Could not initialize GLX -- CLI on virtual machine - qt

Question

I created a Qt app with a GUI that can also run on the command line (by never calling QMainWindow::show()). When I try to run it on a Debian virtual machine I get the error:
$ xvfb-run ./myApp
Could not initialize GLX
Aborted
I built it on Ubuntu 16.04 with dynamic linking to the Qt libs and copied over the needed libraries. It was previously working, but started giving this error after I updated the app. How can I find out if this error is due to missing dependency or some issue with xvfb?

Answer 1

$ xvfb-run ./myApp
Xvfb doesn't support GLX / OpenGL. That's all. Either use a full blown Xorg server with GPU drivers or a headless EGL context.

Answer 2

The problem is most likely because you instantiate QApplication or QGuiApplication: not showing the window is not enough. When you run from the command line, you also need to use QCoreApplication only.
#include <QtWidgets>
#include <memory>
#ifdef Q_OS_WIN
#include <io.h>
int isatty(int fd) { return _isatty(fd); }
#else
#include <unistd.h>
#endif
using MyWindow = QWidget;
bool onCommandLine() {
return isatty(0);
}
int main(int argc, char **argv) {
std::unique_ptr<QCoreApplication> app(
onCommandLine() ? new QCoreApplication(argc, argv)
: new QApplication(argc, argv));
/* common logic goes here, e.g. argument parsing, etc. */
if (!onCommandLine()) {
MyWindow w;
w.show();
return app->exec();
} else
return 0;
}