# Possible VTKHDF bug
**URL:** https://discourse.vtk.org/t/possible-vtkhdf-bug/16255
**Category:** Development
**Tags:** file-formats
**Created:** [January 26, 2026, 3:35pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255 "2026-01-26T15:35:54Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@nncarlson](https://discourse.vtk.org/u/nncarlson)
#### Post date: [January 26, 2026, 3:35pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/1 "2026-01-26T15:35:54Z")
I’ve developed a VTKHDF writer for our code that is working for small development test cases, but which is causing errors in paraview for large application-like cases. I’m generating a MultiBlockDataSet file with 4 blocks. Each block is an UnstructuredGrid with multiple parts written by the individual MPI ranks. One possibly significant feature of this case is that many MPI ranks do not contain a piece of all blocks, so that there are lots of 0-sized parts in the different blocks.
If I only write the mesh (no cell or point arrays), the generated file loads into paraview without error and displays exactly what it should. I can toggle on/off the various blocks using the MultiBlock inspector panel and that does exactly as I expect, however after doing this 6 or 8 times paraview segfaults without any error message.
If I add a scalar cell array to the output I get the following error from paraview:
> vtkDataSet.cxx:757 WARN| vtkUnstructuredGrid (0x37b43410): Cell array FUBAR with 1 components, has 0 tuples but there are only -1 cells.
Despite this the cell array displays exactly as expected.
Any thoughts about what might be going wrong? I’ll be looking into working up some smaller cases that show the same problems.
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### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [January 26, 2026, 3:45pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/2 "2026-01-26T15:45:03Z")
Please share an example data.
FYI @lgivord @Louis_Gombert
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### Author:  [@Louis\_Gombert](https://discourse.vtk.org/u/Louis_Gombert)
#### Post date: [January 26, 2026, 3:51pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/3 "2026-01-26T15:51:45Z")
Can you please try pushing a single 0 value in your `Offsets` array when there are no cells?
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### Author:  [@nncarlson](https://discourse.vtk.org/u/nncarlson)
#### Post date: [January 26, 2026, 4:15pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/4 "2026-01-26T16:15:07Z")
I checked, and I am already doing that as the size of `Offsets` is supposed to be 1 greater than the number of cells. The `NumberOf…`are also 0, and no data is written to the `Connectivity`, `Types`, and `Points` arrays.
Here’s a google drive [download link](https://drive.google.com/file/d/1ncrYxquJTZpDvyTXPF_n08vvcwS_r26g/view?usp=sharing) to my current case; it’s probably larger (10M) than you’d like to see uploaded here. If I can find a small test case I’ll upload it here.
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### Author:  [@nncarlson](https://discourse.vtk.org/u/nncarlson)
#### Post date: [January 27, 2026, 5:01am UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/5 "2026-01-27T05:01:26Z")
Here’s a tiny example. 2 blocks, 1 hex cell in each block, 2 partitions (mpi ranks) for each block, one of which is 0-sized. There is also a cell array. This one doesn’t reproduce the “only -1 cells” error message, but for me it will cause paraview to segfault if the blocks are toggled off/on too many times.
[demo.vtkhdf](https://discourse.vtk.org/uploads/short-url/v31Vew821IXQHBIItCmKsD9m86Z.vtkhdf) (14.8 KB)
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### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [January 27, 2026, 8:24am UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/6 "2026-01-27T08:24:07Z")
Confirmed:
```auto
Thread 1 "paraview" received signal SIGSEGV, Segmentation fault.
