Cross-compile to RPi WITHOUT (RE)COMPILE QT from source

Tibor Csecsei

Hi Everyone!

After a lot of unsuccessful tries I compiled qt 5.15.0 from source on Ubuntu 20.04 (WSL) for cross-compilation to RPi 3B+ (working howto: https://github.com/UvinduW/Cross-Compiling-Qt-for-Raspberry-Pi-4 - but I changed some steps for rpi3 of course).
The ./configure finished within minutes, but make -j4 took more than 20 hours (not minutes, hours) on a business class laptop (16GB ram, i7-7700HQ 2.8GHz CPU...).

Unfortunatelly every of my colleges have to rebuild qmake executable according to the built-in variables (ref.: https://doc.qt.io/qt-5/qmake-environment-reference.html) OR the folder structure have to be the same on every work-stations...

To avoid this, we started to try to change built-in variables, but the qmake.conf structure is not clean enough and qmake -set PROPERTY VALUE not works:

...@...:/mnt/c/_work/xcompile/rpi$ ./qt5.15_mod/bin/qmake -query
QT_SYSROOT:/mnt/c/_work/xcompile/rpi/sysroot
QT_INSTALL_PREFIX:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_PREFIX/dev:/usr/local/qt5.15
QT_INSTALL_ARCHDATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_ARCHDATA/dev:/usr/local/qt5.15
...
QT_HOST_PREFIX:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_HOST_DATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_HOST_BINS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/bin
QT_HOST_LIBS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/lib
QMAKE_SPEC:linux-g++
QMAKE_XSPEC:devices/linux-rasp-pi3-g++
QMAKE_VERSION:3.1
QT_VERSION:5.15.0

...@...:/mnt/c/_work/xcompile/rpi$ ./qt5.15_mod/bin/qmake -set QT_SYSROOT ../sysroot

...@...:/mnt/c/_work/xcompile/rpi$ ./qt5.15_mod/bin/qmake -query
QT_SYSROOT:../sysroot
QT_SYSROOT:/mnt/c/_work/xcompile/rpi/sysroot
QT_INSTALL_PREFIX:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_PREFIX/dev:/usr/local/qt5.15
QT_INSTALL_ARCHDATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_ARCHDATA/dev:/usr/local/qt5.15
...
QMAKE_SPEC:linux-g++
QMAKE_XSPEC:devices/linux-rasp-pi3-g++
QMAKE_VERSION:3.1
QT_VERSION:5.15.0

...@...:/mnt/c/_work/xcompile/rpi$ ./qt5.15_mod/bin/qmake -query QT_SYSROOT
/mnt/c/_work/xcompile/rpi/sysroot

QMake configuration reference says, the changed variables are stored in a QSettings object, but... how? Which qmake.conf files are used by qmake (I think the mkspec/device/... and the mkspec/linux... ones and the included ones)?

[Moreover: Why have to I compile qt? Libraries, runnables are available in the syncronized sysroot folder from RPi (if qt is installed on it) and/or inside the installed qt on the host computer (actually Ubuntu on WSL) and/or inside the cross-compile toolchain(s).
Can I override the qmake's built-in variables somehow (with a custom qmake.conf file or by using qmake -set ...)? Is it enough to cross-compile? If not, why not?]

Tibor Csecsei

I created a qt.conf file with the following:

[General]
QT_SYSROOT=xyz

[Paths]
QT_SYSROOT=xyz
Prefix=..

After that, qmake -query returns with:

QT_SYSROOT:
QT_INSTALL_PREFIX:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_ARCHDATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_DATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_DOCS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/doc
QT_INSTALL_HEADERS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/include
QT_INSTALL_LIBS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/lib
QT_INSTALL_LIBEXECS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/libexec
QT_INSTALL_BINS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/bin
QT_INSTALL_TESTS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/tests
QT_INSTALL_PLUGINS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/plugins
QT_INSTALL_IMPORTS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/imports
QT_INSTALL_QML:/mnt/c/_work/xcompile/rpi/qt5.15_mod/qml
QT_INSTALL_TRANSLATIONS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/translations
QT_INSTALL_CONFIGURATION:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_INSTALL_EXAMPLES:/mnt/c/_work/xcompile/rpi/qt5.15_mod/examples
QT_INSTALL_DEMOS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/examples
QT_HOST_PREFIX:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_HOST_DATA:/mnt/c/_work/xcompile/rpi/qt5.15_mod
QT_HOST_BINS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/bin
QT_HOST_LIBS:/mnt/c/_work/xcompile/rpi/qt5.15_mod/lib
QMAKE_SPEC:linux-g++
QMAKE_XSPEC:devices/linux-rasp-pi3-g++
QMAKE_VERSION:3.1
QT_VERSION:5.15.0

Makefile generation with qmake returs with:

Project MESSAGE: Qt uses sysroot from:

So QT_SYSROOT is really unset, Makefile contains empty sysroots like -sysroot= -WAll ....

Morix Dev

Hello Tibor,
I have the same exact problem here... Have you resolved somehow and you can share the solution with me?

Thanks + regards,
/Moris