Updating cbqt for bullseye

pdo debian sw eng

Back in 2017 I did work to setup a cross-building toolchain for QT Creator, that takes advantage of Debian's packaging for all the dependency ecosystem.

It ended with cbqt which is a little script that sets up a chroot to hold cross-build-dependencies, to avoid conflicting with packages in the host system, and sets up a qmake alternative to make use of them.

Today I'm dusting off that work, to ensure it works on Debian bullseye.

Resetting QT Creator

To make things reproducible, I wanted to reset QT Creator's configuration.

Besides purging and reinstalling the package, one needs to manually remove:

Updating cbqt

Easy start, change the distribution for the chroot:

-DIST_CODENAME = "stretch"
+DIST_CODENAME = "bullseye"

Adding LIBDIR

Something else does not work:

Test$ qmake-armhf -makefile
Info: creating stash file …/Test/.qmake.stash
Test$ make
[...]
/usr/bin/arm-linux-gnueabihf-g++ -Wl,-O1 -Wl,-rpath-link,…/armhf/lib/arm-linux-gnueabihf -Wl,-rpath-link,…/armhf/usr/lib/arm-linux-gnueabihf -Wl,-rpath-link,…/armhf/usr/lib/ -o Test main.o mainwindow.o moc_mainwindow.o   …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Widgets.so …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Gui.so …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Core.so -lGLESv2 -lpthread
/usr/lib/gcc-cross/arm-linux-gnueabihf/10/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lGLESv2
collect2: error: ld returned 1 exit status
make: *** [Makefile:146: Test] Error 1

I figured that now I also need to set QMAKE_LIBDIR and not just QMAKE_RPATHLINKDIR:

--- a/cbqt
+++ b/cbqt
@@ -241,18 +241,21 @@ include(../common/linux.conf)
 include(../common/gcc-base-unix.conf)
 include(../common/g++-unix.conf)

+QMAKE_LIBDIR += {chroot.abspath}/lib/arm-linux-gnueabihf
+QMAKE_LIBDIR += {chroot.abspath}/usr/lib/arm-linux-gnueabihf
+QMAKE_LIBDIR += {chroot.abspath}/usr/lib/
 QMAKE_RPATHLINKDIR += {chroot.abspath}/lib/arm-linux-gnueabihf
 QMAKE_RPATHLINKDIR += {chroot.abspath}/usr/lib/arm-linux-gnueabihf
 QMAKE_RPATHLINKDIR += {chroot.abspath}/usr/lib/

Now it links again:

Test$ qmake-armhf -makefile
Test$ make
/usr/bin/arm-linux-gnueabihf-g++ -Wl,-O1 -Wl,-rpath-link,…/armhf/lib/arm-linux-gnueabihf -Wl,-rpath-link,…/armhf/usr/lib/arm-linux-gnueabihf -Wl,-rpath-link,…/armhf/usr/lib/ -o Test main.o mainwindow.o moc_mainwindow.o   -L…/armhf/lib/arm-linux-gnueabihf -L…/armhf/usr/lib/arm-linux-gnueabihf -L…/armhf/usr/lib/ …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Widgets.so …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Gui.so …/armhf/usr/lib/arm-linux-gnueabihf/libQt5Core.so -lGLESv2 -lpthread

Making it work in Qt Creator

Time to try it in Qt Creator, and sadly it fails:

…/armhf/usr/lib/arm-linux-gnueabihf/qt5/mkspecs/features/toolchain.prf:76: Variable QMAKE_CXX.COMPILER_MACROS is not defined.

QMAKE_CXX.COMPILER_MACROS is not defined

I traced it to this bit in armhf/usr/lib/arm-linux-gnueabihf/qt5/mkspecs/features/toolchain.prf (nonrelevant bits deleted):

isEmpty($${target_prefix}.COMPILER_MACROS) {
    msvc {
        # …
    } else: gcc|ghs {
        vars = $$qtVariablesFromGCC($$QMAKE_CXX)
    }
    for (v, vars) {
        # …
        $${target_prefix}.COMPILER_MACROS += $$v
    }
    cache($${target_prefix}.COMPILER_MACROS, set stash)
} else {
    # …
}

It turns out that qmake is not able to realise that the compiler is gcc, so vars does not get set, nothing is set in COMPILER_MACROS, and qmake fails.

