Building Qt6 for Windows from Linux

[plaristote]

Greetings !

I'm trying to create a Docker machine that would be able to build my Qt applications for Windows.

I've been using the following article which describes how to do it for Qt5 with Ubuntu: https://medium.com/@vladadgad/cross-compile-qt-for-windows-on-linux-platform-57e4b71ed1aa

Expanding on the instructions from this tutorial, here's the command that I used to configure the build:

QT_VERSION=6.4.2
INSTALL_PREFIX=/opt/webapp/builder

mkdir -p $INSTALL_PREFIX/usr/Qt/$QT_VERSION/x86_64-w64-mingw64

./configure -opensource -confirm-license \
  -c++std c++17 \
  -xplatform win32-g++ \
  -device-option CROSS_COMPILE=/usr/bin/x86_64-w64-mingw32 \
  -prefix $INSTALL_PREFIX/usr/Qt/$QT_VERSION/x86_64-w64-mingw32  \
  -nomake examples

The configuration has gone smoothly, but it seems like mingw isn't being used when building. When I run the cmake --build . command with the VERBOSE=1 environment variable, I can see that files are compiled with /usr/bin/c++. Here's an example of the full command:

/usr/bin/c++ -DHAVE_CONFIG_H -DQT_BOOTSTRAPPED -DQT_NO_CAST_FROM_ASCII -DQT_NO_CAST_TO_ASCII -DQT_NO_DEBUG -DQT_NO_FOREACH -DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_TYPESAFE_FLAGS -DQT_USE_QSTRINGBUILDER -DQT_VERSION_MAJOR=6 -DQT_VERSION_MINOR=4 -DQT_VERSION_PATCH=2 -DQT_VERSION_STR=\"6.4.2\" -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/Core_autogen/include -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/include -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/include/QtCore -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/global -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/kernel -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/../3rdparty/tinycbor/src -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/include/QtCore/6.4.2 -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/include/QtCore/6.4.2/QtCore -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/../3rdparty/double-conversion/double-conversion -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/../3rdparty/double-conversion -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/../3rdparty/forkfd -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/.rcc -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/mkspecs/win32-g++ -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/tools/bootstrap/.. -I/opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/tools/bootstrap/../../3rdparty/tinycbor/src -DNDEBUG -O2 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -ffunction-sections -fdata-sections -mshstk -Wsuggest-override -fcf-protection -MD -MT qtbase/src/tools/bootstrap/CMakeFiles/Bootstrap.dir/__/__/corelib/global/qrandom.cpp.o -MF qtbase/src/tools/bootstrap/CMakeFiles/Bootstrap.dir/__/__/corelib/global/qrandom.cpp.o.d -o qtbase/src/tools/bootstrap/CMakeFiles/Bootstrap.dir/__/__/corelib/global/qrandom.cpp.o -c /opt/webapp/builder/Qt/6.4.2/Src/qtbase/src/corelib/global/qrandom.cpp

This fails to compile, claiming the tchar.h header is missing, which makes sense, since this header only exists for Windows (or at least that's what I've read).

I've modified the same command by replacing /usr/bin/c++ with /usr/bin/x86_64-w64-mingw32-g++ -std=c++17, and it seems to work... at least for qrandom.cpp !

I also tried to unlink /usr/bin/{c++,cc,ar,nm} and relink it to the ones provided by mingw, but then the configuration step fails, claiming that some linux-related headers are missing. Well... d'uh ! But why are we looking for these when xplatform is set to win32-g++ ?
Alternatively, I can also configure first, and only then override the system compiler for mingw; but then, the build tries to include some linux-specific headers.

Any guidance on this topic will be very appreciated ! And I'm sure a Docker machine capable of cross-compiling Qt projects for Windows will have its use for many of us, if we can get this thing working !

[KH-219Design]

My reply is going to lead down a bit of a tangent...

If someone knows how to give you a more direct answer, then I would recommend you ignore mine!

I don't have a direct answer. But I have a long-standing curiosity about cross-platform development, so I took a moment to do a brief investigation.

When I need a point of reference for using Qt in a "massively cross platform" way, my "cheatsheet" that I often turn to is the Subsurface divelog project.

They are cross-compiling their Qt app on Linux to target windows. You can see their CI job config here. I know that's not the same as cross-compiling Qt itself.

However, I noticed (at the top of one of their scripts) that they call out how they are leveraging: https://github.com/mxe/mxe

And when I read the MXE page, I see they say: "MXE (M cross environment) is a GNU Makefile that compiles a cross compiler and cross compiles many free libraries such as SDL and Qt. "

Again, this is a very indirect stab and a mostly non-answer.

But if you end up in dead ends with anything you try, maybe MXE would give you a path forward.