0x00007fffe456db40 in ?? () from /usr/lib/libc.so.6
(gdb) bt
#0 0x00007fffe456db40 in ?? () from /usr/lib/libc.so.6
#1 0x00007fffe813ff9a in std::__fill_a1 (__first=0x0, __last=0xffffffffffffffff ,__ x=@0x7fffffffa244: 1 '\001') at /usr/include/c++/15.2.1/bits/stl_algobase.h:951
#2 0x00007fffe80bc6a7 in std::__fill_a (__first=0x0, __last=0xffffffffffffffff ,__ value=@0x7fffffffa244: 1 '\001') at /usr/include/c++/15.2.1/bits/stl_algobase.h:979
#3 std::fill ( __first=0x0,__ last=0xffffffffffffffff , __value=) at /usr/include/c++/15.2.1/bits/stl_algobase.h:1011
#4 vtkAOSDataArrayTemplate::FillValue (this=0x5555604a1220, value=1 '\001') at /home/glow/dev/paraview/pv1/src/VTK/Common/Core/vtkAOSDataArrayTemplate.txx:362
#5 0x00007fffe1242172 in vtkBlockSelector::ComputeSelectedElements (this=0x555560497390, insidednessArray=0x5555604a1220) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkBlockSelector.cxx:125
#6 0x00007fffe13cc439 in vtkSelector::ProcessBlock (this=0x555560497390, inputBlock=0x7fffc00110d0, outputBlock=0x7fffc00070e0, forceFalse=false) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkSelector.cxx:50
#7 0x00007fffe13cceae in vtkSelector::ProcessDataObjectTree (this=0x555560497390, input=0x55555bce51f0, output=0x55556049c8a0, mode=vtkSelector::INCLUDE, compositeIndex=1) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkSelector.cxx:203
#8 0x00007fffe13cce8e in vtkSelector::ProcessDataObjectTree (this=0x555560497390, input=0x55555bc88ef0, output=0x55556102ef10, mode=vtkSelector::EXCLUDE, compositeIndex=0) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkSelector.cxx:198
#9 0x00007fffe13cc719 in vtkSelector::Execute (this=0x555560497390, input=0x55555bc88ef0, output=0x55556102ef10) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkSelector.cxx:96
#10 0x00007fffe12420d5 in vtkBlockSelector::Execute (this=0x555560497390, input=0x55555bc88ef0, output=0x55556102ef10) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkBlockSelector.cxx:118
#11 0x00007fffe1341792 in vtkExtractSelection::RequestData (this=0x55555ca738d0, inputVector=0x55555cbc0bc0, outputVector=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Filters/Extraction/vtkExtractSelection.cxx:368
#12 0x00007fffedcaddab in vtkPVExtractSelection::RequestData (this=0x55555ca738d0, request=0x555560474160, inputVector=0x55555cbc0bc0, outputVector=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTKExtensions/Extraction/vtkPVExtractSelection.cxx:116
#13 0x00007fffeee86da6 in vtkDataObjectAlgorithm::ProcessRequest (this=0x55555ca738d0, request=0x555560474160, inputVector=0x55555cbc0bc0, outputVector=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkDataObjectAlgorithm.cxx:75
#14 0x00007fffeee97f08 in vtkExecutive::CallAlgorithm (this=0x55555cb5bbe0, request=0x555560474160, direction=1, inInfo=0x55555cbc0bc0, outInfo=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkExecutive.cxx:723
#15 0x00007fffeee89b11 in vtkDemandDrivenPipeline::ExecuteData (this=0x55555cb5bbe0, request=0x555560474160, inInfo=0x55555cbc0bc0, outInfo=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkDemandDrivenPipeline.cxx:447
#16 0x00007fffeee7d075 in vtkCompositeDataPipeline::ExecuteData (this=0x55555cb5bbe0, request=0x555560474160, inInfoVec=0x55555cbc0bc0, outInfoVec=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkCompositeDataPipeline.cxx:151
#17 0x00007fffeee89069 in vtkDemandDrivenPipeline::ProcessRequest (this=0x55555cb5bbe0, request=0x555560474160, inInfoVec=0x55555cbc0bc0, outInfoVec=0x55555cb5bcf0) at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkDemandDrivenPipeline.cxx:246
#18 0x00007fffeefeb56f in vtkStreamingDemandDrivenPipeline::ProcessRequest (this=0x55555cb5bbe0, request=0x555560474160, inInfoVec=0x55555cbc0bc0, outInfoVec=0x55555cb5bcf0)
at /home/glow/dev/paraview/pv1/src/VTK/Common/ExecutionModel/vtkStreamingDemandDrivenPipeline.cxx:331
```
Something is wrong in the data itself, not sure what yet, please open an issue @nncarlson: [https://gitlab.kitware.com/vtk/vtk/-/issues](https://gitlab.kitware.com/vtk/vtk/-/issues)
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### Author:  [@nncarlson](https://discourse.vtk.org/u/nncarlson)
#### Post date: [January 27, 2026, 3:11pm UTC](https://discourse.vtk.org/t/possible-vtkhdf-bug/16255/7 "2026-01-27T15:11:27Z")
Done: [https://gitlab.kitware.com/vtk/vtk/-/issues/19923](https://gitlab.kitware.com/vtk/vtk/-/issues/19923). Thanks for the quick feedback!