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[Discuss-gnuradio] QT Throw Bad Alloc on flowgraph startup

From: Tom McDermott
Subject: [Discuss-gnuradio] QT Throw Bad Alloc on flowgraph startup
Date: Fri, 26 Apr 2019 09:25:10 -0700

I've had this problem intermittently for about a year.  On starting the flowgraph it will terminate
with QT Throw : Bad Alloc.   Building different versions from about 3.7.10 ish to 3.7.latest will change
whether or not it happens 100% of the time or 30% of the time.

Today, I noticed that disabling the QT Frequency sink causes it to not throw, and the flowgraph runs
fine.  One input to QT Freq Sink works OK many times, two inputs causes it to throw virtually every

After some playing around:
If I change the initial QT Frequency sink to start with a much smaller FFT size (256), then it does not
QT Throw on startup even with two channels.  The original configuration was to start at FFT size of 1024
(causing throw).  Setting the block to start with FFT size of 256 causes it not to throw.

When the flowgraph starts, the QT Frequency sink does not display any signals.  If, while running
in GRC, I change the FFT size then suddenly the signal traces start displaying.  Also while running
I can then change the FFT size, and it keeps running, and the display shows the effect of changing
the resolution.

So I think the QT Throw problem has something to do with the starting FFT size, and the non-display
seems to fix itself after changing the FFT size all related to QT Frequency Sink.

Ubuntu 16.04,  with gnuradio / GRC

-- Tom, N5EG

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