# QT GUI Segmentation Fault **URL:** \ **Category:** GNU Radio\ **Created:** [October 31, 2014, 6:55pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856 "2014-10-31T18:55:47Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [October 31, 2014, 6:55pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/1 "2014-10-31T18:55:47Z")
Hi all, I recently installed GNURadio on two laptops (installation via PYBOMBs) and everything has been running fine for a few weeks; but I’m suddenly running into issues with QT GUI on one of the laptops. I wasn’t sure what the issue was originally, but after narrowing it down I’m able to see the problem with a simple flow graph: signal source → throttle → QT GUI Sink Starting with a blank slate in GRC and building the flow graph works fine on one device; but the same flow graph on the second laptop opens and closes immediately with return code -11. If I run the python code outside of GRC, I get the segmentation fault. I was looking for a solution and I came across bug 645 ( [http://gnuradio.org/redmine/issues/645](http://gnuradio.org/redmine/issues/645)) that was resolved in 3.7.3, but these laptops have 3.7.6 (git-133-gd814810) installed. I noticed that the bug report mentions the avx volk kernel as the issue and both of my machines indicate that they’re using volk machine avx\_64\_mmx. I also ran volk\_profile and changed “volk\_32fc\_32f\_multiply\_32fc a\_avx generic” to “volk\_32fc\_32f\_multiply\_32fc generic generic” as indicated in the bug report, but I still see the problem. To be honest, I don’t really know how the volk library works and I’m not sure what else I should try changing. If anybody has suggestions, it would be greatly appreciated. Thanks in advance, -Mike ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [October 31, 2014, 7:16pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/2 "2014-10-31T19:16:40Z")
Just a follow up on this - I was able to resolve the issue with the following modifications in the volk\_config file: before: volk\_32fc\_32f\_multiply\_32fc a\_avx generic changed to: volk\_32fc\_32f\_multiply\_32fc generic generic before: volk\_32fc\_deinterleave\_64f\_x2 a\_avx u\_avx changed to: volk\_32fc\_deinterleave\_64f\_x2 generic u\_avx While I’m happy to have it working again, I don’t exactly know what this modification actually does. If anyone with knowledge of the volk library can give me a quick bit of info about what these modifications are actually doing and / or why this would resolve the segfault issue, it would ease my mind a bit. Thanks, -Mike ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [October 31, 2014, 8:52pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/3 "2014-10-31T20:52:41Z")
* * * Discuss-gnuradio mailing list removed\_email\_address@domain.invalid [https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [October 31, 2014, 8:58pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/4 "2014-10-31T20:58:40Z")
Hi Johnathan, Thanks for the reply. I’m using Ubuntu 14.04 on a Dell XPS laptop with 8 cores. Results of the grep call are below: model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz model name : Intel(R) Core™ i7-2630QM CPU @ 2.00GHz I noticed that there are 12 other uses of a\_avx in volk\_config. Is it likely that using any of the kernels with a\_avx will cause a similar issue? -Mike On Fri, Oct 31, 2014 at 2:19 PM, Johnathan C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [October 31, 2014, 9:44pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/5 "2014-10-31T21:44:00Z")
On Thu, Oct 30, 2014 at 4:07 PM, Michael R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > While I’m happy to have it working again, I don’t exactly know what this > modification actually does. If anyone with knowledge of the volk library can > give me a quick bit of info about what these modifications are actually > doing and / or why this would resolve the segfault issue, it would ease my > mind a bit. > > Thanks, > > -Mike Hi Mike, So each VOLK kernel has several internal implementations that are architecture specific (SSE, AVX, NEON, etc). A call to the VOLK library looks like volk\_32fc\_32f\_multiply\_32fc(output\_buffer, input\_buffer0, input\_buffer1, number\_of\_points). Internally VOLK has a dispatcher that uses the implementation that is “best” for your machine. The “best” implementation is determined by running volk\_profile which runs all of the implementations available and write the fastest one to volk\_config. At run-time VOLK reads this file to know which version of each kernel to actually run. Can you tell me your processor model name and flags? A copy of /proc/cpuinfo (just one of the processors) would be useful. Nathan ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [October 31, 2014, 10:03pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/6 "2014-10-31T22:03:05Z")
Hi Nathan, Thanks for the info. In case you didn’t see my previous reply, the processors are all “Intel(R) Core™ i7-2630QM CPU @ 2.00GHz” -Mike On Fri, Oct 31, 2014 at 3:32 PM, West, Nathan [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![roa](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e7dd6/32.png) [@roa](https://www.ruby-forum.com/u/roa)\ **Post date:** [November 2, 2014, 4:30pm UTC](https://www.ruby-forum.com/t/qt-gui-segmentation-fault/232856/7 "2014-11-02T16:30:38Z")
On Fri, Oct 31, 2014 at 4:21 PM, Michael R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi Nathan, > > Thanks for the info. In case you didn’t see my previous reply, the > processors are all “Intel(R) Core™ i7-2630QM CPU @ 2.00GHz” > > -Mike Ok, I think I understand what’s going wrong here. We’re calling an aligned VOLK call directly inside the fft function of that block that works off a buffer that we created (properly aligned) and a buffer that FFTW created. FFTW doesn’t always ship packaged with AVX support, so the FFT buffers are only designed for SSE use – that is, only guaranteed to be 16-byte aligned. AVX requires 32-byte alignment. The fix is simply. Just remove the \_a call from lines 268 and 276 (in sink\_c\_impl.cc). I was thinking that both buffers would be guaranteed to have the right alignment, so I skipped the volk dispatcher and called directly into the aligned kernel. We might just have to ban use of anything but the dispatchers in GNU Radio code from now on. I’m not in a place to fix this right now, so I just reopened issue #645 to remind myself to do this when I can. Tom