Qt 6.2.4 cross-compiled app hangs in ppoll

vinci

Hello

I'm having troubles getting Qt 6.2.4 running when cross-compiling with mingw64 from Ubuntu 20.04. For some reason my app keeps hanging after starting the event loop.

#include <QApplication>
#include <QLabel>

int main(int argc, char* argv[]) {
  QApplication a(argc, argv);
  QLabel* label = new QLabel{"Some test"};
  label->show();
  return a.exec();
}

I can keep the app running with GDB and when I hit pause after some time it's always inside a function called ppoll in the poll2.h header.

__fortify_function int
ppoll (struct pollfd *__fds, nfds_t __nfds, const struct timespec *__timeout,
       const __sigset_t *__ss)
{
  if (__bos (__fds) != (__SIZE_TYPE__) -1)
    {
      if (! __builtin_constant_p (__nfds))
	return __ppoll_chk (__fds, __nfds, __timeout, __ss, __bos (__fds));
      else if (__bos (__fds) / sizeof (*__fds) < __nfds)
	return __ppoll_chk_warn (__fds, __nfds, __timeout, __ss,
				 __bos (__fds));
    }

 // <<< Hangs here >>>
  return __ppoll_alias (__fds, __nfds, __timeout, __ss);
}

Here is the call-stack of the thread of my application

libc.so.6!__libc_disable_asynccancel() (/build/glibc-wuryBv/glibc-2.31/sysdeps/unix/sysv/linux/x86_64/cancellation.S:87)
libc.so.6!__ppoll(struct pollfd * fds, nfds_t nfds, const struct timespec * timeout, const sigset_t * sigmask) (/build/glibc-wuryBv/glibc-2.31/sysdeps/unix/sysv/linux/ppoll.c:44)
libQt6Core.so.6!ppoll(const __sigset_t * __ss, const timespec * __timeout, nfds_t __nfds, pollfd * __fds) (/usr/include/x86_64-linux-gnu/bits/poll2.h:77)
libQt6Core.so.6!qt_ppoll(const timespec * timeout_ts, nfds_t nfds, pollfd * fds) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/kernel/qcore_unix.cpp:132)
libQt6Core.so.6!qt_safe_poll(pollfd * fds, nfds_t nfds, const timespec * timeout_ts) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/kernel/qcore_unix.cpp:155)
libQt6Core.so.6!QEventDispatcherUNIX::processEvents(QEventDispatcherUNIX * const this, QEventLoop::ProcessEventsFlags flags) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/tools/qarraydatapointer.h:140)
libQt6XcbQpa.so.6!QXcbUnixEventDispatcher::processEvents(QXcbUnixEventDispatcher * const this, QEventLoop::ProcessEventsFlags flags) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/plugins/platforms/xcb/qxcbeventdispatcher.cpp:60)
libQt6Core.so.6!QEventLoop::processEvents(QEventLoop * const this, QEventLoop::ProcessEventsFlags flags) (/usr/include/c++/9/bits/atomic_base.h:734)
libQt6Core.so.6!QEventLoop::exec(QEventLoop * const this, QEventLoop::ProcessEventsFlags flags) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/global/qflags.h:110)
libQt6Core.so.6!QCoreApplication::exec() (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/global/qflags.h:109)
libQt6Gui.so.6!QGuiApplication::exec() (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/gui/kernel/qguiapplication.cpp:1875)
libQt6Widgets.so.6!QApplication::exec() (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/widgets/kernel/qapplication.cpp:2617)
main(int argc, char ** argv) (/home/vinci/Downloads/QtPlayground/src/main.cpp:8)

And here the one from the QXcbEventQueue

libc.so.6!__GI___poll(struct pollfd * fds, nfds_t nfds, int timeout) (/build/glibc-wuryBv/glibc-2.31/sysdeps/unix/sysv/linux/poll.c:29)
libxcb.so.1![Unknown/Just-In-Time compiled code] (Unknown Source:0)
libxcb.so.1!xcb_wait_for_event (Unknown Source:0)
libQt6XcbQpa.so.6!QXcbEventQueue::run(QXcbEventQueue * const this) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/plugins/platforms/xcb/qxcbeventqueue.cpp:228)
libQt6Core.so.6!QThreadPrivate::<lambda()>::operator()(const QThreadPrivate::<lambda()> * const __closure) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/thread/qthread_unix.cpp:356)
libQt6Core.so.6!(anonymous namespace)::terminate_on_exception<QThreadPrivate::start(void*)::<lambda()> >(QThreadPrivate::<lambda()> &&)(QThreadPrivate::<lambda()> && t) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/thread/qthread_unix.cpp:292)
libQt6Core.so.6!QThreadPrivate::start(void * arg) (/home/vinci/Downloads/QtPlayground/build/_deps/qt6-src/qtbase/src/corelib/thread/qthread_unix.cpp:315)
libpthread.so.0!start_thread(void * arg) (/build/glibc-wuryBv/glibc-2.31/nptl/pthread_create.c:477)
libc.so.6!clone() (/build/glibc-wuryBv/glibc-2.31/sysdeps/unix/sysv/linux/x86_64/clone.S:95)

Any ideas what might be going on here? Upon request I can also provide further installation details, but I have no clue if these are even of interest at this point?

vinci

What the... ok, in case anyone ever runs into this. This is caused by compiling Qt 6.4.2 without harfbuzz. Why? Don't ask me.