The CMake update offers an opportunity to revisit some topics that came up during the last few years.
* The Qt build system does not support building host tools during a cross-compilation run. You need to build a Qt for your host machine first and then use the platform tools from that version. The decision to do this was reached independent of cmake: This does save resources on build machines as the host tools will only get built once.
* 3rd-party dependencies are no longer built as part of Qt. zlib, libpng, etc. from src/3rdparty need
to be supplied from the outside to the build now. You may find apt-get/brew/etc. useful for this. Otherwise you may consider using vcpkg as in the next section. The decision to remove 3rd party dependencies from Qt repositories was reached independent of the decision to use cmake, we just use the opportunity to implement this decision.
* There is less need for bootstrapping. Only moc and rcc (plus the lesser known tracegen and qfloat16-tables) are linking against the bootstrap Qt library. Everything else can link against the full QtCore. This will include qmake, which is currently missing from a cmake build. This will change: Qmake is supported as a build system for applications *using* Qt going forward and will not go away anytime soon.
* For the time being we try to keep qmake working so that we do not interfere too much with ongoing development.
# Building against VCPKG
You may use vcpkg to install dependencies needed to build QtBase.
* When running cmake in qtbase, pass ``-DCMAKE_PREFIX_PATH=/path/to/your/vcpkg/installed/$VCPKG_DEFAULT_TRIPLET`` or ``-DCMAKE_PREFIX_PATH=/path/to/your/vcpkg/installed/%VCPKG_DEFAULT_TRIPLET%`` on Windows.
# Building
The basic way of building with cmake is as follows:
```
cd {build directory}
cmake {path to source directory}
cmake --build .
```
``cmake --build`` is just a simple wrapper around the basic build tool that CMake generated a build system for. It works with any supported build backend supported by cmake, but you can also use the backend build tool directly, e.g. by running ``make`` in this case.
CMake has a ninja backend that works quite well and is noticeably faster than make, so you may want to use that:
```
cd {build directory}
cmake -GNinja {path to source directory}
cmake --build . # ... or ninja ;-)
```
You can look into the generated ``build.ninja`` file if you're curious and you can also build targets directory such as ``ninja lib/libQt5Core.so``.
When you're done with the build, you may want to install it, using ``ninja install`` or ``make install``. The installation prefix is chosen when running cmake though:
```
cd {build directory}
cmake -GNinja -DCMAKE_INSTALL_PREFIX=/path/where/to/install {path to source directory}
ninja
ninja install
```
You can use ``cmake-gui {path to build directory}`` or ``ccmake {path to build directory}`` to configure the values of individual cmake variables or Qt features. After changing a value, you need to choose the *configure* step (usually several times:-/), followed by the *generate* step (to generate makefiles/ninja files).
# Debugging CMake files
CMake allows specifying the ``--trace`` and ``--trace-expand`` options, which work like ``qmake -d -d``: As the cmake code is evaluated, the values of parameters and variables is shown. This can be a lot of output, so you may want to redirect it to a file.
# Porting Help
We have some python scripts to help with the conversion from qmake to cmake. These scripts can be found in ``utils/cmake``.
## configurejson2cmake.py
This script converts all ``configure.json`` in the Qt repository to ``configure.cmake`` files for use with CMake. We want to generate configure.cmake files for the foreseeable future, so if you need to tweak the generated configure.cmake files, please tweak the generation script instead.
``configurejson2cmake.py`` is run like this: ``util/cmake/configurejson2cmake.py .`` in the top-level source directory of a Qt repository.
## pro2cmake.py
``pro2cmake.py`` generates a skeleton CMakeLists.txt file from a .pro-file. You will need to polish the resulting CMakeLists.txt file, but e.g. the list of files, etc. should be extracted for you.
``pro2cmake.py`` is run like this: ``/path/to/pro2cmake.py some.pro``.
## run_pro2cmake.py
`` A small helper script to run pro2cmake.py on all .pro-files in a directory. Very useful to e.g. convert all the unit tests for a Qt module over to cmake;-)
``run_pro2cmake.py`` is run like this: ``/path/to/run_pro2cmake.py some_dir``.
