CERN ROOT on Windows

johanr · July 23, 2026, 12:30pm

Here is the build log after I applied Gemini’s fix. Everything else is the same as in build-log-6

bellenot · July 23, 2026, 12:39pm

It’s the configure log. Then you have to build:
cmake --build C:\Root\root-6.40.02-build --config Release

johanr · July 23, 2026, 12:46pm

I asked Gemini the follow-up question “Are you sure?”.
Here is the answer. It confirms the first answer but adds some details.

Gemini_answer-2.txt (4.4 KB)

bellenot · July 23, 2026, 12:47pm

OK, thanks

johanr · July 24, 2026, 6:41am

I tested handling Python your way, using PATH instead of -DPython3_EXECUTABLE.
But it made no difference. The original CMakeLists.txt still fails and the patched works.

bellenot · July 24, 2026, 6:49am

OK, but that allows you to go on, right? Or did I miss something?

johanr · July 24, 2026, 6:57am

I’m able to go on. No problem. But I thought it would be good to fix the underlying problem.

bellenot · July 24, 2026, 7:03am

Sure! I’ll take care of it, thanks!

johanr · July 24, 2026, 7:04am

If there is anything I can do to help you, do not hesitate to ask me.

bellenot · July 24, 2026, 7:18am

Thanks!

johanr · July 27, 2026, 10:59am

I’m making progress, but I’m not quite there yet. Here is what I did:

Downloaded and ran the the miniconda installer Miniconda3-py311_26.5.3-1-Windows-x86_64 from Index of /miniconda
Opened an Anaconda Prompt and ran the commands (are both needed?)
pip install jupyter
pip install notebook
Downloaded root_v6.40.02.source.tar.gz from Release 64002 - ROOT
Unzipped. Untarred. Moved and renamed as C:\Root\root-6.40.02-source.
Patched C:\Root\root-6.40.02-source\CMakeLists.txt with the Gemini patch
Opened a Developer Command Prompt for VS 2022 and ran the install script below.
The script finished without any errors after about 80 minutes.
Opened an Anaconda Prompt and ran the commands
call C:\Root\root-6.40.02\bin\thisroot.bat
root --notebook

Then I got the error message
Error starting ROOT notebook – please check that Jupyter is installed

Here is my install script:

set SOURCE_DIR=C:/Root/root-6.40.02-source
set BUILD_DIR=C:/Root/root-6.40.02-build
set INSTALL_DIR=C:/Root/root-6.40.02
set PYTHON_DIR=C:/Python/miniconda3
if exist “%BUILD_DIR%” rmdir /s /q “%BUILD_DIR%”
cmake -S “%SOURCE_DIR%” -B “%BUILD_DIR%” -G “Visual Studio 17 2022” -A x64 -T host=x64 -Wno-author ^
-DPython3_EXECUTABLE=“%PYTHON_DIR%/python.exe” ^
-Dbuiltin_freetype=ON -Dbuiltin_ftgl=ON -Dbuiltin_gif=ON -Dbuiltin_gl2ps=ON -Dbuiltin_jpeg=ON ^
-Dbuiltin_png=ON -Dbuiltin_gtest=ON -Dbuiltin_lzma=ON -Dbuiltin_lz4=ON -Dbuiltin_pcre=ON ^
-Dbuiltin_tiff=ON -Dbuiltin_unuran=ON -Dbuiltin_xxhash=ON -Dbuiltin_zlib=ON -Dbuiltin_zstd=ON
cmake --build “%BUILD_DIR%” --config Release
cmake --install “%BUILD_DIR%” --prefix “%INSTALL_DIR%”

bellenot · July 27, 2026, 11:17am

You must build ROOT in the environment where the jupyter notebook can be found.

johanr · July 28, 2026, 9:13am

I tried building ROOT from an Anaconda Prompt. That should have the required environment. The build ran without any errors, but when I tried to start the notebook I got the same error as before.

Here are the details of what I did. I have also attached a complete log and the environment in the Anaconda Prompt.

