# Mac extension builds failing with cout error
**URL:** https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685
**Category:** Development
**Tags:** extensions
**Created:** [April 8, 2026, 2:18pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685 "2026-04-08T14:18:20Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@pieper](https://discourse.slicer.org/u/pieper)
#### Post date: [April 8, 2026, 2:18pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/1 "2026-04-08T14:18:20Z")
We noticed SlicerDMRI and other mac builds today. Looks like python extensions are okay but not C++ ones.
Maybe related to the [clipboard locale change](https://discourse.slicer.org/t/2026-04-07-weekly-meeting/46673/2#p-133052-failing-macos-tests-sam-2)? @Sam_Horvath @ebrahim ?
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 2:38pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/2 "2026-04-08T14:38:10Z")
DMRI is failing across all platforms and has been since before the locale change
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 2:39pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/3 "2026-04-08T14:39:19Z")
Other C++ builds, such as SlicerIGT are fine
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 2:42pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/4 "2026-04-08T14:42:35Z")
C++ builds begin to fail after: [ENH: Add support for building against VTK version 9.6.0 · Slicer/Slicer@1b21266 · GitHub](https://github.com/Slicer/Slicer/commit/1b21266bf47f126ece58d12b87160e38f55ad258)
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### Author:  [@pieper](https://discourse.slicer.org/u/pieper)
#### Post date: [April 8, 2026, 2:45pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/5 "2026-04-08T14:45:51Z")
Thanks @Sam_Horvath !
@jamesobutler do you see why this change would be breaking some extensions? Maybe we can revert until the underlying issue is understood and fixed?
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 2:52pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/6 "2026-04-08T14:52:35Z")
Looking at the errors, many/most are complaining about `cout` missing from `std`. Looking at a file that has errors in SlicerDMRI: [SlicerDMRI/Libs/vtkDMRI/vtkSeedTracts.cxx at master · SlicerDMRI/SlicerDMRI · GitHub](https://github.com/SlicerDMRI/SlicerDMRI/blob/master/Libs/vtkDMRI/vtkSeedTracts.cxx), `` is not included but `cout` is used, so perhaps a transitive include was cleaned up.
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### Author:  [@pieper](https://discourse.slicer.org/u/pieper)
#### Post date: [April 8, 2026, 3:00pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/7 "2026-04-08T15:00:50Z")
Yes, that makes sense @Sam_Horvath - maybe we should add that include to a core slicer header to fix all the extensions that relied on that at once and avoid the backwards incompatibility.
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 3:11pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/8 "2026-04-08T15:11:45Z")
I am not sure how much of the problem that would actually fix? The `vtkSeedTracts` one I linked above is a pure vtkObject, it doesn’t include any Slicer headers.
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### Author:  [@jamesobutler](https://discourse.slicer.org/u/jamesobutler)
#### Post date: [April 8, 2026, 3:16pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/9 "2026-04-08T15:16:37Z")
The related items is a simple fix.
See the following comment:
>
>
> Slicer extension compatibility based on https://github.com/Slicer/Slicer/commit/…cf471ffd35bd45705a15557001db1aa9e7d263e4:
> \- \[x\] https://github.com/lassoan/SlicerSegmentEditorExtraEffects/pull/69
> \- \[x\] https://github.com/IGSIO/IGSIO/pull/44
> \- \[x\] https://github.com/IGSIO/SlicerIGSIO/pull/33
and the following for reference:
> **[Important Update: Standard iostream availability from VTK](https://discourse.vtk.org/t/important-update-standard-iostream-availability-from-vtk/16171)**
>
> Please note a significant change in VTK. In the changeset from Merge Request !11993, the inclusion of has been removed from a core header. This change previously caused global constructors to be triggered in every downstream translation unit. ...
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 3:38pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/10 "2026-04-08T15:38:54Z")
Thanks James!
Steve, I don’t think we can avoid doing the fixes at the extension level.
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### Author:  [@pieper](https://discourse.slicer.org/u/pieper)
#### Post date: [April 8, 2026, 3:47pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/11 "2026-04-08T15:47:13Z")
Yes, that’s very clear now, thanks!
Odd to make a backwards incompatible change on a minor release, but that’s the way it goes I guess.
I’ll try to fix SlicerDMRI later today.
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 8, 2026, 3:52pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/12 "2026-04-08T15:52:06Z")
I’ll get SlicerRT today as well
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### Author:  [@pieper](https://discourse.slicer.org/u/pieper)
#### Post date: [April 8, 2026, 10:39pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/13 "2026-04-08T22:39:41Z")
Okay, SlicerDMRI should be good to go tomorrow.
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 10, 2026, 3:04pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/14 "2026-04-10T15:04:29Z")
Fix log (unchecked are in progress):
SlicerDMRI
SlicerRT : [Iostream missing include fixes by sjh26 · Pull Request #314 · SlicerRt/SlicerRT · GitHub](https://github.com/SlicerRt/SlicerRT/pull/314)
MeshToLabelMap
SlicerVMTK: Pending this vmtk fix, [Include iostream to build against VTK 9.6 by chir-set · Pull Request #481 · vmtk/vmtk · GitHub](https://github.com/vmtk/vmtk/pull/481), my branch here: [GitHub - sjh26/SlicerExtension-VMTK at fix-iostream · GitHub](https://github.com/sjh26/SlicerExtension-VMTK/tree/fix-iostream)
VASSTAlgorithms: [BUG: Add missing #include \ to C++ files using cerr/endl by sjh26 · Pull Request #5 · VASST/VASSTAlgorithms · GitHub](https://github.com/VASST/VASSTAlgorithms/pull/5)
SlicerAutoscoperM: Autoscoper PR: [COMP: Fix build with VTK 9.6 by sjh26 · Pull Request #328 · BrownBiomechanics/Autoscoper · GitHub](https://github.com/BrownBiomechanics/Autoscoper/pull/328)
CarreraSlice: [COMP: Fix iostream include hygiene across C++ sources (VTK 9.6 compat) by sjh26 · Pull Request #14 · ikolesov/CarreraSlice · GitHub](https://github.com/ikolesov/CarreraSlice/pull/14)
CMFreg: [COMP: VTK 9.6 build fix - iostream include hygiene by sjh26 · Pull Request #41 · DCBIA-OrthoLab/CMFreg · GitHub](https://github.com/DCBIA-OrthoLab/CMFreg/pull/41)
PortPlacement: [COMP: VTK 9.6 build fix - add missing \ include by sjh26 · Pull Request #4 · SlicerIGT/PortPlacement · GitHub](https://github.com/SlicerIGT/PortPlacement/pull/4)
ShapeVariationAnalyzer: [COMP: VTK 9.6 Build Fix - Fix iostream include hygiene by sjh26 · Pull Request #72 · DCBIA-OrthoLab/ShapeVariationAnalyzer · GitHub](https://github.com/DCBIA-OrthoLab/ShapeVariationAnalyzer/pull/72)
SlicerFreeSurfer: [COMP: VTK 9.6 Build Fix - Fix iostream include hygiene by sjh26 · Pull Request #22 · PerkLab/SlicerFreeSurfer · GitHub](https://github.com/PerkLab/SlicerFreeSurfer/pull/22)
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### Author:  [@Sam\_Horvath](https://discourse.slicer.org/u/Sam_Horvath)
#### Post date: [April 10, 2026, 7:20pm UTC](https://discourse.slicer.org/t/mac-extension-builds-failing-with-cout-error/46685/15 "2026-04-10T19:20:50Z")
This appears to be everything for current failures caused by VTK 9.6 transition