[Difx-users] Lack of mixed-mode fringes with 2.6.3, then segfault with 2.8.1? {External} {External}

[Eskil Varenius]

Hey Adam,
Jan Wagner and Walter Brisken both offered to check, and concluded the segfault is a bug in 2.8.1 which was then fixed in the repo later. So the official 2.8.1 release has a confirmed bug causing a segfault in vex2difx. Jan wrote:
The fix was r10990 (trunk) and r10991 (2.8): "vex2difx.cpp fix segfault
caused by missing realloc() of freqOffset[] and recFreqDestId[] - the
associated code in vex2difx fixDatastreamTable() should really be moved into
difxio".

Pulling the trunk version sorted this, but there are other issues. Ufortunately, my attempts to correlate the data to get at my original problem – lack of fringes at one specific baseline and polarisation combination – did not get further with 2.8.1 or trunk. My datastream formats get misinterpreted some how to wrong values, despite working with with 2.6.3 vex2difx. I tried some combinations of vex2difx, startdifx, and difx2mark4 from 2.6.3 and 2.8.1 with –override-version but so far no solution.

Simon C at OSO offered to look into the matter a bit more with the data we have, so I’ll await his findings and then maybe ask here again for additional advice.

Kind regards
Eskil

From: Adam Deller <adeller at astro.swin.edu.au>
Date: Sunday, 13 August 2023 at 13:03
To: Eskil Varenius <eskil.varenius at chalmers.se>
Cc: difx-users at nrao.edu <difx-users at nrao.edu>
Subject: Re: [Difx-users] Lack of mixed-mode fringes with 2.6.3, then segfault with 2.8.1? {External}
Hey Eskil,

Have you tried vex2difx -v -v and/or valgrind vex2difx to see where the segfault is occurring?  Obviously, it shouldn't be segfaulting, so presumably finding that and fixing it should go a fair way towards solving the problem.

Cheers,
Adam

On Sat, 12 Aug 2023 at 04:22, Eskil Varenius via Difx-users <difx-users at listmgr.nrao.edu<mailto:difx-users at listmgr.nrao.edu>> wrote:
Dear Difx-users,

Background: I have mixed-mode continuum data from 3 stations: Ns(16 MHz VDIF), Ny(16 MHz M5B), Nn 32 MHz VDIF). Ns and Ny observe standard S/X with R-pol only, and Nn (VGOS) observe dual linear X/Y (strictly speaking H/V but I'm using X/Y here since that's the VEX labels). Correlating Ns-Ny baseline with DiFX 2.6.3, is all good. I use zoom windows (to get 16 MHz from all stations) and FreqId selection (to only correlate X-band and skip S-band channels). But when trying to correlate all three stations I run into problems.

Problem 1: With DiFX 2.6.3 I can correlate all three stations, and then run difx2mark4. Fourfit then gives me nice fringes on the Ns-Ny baseline (R-R pol), and the Ns-Nn baseline (RX and RY pol). But while I do get nice XR fringes on the Nn-Ny baseline, I do not get any YR fringes on this baseline. I know the data are good, since I get fringes to these polarisations on another baseline. So I suspect I have fallen into the "Warning: Limited support for mixed-mode..." here, or do you have other suggestions?

I saw there are improvements for mixed-mode in DiFX 2.8.1, so I decided to try it, but:

Problem 2: With DiFX 2.8.1, I get "segfault" when I run ”vex2difx” (this worked with 2.6.3). Any ideas?

I guess hunting the "segfault in 2.8.1" is the most profitable route for the future? If so, please let me know what info you may need to speculate further. I am happy to share output, v2d, vex, and even raw data for one scan for someone to reproduce this if you would be so kind to help.

I am very puzzled by this!

Kind regards
Eskil
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Prof. Adam Deller
Centre for Astrophysics & Supercomputing
Swinburne University of Technology
John St, Hawthorn VIC 3122 Australia
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