# Build Error: \_ffi\_type\_longdouble symbol not found for arm64 during Python build

**URL:** <https://discourse.slicer.org/t/build-error-ffi-type-longdouble-symbol-not-found-for-arm64-during-python-build/45650>\
**Category:** Development\
**Created:** [January 1, 2026, 10:40am UTC](https://discourse.slicer.org/t/build-error-ffi-type-longdouble-symbol-not-found-for-arm64-during-python-build/45650 "2026-01-01T10:40:42Z")\
**Posts on this page:** 1\
**Showing post:** 6

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**Author:** ![jamesobutler](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/jamesobutler/32/7511_2.png) [@jamesobutler](https://discourse.slicer.org/u/jamesobutler)\
**Post date:** [January 6, 2026, 12:18pm UTC](https://discourse.slicer.org/t/build-error-ffi-type-longdouble-symbol-not-found-for-arm64-during-python-build/45650/6 "2026-01-06T12:18:38Z")

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As a first try you should let the Slicer super build build all the dependencies it wants including the python version which becomes an embedded python in the application. It will have to build an x86\_64 python for use in this x86\_64 application.

The following linked post was a good one that helped me build an x86\_64 build on arm mac successfully.

> [@Developing for Slicer on Apple Silicon (build targeting x86\_64)](https://discourse.slicer.org/t/developing-for-slicer-on-apple-silicon-build-targeting-x86-64/40686/6):
>
> This is really helpful @jamesobutler , kudos!

The Slicer factory build machine builds successfully on macOS with the following environment:

> [@Maintenance of macOS dashboard in progress - upgrading from Ventura to Sequoia](https://discourse.slicer.org/t/maintenance-of-macos-dashboard-in-progress-upgrading-from-ventura-to-sequoia/44950/4):
>
> Updates: XCode has been updated to 26.1 and clang version is now 17.0.0. Issue related to the build of Qt 5.15.18 should now all be sorted our and the build is in progress rocket For more details: [[5.15.18] Fix detection of OpenGL Qt backend and OpenSSL linkage on macOS by jcfr · Pull Request #80 · commontk/qt-easy-build · GitHub](https://github.com/commontk/qt-easy-build/pull/80)

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