[Development] wip/cmake status information

Tobias Hunger Tobias.Hunger at qt.io
Mon Oct 29 13:31:34 CET 2018


Some technical details on the wip/cmake branch in Gerrit. You can also 
find this information in cmake/README.md there.

# Status

Initial port is on-going. Some modules of QtBase are ported, incl. some 
of the platform modules. Most are missing still.

Basic functionality is there (moc, uic, etc.), but documentation, 
translations, qdbusxml2cpp, etc. are missing.

# Intro

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 

* 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.

   * ```git clone -b qt https://github.com/tronical/vcpkg```
   * Run ```bootstrap-vcpkg.bat``` or ```bootstrap-vcpkg.sh```
   * Set the ``VCPKG_DEFAULT_TRIPLET`` environment variable to
     * Linux: ``x64-linux``
     * Windows: ``qt-x86-windows-static``
   * Build Qt dependencies:  ``vcpkg install zlib pcre2 
double-conversion harfbuzz``
   * When running cmake in qtbase, pass 
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 

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 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 

## How to convert certain constructs

| qmake                 | CMake                   |
| ------                | ------                  |
| ``qtHaveModule(foo)`` | ``if(TARGET Qt::foo)``  |
| ``qtConfig(foo)``     | ``if (QT_FEATURE_foo)`` |

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