symbol lookup error: undefined symbol: _ZN7QWidget8qwsEventEP8QWSEvent

Question

I have cross-compiled executable file from QT 4.8.7. I am running it on a Raspberry PI (Ubuntu), with QT 4.8.7 and QT declarative module installed. When i run the executable file on my raspberry pi, i got the error message : undefined symbol: _ZN7QWidget8qwsEventEP8QWSEvent. I don't know why I am getting this error as both installed QT versions are the same and arm-linux-gcc toolchain is same. Note : I am using Raspberry pi 2 Model B ver 1.1 and Ubuntu Mate Os.

• The desktop Ubuntu which I am cross-compiling from as shown via lsb_release:

$ lsb_release -id
???

• The exact version of Ubuntu MATE 16.04 installed on the Raspberry Pi 2B v1.1:

$ lsb_release -id
???

• The exact cross-compiler version on my build-system:

$ arm-linux-gnueabihf-g++ --version
???

• On the Pi my Qt 4 package version is as follows:

$ apt-cache policy libqt4-declarative
  Installed: 4:4.8.7-dfsg-5ubuntu2
  Candidate: 4:4.8.7-dfsg-5ubuntu2

• On the Ubuntu 16.04 desktop build environment the package version is:

$ apt-cache policy libqt4-declarative:armhf
???

• On the Pi, checking the Qt library binary shows it using X11, not QWS:

$ strings /usr/lib/arm-linux-gnueabihf/libQtDeclarative.so.4 | egrep 'qwsEvent|x11Event'
_ZN7QWidget8x11EventEP7_XEvent

• On the desktop build environment:

$ strings /usr/lib/arm-linux-gnueabihf/libQtDeclarative.so.4 | egrep 'qwsEvent|x11Event'
???

• To install the Qt library in my cross-compiling build environment I did (???select the correct one and remove the other two):

  • Configured and cross-compiled the Qt libraries from source (bad, and almost guaranteed to cause the windowing mismatch problem @jdonald linked)
  • Ran sudo dpkg --add-architecture armhf, ensured my /etc/apt/sources.list had the correct Xenial armhf ports sources, and ran sudo apt-get update; sudo apt-get install libqt4-dev:armhf (good)
  • Installed from a different source (???)

• Searching for references to different windowing systems in my build directory turns up the following:

$ egrep -r qwsEvent my_build_dir/*
???
$ egrep -r x11Event my_build_dir/*
???

Answer 1

$ c++filt _ZN7QWidget8qwsEventEP8QWSEvent
QWidget::qwsEvent(QWSEvent*)

If you look at the documentation, this method is only available if Qt is built under the windowing configuration of Qt for Embedded Linux (Q_WS_QWS), a legacy system that was removed in Qt 5. If Ubuntu is anything like Raspbian, Qt is built for X11 instead and you only have QWidget::x11Event(_XEvent*) available.

Possibly the reason you're even able to hit this problem may be due to cross-compiling Qt yourself, which allows you to set a different configuration from the target platform. If you use the standard Ubuntu armhf packages on your build machine then this would avoid a mismatch and show linker errors at build time rather than postponing until load time. If you've done the required dpkg --add-architecture and installed the appropriate APT sources, the installation command on the host PC would be:

sudo apt install libqt4-declarative:armhf

^[1] http://m-embedded.blogspot.com/2015/07/build-qt4-embedded-on-host-pc.html shows a similar example of a developer who cross-compiled Qt itself for QWS while the installed libraries on a BeagleBone Black are for X11 and thus a mismatch.