Qt Build System Glossary

To be on the same page when talking about Qt build system related topics, we're offering here a collection of used terms.

Build System

The build system of Qt is the collection of files that describe how to build Qt from source.

This includes, among others, the project files and all configure-related files.

It does not include QMake, CMake, make or ninja.

Build Tool

In the widest sense, those are tools that contribute to the build.

Common build tools are ninja, make, nmake and jom.

We can count build system generators to the the build tools.

Build System Generator

Build system generators are tools that take a high-level project description as input (e.g. a qmake project file) and output a lower level build system (e.g. a Makefile). Examples are QMake, CMake, gyp or gn.

In-source Build

This means that your build directory is the same as your source directory.

$ cd qt-source-dir
$ ./configure
$ cmake --build .
$ cmake --install .

Out-of-source Build

This means that your build directory is outside your source directory.

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure
$ cmake --build .
$ cmake --install .

This has the advantage that you can have multiple builds of the same source.

Also, it's very easy to do a fresh build by purging your build directory.

Shadow Build

This is just another name for out-of-source build.

Developer Build

A developer build of Qt is meant for developers of Qt.

It differs from a non-developer build in two details:

• It's a non-prefix build if the -prefix configure option is omitted. See below for non-prefix builds.
• It enables private tests. Those are autotests that require symbols to be exported that would be hidden under normal circumstances.

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -developer-build
$ cmake --build .

It's perfectly possible to have a developer prefix build:

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -developer-build -prefix ~/Qt
$ cmake --build .
$ cmake --install .

Prefix Build

This means that you configured Qt with an installation prefix.

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -prefix /opt/qt6
$ cmake --build .
$ cmake --install .

Prefix builds require that you install after building.

Note that, by default, configure uses /usr/local as installation prefix. Unless -developer-build is specified. See below.

Non-prefix Build

This means that you configured Qt without an installation prefix.

This is only possible for developer builds.

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -developer-build
$ cmake --build .

There's no point in installing a non-prefix build.

Technically, the install prefix is set to the build dir, and care is taken by the build system that the necessary files are copied/built into the right place.

Top-level Build

As you probably know, Qt is split into several repositories: qtbase, qtdeclarative, qttools and so on. There's also a "super" repository called "qt5" that bundles everything as git submodules.

A top-level build means building the "super" repository.

Dependencies between the single repositories are automatically resolved by the build system.

Per-repository Build

This is building Qt's single repositories one by one.

Dependencies have to be resolved by the user doing the build.

Per-repository builds can be configured as either prefix or non-prefix builds.

An out-of-source non-prefix build example:

$ mkdir qtbase-build-dir
$ cd qtbase-build-dir
$ ../qtbase-source-dir/configure -developer-build
$ cmake --build .
$ cd ..
$ mkdir qtdeclarative-build-dir
$ cd qtdeclarative-build-dir
$ ../qtbase-build-dir/bin/qt-configure-module ../qtdeclarative
$ cmake --build .
# Repeat for other needed repositories

An out-of-source prefix build example:

$ mkdir qtbase-build-dir
$ cd qtbase-build-dir
$ ../qtbase-source-dir/configure -prefix /opt/qt6
$ cmake --build .
$ cmake --install .
$ cd ..
$ mkdir qtdeclarative-build-dir
$ cd qtdeclarative-build-dir
$ /opt/qt6/bin/qt-configure-module ../qtdeclarative
$ cmake --build .
$ cmake --install .
# Repeat for other needed repositories

Qt tests

Starting with Qt 6, Qt's tests can be built in a few ways:

• all tests defined in a repository are built in-tree with the Qt build directory (henceforth called 'in-tree')
• all tests defined in a repository are built in a standalone build directory (called 'standalone tests')
• one specific test is built in a standalone build directory (called 'standalone test')

In-tree tests

To build tests in-tree with the Qt build itself, you need to pass the following configure argument when configuring Qt

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -make tests
$ cmake --build .
$ ctest # to run the tests

The advantage of this approach is that you can build and run the tests from the same build directory as your Qt build directory. The disadvantage is that configuration time and total build time will be longer.

Standalone tests

To build standalone tests in a separate build directory you first need to build at least qtbase, and then you need to use a special shell script provided by the qtbase installation:

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -developer-build
$ cmake --build .
$ cd ..
$ mkdir qtbase-standalone-tests
$ cd qtbase-standalone-tests
$ ../qt-build-dir/bin/qt-internal-configure-tests ../qt-source-dir
$ cmake --build .
$ ctest # to run the tests

The advantage of this approach is that you can build and run the tests from a separate build directory, without affecting overall Qt build times. This approach is what Qt's CI is currently using. The disadvantage is that you have to go through a few steps more than usual.

Standalone test

Sometimes you just want to build one single test, outside your Qt build directory. You can do that with a special 'qt-cmake-standalone' shell script.

$ mkdir qt-build-dir
$ cd qt-build-dir
$ ../qt-source-dir/configure -developer-build
$ cmake --build .
$ cd ..
$ mkdir tst_qwidget-build-dir
$ cd tst_qwidget-build-dir
$ ../qt-build-dir/bin/qt-cmake-standalone-test ../qt-source-dir/tests/auto/widgets/kernel/qwidget
$ cmake --build .
$ ctest # to run the test

The advantage of this approach is that the configuration time and build time of one test is very fast.