Cannot run Qt5 applications.

[David Stacey]

I'm having difficulty running any Qt5 application. These are the 
commands I'm issuing:

    XWin -multiwindow &
    export DISPLAY=:0.0
    xclock &
and I see the clock, so X is up and running. Then:

    /usr/lib/qt5/examples/gui/analogclock/analogclock
    QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource 
id: 0, major c
    ode: 140 (Unknown), minor code: 20
    Bad system call (core dumped)

and no clock.

I've tried compiling a really trivial Qt5 example (attached):

    g++ -I/usr/include/qt5 -I/usr/include/qt5/QtWidgets 
-I/usr/include/qt5/QtGui \
    -o qt5-demo qt5-demo.cpp -lQt5Widgets -lQt5Core

And then running ./qt5-demo gives:

    QObject::connect: signal not found in QPushButton
    QObject::connect: signal not found in QWidget
    QObject::connect: signal not found in QWidget
    QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource 
id: 0, major
    code: 140 (Unknown), minor code: 20
    Bad system call (core dumped)

The demo runs fine in Fedora 21. It also runs in Cygwin when compiled 
with Qt4:

    g++ -I/usr/include/qt4 -I/usr/include/qt4/QtGui -o qt5-demo 
qt5-demo.cpp \
    -lQtGui -lQtCore

And now running the (incorrectly-named!) ./qt5-demo launches the 
application.

Please could you advise what I'm doing wrong.

Many thanks in advance,

Dave.

#include <QApplication>
#include <QPushButton>

int main(int argc, char** argv)
{
  QApplication app(argc, argv);
  QPushButton *button = new QPushButton("Hello!");
  QObject::connect(button, SIGNAL(clicked()), &app, SLOT(quit()));
  button->show();
  return app.exec();
}
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