Install Python
- Download and run the the miniconda installer Miniconda3-py311_26.5.3-1-Windows-x86_64 from Index of /miniconda . Make it the default Python installation but do not add to PATH.
- Open an Anaconda Prompt an run the command
  pip install notebook
Install ROOT
- Download, unzip and untar root_v6.40.02.source.tar.gz from Release 64002 - ROOT
- Move and rename root-6.40.02 as C:\Root\root-6.40.02-source .
- Apply the Gemini patch.
- Open an Anaconda Prompt and run the script below.
Launch Jupyter notebook with ROOT kernels
- Open an Anaconda Command Prompt and run the commands
  call C:\Root\root-6.40.02\bin\thisroot.bat
  root --notebook

Configure-build-install script:

set SOURCE_DIR=C:/Root/root-6.40.02-source
set BUILD_DIR=C:/Root/root-6.40.02-build
set INSTALL_DIR=C:/Root/root-6.40.02
if exist “%BUILD_DIR%” rmdir /s /q “%BUILD_DIR%”

cmake -S “%SOURCE_DIR%” -B “%BUILD_DIR%” -G “Visual Studio 17 2022” -A x64 -T host=x64 -Wno-author ^
-Dbuiltin_freetype=ON -Dbuiltin_ftgl=ON -Dbuiltin_gif=ON -Dbuiltin_gl2ps=ON -Dbuiltin_jpeg=ON ^
-Dbuiltin_png=ON -Dbuiltin_gtest=ON -Dbuiltin_lzma=ON -Dbuiltin_lz4=ON -Dbuiltin_pcre=ON ^
-Dbuiltin_tiff=ON -Dbuiltin_unuran=ON -Dbuiltin_xxhash=ON -Dbuiltin_zlib=ON -Dbuiltin_zstd=ON
cmake --build “%BUILD_DIR%” --config Release
cmake --install “%BUILD_DIR%” --prefix “%INSTALL_DIR%”

Environment.txt (2.3 KB)

Root-log.txt (1.5 MB)

bellenot · July 28, 2026, 10:47am

So it’s maybe an issue with Anaconda… Building ROOT (master) with the exact same config parameters, using Python and notebook from the system, works just fine for me…

C:\root-dev\build\x64\test>root --notebook

C:\root-dev\build\x64\test>[I 2026-07-28 12:43:25.317 ServerApp] jupyter_lsp | extension was successfully linked.
[I 2026-07-28 12:43:25.321 ServerApp] jupyter_server_terminals | extension was successfully linked.
[I 2026-07-28 12:43:25.325 ServerApp] jupyterlab | extension was successfully linked.
[W 2026-07-28 12:43:25.327 JupyterNotebookApp] 'extra_static_paths' was found in both NotebookApp and ServerApp. This is likely a recent change. This config will only be set in NotebookApp. Please check if you should also config these traits in ServerApp for your purpose.
[I 2026-07-28 12:43:25.329 ServerApp] notebook | extension was successfully linked.
[I 2026-07-28 12:43:26.326 ServerApp] notebook_shim | extension was successfully linked.
[I 2026-07-28 12:43:26.388 ServerApp] notebook_shim | extension was successfully loaded.
[I 2026-07-28 12:43:26.391 ServerApp] jupyter_lsp | extension was successfully loaded.
[I 2026-07-28 12:43:26.392 ServerApp] jupyter_server_terminals | extension was successfully loaded.
[I 2026-07-28 12:43:26.398 LabApp] JupyterLab extension loaded from C:\Python314\Lib\site-packages\jupyterlab
[I 2026-07-28 12:43:26.398 LabApp] JupyterLab application directory is C:\Python314\share\jupyter\lab
[I 2026-07-28 12:43:26.399 LabApp] Extension Manager is 'pypi'.
[I 2026-07-28 12:43:26.497 ServerApp] jupyterlab | extension was successfully loaded.
[I 2026-07-28 12:43:26.501 ServerApp] notebook | extension was successfully loaded.
[I 2026-07-28 12:43:26.503 ServerApp] Serving notebooks from local directory: C:\root-dev\build\x64\test
[I 2026-07-28 12:43:26.503 ServerApp] Jupyter Server 2.20.0 is running at:
[I 2026-07-28 12:43:26.503 ServerApp] http://localhost:8888/tree?token=3ff36ef3c311726f2b62eee57e3c5b866fe70054118afe33
[I 2026-07-28 12:43:26.503 ServerApp]     http://127.0.0.1:8888/tree?token=3ff36ef3c311726f2b62eee57e3c5b866fe70054118afe33
[I 2026-07-28 12:43:26.503 ServerApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[C 2026-07-28 12:43:26.547 ServerApp]

