Re: [Discuss-gnuradio] Segmentation fault due to thread unsafety?

[Marcus Müller]

Hi Jeroen,

>
> I suspect that a solution is that I drag all plotting into my own
> block, and that also calling the plot_all() function must be done from
> within the work() function (say every 100 ms). 
I'd say quite the opposite is true!

So the point is that GUIs need to have their own loops to update the
display, so the GUI routines inherently run in a different thread than
your block. What you're doing by plotting in your work() is that you
change the state (e.g. the buffer that is displayed on screen) that the
GUI thread works on.
Since changing that state is, like you've guessed, usually not
thread-safe, you get bad effects.

Typically, you let your GUI toolkit define callbacks or functions that a
programmer must overload to fill a specific framebuffer with image data,
*when the toolkit calls it*.

The trick is hence to not do anything graphical in your work(). Instead,
over some thread safe mechanism (e.g. Queue with a mutex, or if you're
using Qt, Signals) copy the data over to your GUI logic. In that logic,
take the data and plot it -- but that has to be done from within the GUI
toolkit!
E.g. some GUI toolkits let you override a "paint" method to draw
something on the screen. Do that, based on the data you get from a Queue.
That paint method will automatically be called by the GUI when e.g. the
window size changed. But you want to update every N samples, so after
passing the data to the Queue, the work() calls an "invalidate" function
from the toolkit -- these are typically thread-safe, and trigger
updating of the screen, and in turn call the "paint" method.
I'd point you to gr-qtgui, but I think it's fair to mention that you
should first read up a bit on Qt signals and slots -- its source code
can be hard to understand, otherwise.

Best regards,
Marcus


On 07/04/2015 08:45 AM, address@hidden wrote:
> Hi all,
>
> I suspect I'm having problems with different threads which gives me a
> segmentation fault. Hopefully someone with knowledge on how this works
> with GNU radio can give me a hint for a solution.
>
> What I did is as follows (C++ implementation code is quite long, so I
> hope the following sketch of the situation is sufficient):
>
> 1) I made a pure C++ test-application, so not using GRC.
> 2) I derived my own block from the sync-block.
> 3) In the work()-function of my own block, variables are prepared to
> be plotted (simple xy-graphs). These variables are members of my
> block-class.
> 4) My own block also has a member function to plot the xy variables,
> say void plot_all(void).
> 5) In the main loop (so within main()...) there is a loop which waits
> for CTRL-C, and lets the GNU Radio thread sleep for 10 ms each
> iteration. In that loop, the plotting routine is called when more than
> 100 ms elapsed since the last time: my_block->plot_all().
> 6) If the plot_all() function now not only reads or only writes a
> member variable of my own block but both reads and writes, then I get
> a segmentation fault.
> 7) I even get a segmentation fault if the plotting variables are made
> global, so outside my own block.
>
> I suspect that a solution is that I drag all plotting into my own
> block, and that also calling the plot_all() function must be done from
> within the work() function (say every 100 ms).
>
> a) Would that work? (if I would try and test it and it works I may
> still not know if the solution is foolproof, so I ask therefore)
> b) Does that give me additional chance for buffer overruns because of
> required plotting CPU time within work()?
> c) Do I have to pull GLUT initializations as well to the constructor
> of my own block, or can I leave them in main()?
> d) Is there a better solution?
>
> Sorry if I ask much, but I've spent many hours on this problem already
> and an expert may just have a simple solution for me :-)
>
> hanks and best regards,
>
>    Jeroen
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio