Qt5 custom build for amd64

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.

First step, build Qt5 5.15 packages for amd64.

To prevent conflicting with Debian Qt packages, we'll install everything in /opt.

We can install qtchooser configuration files to allow developers to easily switch between Debian's standard Qt version or the custom version, at will.

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-amd64 qt-everywhere-src-5.15.0/debian

If needed, install the Qt license:

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

Install build dependencies

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

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

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

apt install devscripts
mk-build-deps debian-amd64/control
apt -f install qt-everywhere-src-build-deps_5.15.0-1_amd64.deb

Package build

The package is built by debian/rules base on the excellent work done by Debian Qt5 maintainers

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.

Build output

Building sources generates 4 packages:

qtbase5custom-dev and libqt5custom are needed for development; only libqt5custom is needed to run build programs.

Using custom Qt for amd64

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

One needs to be careful not to create programs that link, either directly or indirectly, with both the Debian Qt and the custom 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

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

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

Building software using custom Qt

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

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


export QT_SELECT=qt5custom
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.