From: Eleanor McHugh <eleanor@...>
Date: 2009-09-08T20:28:43+09:00
Subject: Re: Problems with cross-thread violation

On 8 Sep 2009, at 05:09, Gin Mendi wrote:
> I'm only learning C now as I was trying to look at the source code. At
> first glance I couldn't understand anything there but I'll study it  
> if I
> run out of ideas. Anything in particular that I should study?

If C is a new language to you then the Ruby sources are definitely not  
the place to learn it.

> what about the pointers in the callback will those remain  
> unreferrenced
> or are they automatically freed when the callback ends?

They should also be freed. There are caveats and edge cases where this  
might not be the case, but based on the code you've posted none of  
those seem to apply.

> Well I tried disabling GC as you suggested but I got the same result.
> Here's what I did:
>
> require 'feed_library'
>
> module live_feed
>
>  # my callback for the live feed
>  fc = DL.callback('0PL') { |result_list, count| }
>    if result_list
>      record = FeedRecord.new(result_list)
>      puts record.code
>    end
>  }
>
>  # handle used to request for a feed
>  handle = FeedLibrary::log_on("user", "password")

Are you certain that in the case of no feed being available, you're  
receiving a nil rather than a zero? Because if not then the following  
condition will always be true and then there'll be times when you try  
and access a non-existent feed.

>  if handle
>    GC.disable
>
>    session = get_live_feed(handle, fc)
>
>    loop do
>    end
>
>    GC.enable
>  end
>
> end
>
> I still got the cross-thread violation on rb_thread_schedule()
>
> I tried changing the callback. Instead of using the struct class
> FeedRecord I instead used the struct! method. So I made it simple just
> to access 3 fields from the data structure:
>
>  fc = DL.callback('0PL') { |result_list, count| }
>    if result_list
>      result_list.struct('C16FF', :code, :value, :stat)
>      puts result_list[:code].pack('C16')
>      puts result_list[:value]
>    end
>  }
>
> I instead got a crash where I would see the printout of the callback
> with mixed in letters forming segmentation fault. Other runs would  
> just
> flat out crash back to the command prompt.


You could try installing Tenderlove's Never Say Die library from  
github, it allows you to rescue segfaults which might allow you to  
extract some useful program state or else continue as if nothing had  
gone wrong (but that's a really bad idea in the majority of cases).

You might also try coding your wrapper with ruby-ffi rather than ruby/ 
dl.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-brains.net
----
raise ArgumentError unless @reality.responds_to? :reason