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. This is also called in-tree build.

$ 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 the default 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 (or <build-dir>/qtbase for a top-level build), and care is taken by the build system that the necessary files are copied/built into the right place.

For non-developer builds, one can force a non-prefix build by setting the prefix explicitly to the build directory. For a top-level build, one would do:

$ ../qt-source-dir/configure -prefix <full-path-to-build-dir>/qtbase

For a build of qtbase only, one would do:

$ ../qt-source-dir/qtbase/configure -prefix <full-path-to-build-dir>

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

debug-and-release build

This means that Qt (or some other project) is configured for building the Debug and Release configurations in the same build directory.

QMake has in-built support for that. Out of the box, two fixed configs are available: debug and release.

For CMake projects, Multi-Config generators (e.g. Ninja Multi-Config) support the build of multiple configurations per build directory.

Framework Build

On Apple platforms, the Qt libraries are built by default as frameworks.

The <install-prefix>/lib/Qt6Core.framework then contains the library and all related header files.

Qt can also be configured as non-framework build by passing -no-framework to configure.

In this case, <install-prefix>/lib contains the libraries as .dylib files, and the header files are placed below <install-prefix>/include.

Documentation-only Build

Packagers usually build Qt per repo, but the documentation must be built from the full Qt source archive to ensure that inter-repo links are set up correctly. If building the documentation for a single repo, you can consider setting the QDOC_NOLINKERRORS environment variable to suppress QDoc's "Can't link to" warnings. When building the documentation, it's desirable to avoid building tools and re-use to tools of an existing Qt build.

Prior to configuring, make sure you have LLVM/libclang v10.0+ installed and LLVM_INSTALL_DIR set to the base of the installation. This is required to build the qdoc tool which is required for building documentation.

One now can do a documentation-only build by configuring Qt like this:

cmake /path/to/Qt/source -GNinja -DQT_HOST_PATH=/path/to/Qt/installation ...other flags...

and build the documentation with

ninja docs

Introduced in https://codereview.qt-project.org/c/qt/qtbase/+/346155

To just build the documentation for one module after you have built it for all modules once:

ninja docs_<module_name>

For example, to build the docs for the qtdoc module, enter:

ninja docs_qtdoc

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 -V     # to run all 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.

To build and run one single test, use

$ ninja tst_qprocess_check

To run tests matching a certain name, use

$ ctest -V -R ^tst_qprocess

To run all tests in a subdirectory, use

$ ctest -V -L tests/auto/corelib

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 -nomake tests
$ cmake --build .
$ cd ..
$ mkdir qtbase-standalone-tests
$ cd qtbase-standalone-tests
$ ../qt-build-dir/libexec/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. It also circumvents some catch-22 issues regarding qmlimportscanner and QML modules. 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.

Note that qt-internal-configure-tests might be located in the "bin" directory instead of "libexec" (on Windows hosts and in older Qt versions).

Single 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/libexec/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.

Note that qt-cmake-standalone-test might be located in the "bin" directory instead of "libexec" (on Windows hosts and in older Qt versions).

test batching

On some platforms, for example WASM, linking is slow and/or the resulting binaries are huge. Test batching can be enabled to mitigate these problems: multiple tests are built into one binary.

See https://codereview.qt-project.org/c/qt/qtbase/+/421446 which introduced this feature.

qmake trace file

A qmake trace file for a qmake-based project is produced by passing three -d arguments to qmake:

$ qmake /my/project/sources -d -d -d >qmake.trace 2>&1

CMake trace file

A CMake trace file for a CMake-based project is produced like this:

$ cmake /my/project/sources --trace-expand --trace-redirect=cmake_trace.txt --log-level=STATUS

For a Qt build you would do

$ configure ...more options... -- --trace-expand --trace-redirect=cmake_trace.txt --log-level=STATUS

CMake find_package debugging

To make CMake output where it looks for files when running commands like find_package / find_path / find_library run:

$ cmake /my/project/sources --debug-find

For a Qt build you would do

$ configure ...more options... -- --debug-find

This will generate a lot of output. To redirect it into a file use:

$ cmake /my/project/sources --debug-find > config_debug.txt 2>&1

This works across bash, powershell and batch files.

Repo Target Sets

A repo target set is a named set of targets within a Qt repository. One can build a repo target set by passing QT_BUILD_SINGLE_TARGET_SET=target-set-name to cmake when configuring the Qt repository.

The concept was introduced by this change for aiding building conan packages, but it may be useful for any kind of Qt distribution that wishes to create packages of a finer granularity than Qt repository level.