bellenot · July 28, 2026, 11:59am

And BTW, see [cmake] Quote list-valued cache variable in root-config generation · root-project/root@227e075 · GitHub
and [cmake] Quote list-valued cache variable in root-config generation · root-project/root@1cc376e · GitHub

johanr · July 28, 2026, 1:08pm

So you are not using Anaconda? What Python distribution do you use?

bellenot · July 28, 2026, 1:10pm

I simply installed Python (3.14) on my computer (in C:\Python314)

johanr · July 29, 2026, 9:27am

I tried to do it exactly the way you do it: I downloaded Python from python.org, installed it in C:\Python314 and added Python to PATH.
Now the command root --notebook succeeded and I could create a notebook. But when I selected the ROOT C++ kernel, then I got the error message

The ‘root’ kernel could not be started: ‘[WinError 2] The system cannot find the file specified’

I have attached the output from the console. At the top you find the environment.

Log.txt (22.7 KB)

bellenot · July 29, 2026, 11:00am

[E 2026-07-29 11:18:13.317 ServerApp] Failed to run command:
    ['python3.11', '-m', 'ROOT._jupyroot.kernel.rootkernel', '-f', 'C:\\Users\\johan\\AppData\\Roaming\\jupyter\\runtime\\kernel-cef46120-6325-4ad3-b115-4b43d3704ac9.json']
        PATH='C:/Root/root-6.40.02/bin;C:/Root/root-6.40.02/bin/bin;C:\\Root\\root-6.40.02\\bin;C:\\Root\\root-6.40.02\\bin;C:\\Root\\root-6.40.02\\bin;C:\\Python314\\Scripts\\;C:\\Python314\\;C:\\Windows\\system32;C:\\Windows;C:\\Windows\\System32\\Wbem;C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\;C:\\Windows\\System32\\OpenSSH\\;C:\\Program Files (x86)\\NVIDIA Corporation\\PhysX\\Common;C:\\Program Files\\Git\\cmd;C:\\Program Files\\dotnet\\;C:\\WINDOWS\\system32;C:\\WINDOWS;C:\\WINDOWS\\System32\\Wbem;C:\\WINDOWS\\System32\\WindowsPowerShell\\v1.0\\;C:\\WINDOWS\\System32\\OpenSSH\\;C:\\Program Files\\NVIDIA Corporation\\NVIDIA app\\NvDLISR;C:\\Program Files (x86)\\Windows Kits\\10\\Windows Performance Toolkit\\;C:\\Program Files\\TortoiseGit\\bin;C:\\Program Files\\CMake\\bin;C:\\Users\\johan\\AppData\\Local\\Microsoft\\WindowsApps;C:\\Users\\johan\\AppData\\Local\\Programs\\Microsoft VS Code\\bin;'

'python3.11' ???
And you still have:

PYTHON_DIR=C:\Users\johan\anaconda3

in your environment…

johanr · July 30, 2026, 7:18am

I have tried to purge my system (filesystem, environment) of all traces of other Python installations than 3.14.
I have deleted the ROOT build and install directories and rebuilt ROOT from sources.
But I still get the same error when selecting the ROOT C++ kernel:

The ‘root’ kernel could not be started: ‘[WinError 2] The system cannot find the file specified’

And the console output still contains

Failed to run command: [‘python3.11’, ‘-m’, ‘ROOT._jupyroot.kernel.rootkernel’,‘-f’, ‘C:\Users\johan\AppData\Roaming\jupyter\runtime\kernel-28ce1bde-bd99-4381-ae0d-8877598fd496.json’]

I have attached the console output.

Log.txt (15.7 KB)