[Solved] How to redirect the QT Application display to /dev/fb0

[Sidii]

Dear All,

I have a very simple QT GUI application like shown below:

#include <QApplication>
#include <QtWidgets/QLabel>
int main(int argc, char** argv)
{

    QApplication app(argc, argv);
    QLabel *label = new QLabel("Hello World!");
    label->show();
    return app.exec();

}

Now when i run the above command like this:
./myApp -qws -display "linuxfb:/dev/fb0"

I get following error:

This application failed to start because it could not find or load the Qt platform plugin "xcb".

Available platform plugins are: linuxfb (from :/home/GUI).

Reinstalling the application may fix this problem.
Aborted (core dumped)

Kindly help me out. I compiled QT from source code and there was no XCB support. Only Support that is available is LinuxFB.

Thanks
Sid

[lutzhell]

Hi Sid,

i assume you use Qt5? The QWS system does not exist anymore and is replaced by the QPA system which by now is not documented very well.

http://www.kdab.com/porting-from-qt-4-to-qt-5/

However you should be fine with the linuxfb plugin. What you need to change is:

• change your application to be a QGuiApplication

@#include <QGuiApplication>@

...

@QGuiApplication theApp(argc, argv);@

• remove @-qws -display “linuxfb:/dev/fb0”@

command line parameters

• copy or move the platforms directory created by qt install to the directory containing your application (maybe it is also ok to just install the platforms directory's contents to /lib or /usr/lib?)

Cheers,
Lutz

[lutzhell]

Oh, and dont't forget to pass

@-qpa linuxfb@

as an argument to configure when you build Qt5 to assure that the linuxfb plugin is used as default.

[Sidii]

Dear Lutzhell,

Thanks for your quick reply.
Actually while building QT from source i got following Config summary:

QPA backends:
DirectFB ............. no
EGLFS ................ no
KMS .................. no
LinuxFB .............. yes
XCB .................. no

so i think there is no need of doing QT Configure with -qpa linuxfb.

So i want to pass the command line argument. So to run the application what arguments i should use so that it is redirected to LinuxFb.

Looking forward for your help.

Thanks lot.
Sid

[Sidii]

Forgot to mention that i am using QT 5.2.1.

[lutzhell]

Hi Sid,

my application starts nicely without any additional parameters.

This is possible because with the configure flag i say that i want to use "offscreen" (i.e. in my case: -qpa offscreen, you would use -qpa linuxfb) as default QPA platform plugin.

from ./configure --help

@-qpa <name> ......... Sets the default QPA platform (e.g xcb, cocoa, windows).@

If you don't want to use linuxfb as default you can still pass the platform parameter to your programme:

@-platform linuxfb@

[lutzhell]

BTW i'm using Qt 5.1.1 but the instructions should do for your version as well

[Sidii]

Dear Lutzhell,

Thanks for your quick reply.

I will work on your inputs and will update you about the progress :)

[lutzhell]

Good luck :)

The doc for QGuiApplication gives some clues which arguments can be passed to a QGuiApplication:

http://qt-project.org/doc/qt-5/qguiapplication.html#QGuiApplication

very interesting is:

@-platform platformName[:options], specifies the Qt Platform Abstraction (QPA) plugin.

Overridden by the QT_QPA_PLATFORM environment variable.
@

[Sidii]

Hi lutzhell,

Thanks for the help. I regenerated all the libraries by passing

@-qpa LinuxFB @

during the configure.

Now i am able to see the output on LinuxFB by giving the following command:
@
./gui_qt -platform linuxfb@

Cheers :)

[Sidii]

Marking it as solved.