Bryan O'Sullivan and Johan Tibell are doing on a new IO manager: http://github.com/bos/event/ There's no Win32 support yet, but it's designed to allow multiple backends. Cheers, Simon