Repo target sets are defined in the top-level CMakeLists.txt of a repository. The definitions must be followed by a call to qt_internal_prepare_single_repo_target_set_build().

Let's walk through what has been done for the qtscxml repository.

# CMakeLists.txt

# Define the repo target set "qtscxml". 
# That matches the conan package name but can be an arbitrary string.
qt_internal_define_repo_target_set(qtscxml)

# Define the repo target set "qtscxmlqml".
# The target ScxmlQml will be part of this set, and building it requires a call to
# find_package(Qt6 COMPONENTS Scxml).
# Denote this requirement with the DEPENDS argument.
qt_internal_define_repo_target_set(qtscxmlqml DEPENDS Scxml)

qt_internal_prepare_single_repo_target_set_build()

By now, all those repo target sets contain all targets of the repository. We now assign every module, plugin and tool to its target set.

# src/scxml/CMakeLists.txt

qt_internal_include_in_repo_target_set(qtscxml)

qt_internal_add_module(Scxml
...

# src/scxmlqml/CMakeLists.txt

qt_internal_include_in_repo_target_set(qtscxmlqml)

qt_internal_add_qml_module(ScxmlQml
...

qt_internal_include_in_repo_target_set operates on file level and simply skips the rest of the file if we're currently building a repo target set that does not match.

Apps and tools shipped by Qt

https://bugreports.qt.io/browse/QTBUG-100047

Qt ships a variety of executables(tools and apps) that vary by the following parameters:

1. user-facing vs. internal
2. maybe built when cross-compiling
3. maybe creates a target in host Qt that can be imported in target Qt
4. maybe required in host Qt when building target Qt

They can broadly be categorised into the following groups:

1. build tools - they contribute to the Qt build, e.g code generators (moc, uic, cmake_automoc_parser, qsb, repc, qmltc, qmlcachegen, qmlimportscanner, qdoc?, lupdate?, lrelease?, qvkgen) these usually go into $prefix/libexec, unless they are also user-facing, which means they'd go into $prefix/bin
2. target apps (qml, qmljs, qwebengineprocess), they don't contribute to the build and are also built for the host, but are intended to run on the target platform / device
3. host-only development tools (qmake, qmllint?, qmlformat, qmldom, qmljsrootgen, qwebengine_convert_dict?, Linguist, Assistant, windeployqt, macdeployqt, androiddeployqt) usually used for development of Qt projects

Tools should be added using qt_internal_add_tool. They are usually built as part of the host build (aside from exceptions like windeployqt when cross-compiling from Linux to Windows). They will also be built during a target build, but only if -DQT_BUILD_TOOLS_WHEN_CROSSCOMPILING=ON is passed. A typical case when this is useful is for creating a Yocto SDK where a user might want to be able to build Qt code directly on the device.

Apps should be added with qt_internal_add_app.

CMake verbosity

Starting with Qt 6.2.4 and Qt 6.3.0, a non-developer-build of Qt by default sets CMAKE_MESSAGE_LOG_LEVEL to NOTICE. To restore the more verbose output either configure Qt with -DCMAKE_MESSAGE_LOG_LEVEL=STATUS or --log-level=STATUS.

$ cmake /my/project/sources --log-level=STATUS

$ configure ...more options... -- --log-level=STATUS

https://cmake.org/cmake/help/latest/variable/CMAKE_MESSAGE_LOG_LEVEL.html

Reporting build system issues

If you encounter an issue building Qt or your own project that uses Qt, please provide the following general information as well as any additional information as requested below:

• Operating system used:
• Target platform:
• Qt configure line / project configure line:
• Qt version / branch:
• CMake version:
• Ninja version:
• qmake version:
• Compiler version:
• macOS / iOS Xcode version:
• Android NDK / SDK version:
• WebAssembly Emscripten version:

Ninja: somethings rebuilds when it shouldn't

Try to replicate the scenario one more time, but make sure to add the -d explain flag when calling ninja. Share the standard output.

If Qt fails to configure

Follow the steps at CMake trace file, share the trace file, the CMakeCache.txt file and the stdout of running configure/cmake.

If Qt fails to build

Share the standard output of the build failure and the full command line of the failed build step.

If a CMake project fails to configure

Share your project CMakeLists.txt files, follow the steps at CMake trace file, share the trace file, CMakeCache.txt file and the stdout of running CMake.

If a qmake project fails to configure

Share your project .pro/pri files, follow the steps at qmake trace file, share the trace file and the stdout of running qmake.