This is part of a series of posts on compiling a custom version of Qt5 in order to develop for both amd64 and a Raspberry Pi.

I split building Qt5 for armhf development in two parts: one cross-build environment to be installed on amd64 develpment systems, and a runtime part to be installed on the target armhf hardware.

Building a Qt5 cross-building environment builds a mix of armhf and amd64 binares: the amd64 tools to use for cross-building, like moc, qmake, plugins for Qt Creator, and so on; armhf headers and libraries to use at cross-build time; armhf shared libraries to use at runtime.

The procedure I came up with builds a devel package for amd64 development machines, which contains everything, and a second package that extracts from it only what is needed at runtime.

The cross-build environment is coinstallable both with the version of Qt distributed with Debian, and with the amd64 custom Qt development package.

The current build is sadly using -skip qtwebengine, because I have had no success so far getting QtWebEngine to compile as part of a cross-build Qt setup (the last road bump I can't overcome is nss and nspr not being coinstallable on amd64 and armhf, while both seem to be needed for it).

The resulting packaging is at https://github.com/Truelite/qt5custom.

Set up sources

Open the source tarball, and add the amd64 packaging:

tar axf qt-everywhere-src-5.15.0.tar.xz
cp -a debian-cross qt-everywhere-src-5.15.0/debian

If needed, install the Qt license:

cp qt-license.txt ~/.qt-license

If debugging information are not needed in armhf development, remove --no-strip from ./configure invocation in the rules file, to build significantly smaller .deb packages.

Install build dependencies

Install cross-compilers:

dpkg --add-architecture armhf
apt install crossbuild-essential-armhf

You can use apt build-dep to install dependencies manually:

cd qt-everywhere-src-5.15.0
apt build-dep .
apt -a armhf build-dep .

Alternatively, you can create installable .deb metapackages that depends on the build dependencies:

apt install devscripts
mk-build-deps --host-arch amd64 debian-cross/control
mk-build-deps --host-arch armhf debian-cross/control
apt -f install qt-everywhere-cross-build-deps_5.15.0-1_amd64.deb qt-everywhere-cross-cross-build-deps_5.15.0-1_armhf.deb

Note that there are two sets of dependencies: one of amd64 packages, and one of armhf packages.

Building the cross-build environment

After installing the build dependencies, you can build like this:

cd qt-everywhere-src-5.15.0
fakeroot debian/rules binary

In debian/rules you can configure NUMJOBS with the number of available CPUs in the machine, to have parallel builds.

This will build a package with the cross-build development environment for amd64, called qtbase5custom-armhf-dev

Building the runtime environment

To generate the runtime package for armhf, one needs to have the cross-build package (qtbase5custom-armhf-dev) installed in the system together with its build dependencies.

At that point, the armhf runtime package can be built using the debian-armhf directory without further sources:

apt install crossbuild-essential-armhf debhelper qtbase5custom-armhf-dev*_amd64.deb qt-everywhere-src-cross-build-deps*_armhf.deb
mkdir runtime
cp -a debian-armhf runtime/debian
cd runtime
dpkg-buildpackage -a armhf

Building the runtime environment generates:

• a libqt5custom package for armhf, installable on the target devices, containing the runtime Qt libraries and depending on the packages that they need to run;
• a libqt5custom-dbgym package for armhf with debugging symbols, to use for debugging on the target hardware.

If, while generating the cross-build environment, --no-strip was removed, the libqtcustom-dbgsym package with debugging symbols will not be generated.

Using the cross-build environment

These install their content in /opt, and are coninstallable with the version of Qt distributed in Debian, and with the custom Qt packages for amd64.

One needs to be careful not to create programs that link, either directly or indirectly, with more than one of these coinstalled Qt, because the in memory layout of objects could be different and incompatible, causing unexpected results.

Selecting which Qt version to use: qtchooser

These Qt custom packages integrate with qtchooser to select the version of Qt to use at compile time.

qtchooser --list-versions lists available versions. One can choose what to use by exporting QT_SELECT:

# apt install qtchooser qt5-qmake qt5-default
$ qtchooser --list-versions
4
5
qt4-x86_64-linux-gnu
qt4
qt5-x86_64-linux-gnu
qt5
qt5custom-x86_64-linux-gnu
qt5custom
qt5custom-armhf-x86_64-linux-gnu
qt5custom-armhf

$ qmake --version
QMake version 3.1
Using Qt version 5.11.3 in /usr/lib/x86_64-linux-gnu

$ export QT_SELECT=qt5custom-armhf
$ qmake --version
QMake version 3.1
Using Qt version 5.15.0 in /opt/qt5custom-armhf/lib

Cross-building software using custom Qt

One just needs to export QT_SELECT=qt5custom-armhf in the environment, then proceed to build normally:

export QT_SELECT=qt5custom-armhf
fakeroot ./debian/rules clean binary

Or:

export QT_SELECT=qt5custom-armhf
qmake file.pro

If switching from one Qt to another, it is possible that the makefiles created by one qmake are not working well with the other. In that case, one can just remove them and regenerate them.

The build result is ready to be copied into, and run in, the target armhf device.