Reproducing it on the command line

When run manually, however, qmake-armhf worked, so it would be good to know how Qt Creator is actually running qmake. Since it frustratingly does not show what commands it runs, I'll have to strace it:

strace -e trace=execve --string-limit=123456 -o qtcreator.trace -f qtcreator

And there it is:

$ grep qmake- qtcreator.trace
1015841 execve("/usr/local/bin/qmake-armhf", ["/usr/local/bin/qmake-armhf", "-query"], 0x56096e923040 /* 54 vars */) = 0
1015865 execve("/usr/local/bin/qmake-armhf", ["/usr/local/bin/qmake-armhf", "…/Test/Test.pro", "-spec", "arm-linux-gnueabihf", "CONFIG+=debug", "CONFIG+=qml_debug"], 0x7f5cb4023e20 /* 55 vars */) = 0

I run the command manually and indeed I reproduce the problem:

$ /usr/local/bin/qmake-armhf Test.pro -spec arm-linux-gnueabihf CONFIG+=debug CONFIG+=qml_debug
…/armhf/usr/lib/arm-linux-gnueabihf/qt5/mkspecs/features/toolchain.prf:76: Variable QMAKE_CXX.COMPILER_MACROS is not defined.

I try removing options until I find the one that breaks it and... now it's always broken! Even manually running qmake-armhf, like I did earlier, stopped working:

$ rm .qmake.stash
$ qmake-armhf -makefile
…/armhf/usr/lib/arm-linux-gnueabihf/qt5/mkspecs/features/toolchain.prf:76: Variable QMAKE_CXX.COMPILER_MACROS is not defined.

Debugging toolchain.prf

I tried purging and reinstalling qtcreator, and recreating the chroot, but qmake-armhf is staying broken. I'll let that be, and try to debug toolchain.prf.

By grepping gcc in the mkspecs directory, I managed to figure out that:

Sadly, I failed to find reference documentation for QMAKE_COMPILER's syntax and behaviour. I also failed to find why qmake-armhf worked earlier, and I am also failing to restore the system to a situation where it works again. Maybe I dreamt that it worked? I had some manual change laying around from some previous fiddling with things?

Anyway at least now I have the fix:

--- a/cbqt
+++ b/cbqt
@@ -248,7 +248,7 @@ QMAKE_RPATHLINKDIR += {chroot.abspath}/lib/arm-linux-gnueabihf
 QMAKE_RPATHLINKDIR += {chroot.abspath}/usr/lib/arm-linux-gnueabihf
 QMAKE_RPATHLINKDIR += {chroot.abspath}/usr/lib/

-QMAKE_COMPILER          = {chroot.arch_triplet}-gcc
+QMAKE_COMPILER          = gcc {chroot.arch_triplet}-gcc

 QMAKE_CC                = /usr/bin/{chroot.arch_triplet}-gcc

Fixing a compiler mismatch warning

In setting up the kit, Qt Creator also complained that the compiler from qmake did not match the one configured in the kit. That was easy to fix, by pointing at the host system cross-compiler in qmake.conf:

 QMAKE_COMPILER          = {chroot.arch_triplet}-gcc

-QMAKE_CC                = {chroot.arch_triplet}-gcc
+QMAKE_CC                = /usr/bin/{chroot.arch_triplet}-gcc

 QMAKE_LINK_C            = $$QMAKE_CC
 QMAKE_LINK_C_SHLIB      = $$QMAKE_CC

-QMAKE_CXX               = {chroot.arch_triplet}-g++
+QMAKE_CXX               = /usr/bin/{chroot.arch_triplet}-g++

 QMAKE_LINK              = $$QMAKE_CXX
 QMAKE_LINK_SHLIB        = $$QMAKE_CXX

Updated setup instructions

Create an armhf environment:

sudo cbqt ./armhf --create --verbose

Create a qmake wrapper that builds with this environment:

sudo ./cbqt ./armhf --qmake -o /usr/local/bin/qmake-armhf

Install the build-dependencies that you need:

# Note: :arch is added automatically to package names if no arch is explicitly specified
sudo ./cbqt ./armhf --install libqt5svg5-dev libmosquittopp-dev qtwebengine5-dev

Build with qmake

Use qmake-armhf instead of qmake and it works perfectly:

qmake-armhf -makefile
make

Set up Qt Creator

Configure a new Kit in Qt Creator:

  1. Tools/Options, then Kits, then Add
  2. Name: armhf (or anything you like)
  3. In the Qt Versions tab, click Add then set the path of the new Qt to /usr/local/bin/qmake-armhf. Click Apply.
  4. Back in the Kits, select the Qt version you just created in the Qt version field
  5. In Compilers, select the ARM versions of GCC. If they do not appear, install crossbuild-essential-armhf, then in the Compilers tab click Re-detect and then Apply to make them available for selection
  6. Dismiss the dialog with "OK": the new kit is ready

Now you can choose the default kit to build and run locally, and the armhf kit for remote cross-development.

I tried looking at sdktool to automate this step, and it requires a nontrivial amount of work to do it reliably, so these manual instructions will have to do.

Credits

This has been done as part of my work with Truelite.