Starting QT app as a Systemd service

[SGaist]

Hi,

Do you even need to have X running ? Wouldn't using the eglfs backend be simpler and less power hungry on your board ?

That said, you should start your application with the QT_DEBUG_PLUGINS environment variable set to 1 to see exactly what is going wrong.

For example, the DISPLAY environment variable might be set to an invalid value.

[vmetodiev]

@Pl45m4, @KroMignon

Well, here is a snippet from my main function:

int main(int argc, char *argv[])
{
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

     QGuiApplication app(argc, argv);
     QQmlApplicationEngine engine;
     ...
}	

I will read a bit more about QCore and QGui... now it is some kind of a Frankenstein :D

@SGaist

Yes, I always start the X server before attempting to launch the QT application.
It also starts correctly as a systemd service.

About "eglfs" - thanks for mentioning this, I will read about it as well and write another post. I have no idea what you are taking about right now.

Thank you guys for your support and informative directions!

[JonB]

@vmetodiev
I am not an expert, but how can you have a QGuiApplication and/or a QQmlApplicationEngine if you're not going to have any UI or interaction?

[vmetodiev]

@JonB

I agree! My goal is to make the GUI application start as a service with only the X server running beneath it, without LXDE or any other desktop environment.

I am just struggling how to achieve it no additional overhead...

[KroMignon]

@vmetodiev said in Starting QT app as a Systemd service:

I agree! My goal is to make the GUI application start as a service with only the X server running beneath it, without LXDE or any other desktop environment.
I am just struggling how to achieve it no additional overhead...

So you want to start you Qt QML/QWidget application with SystemD and X-Server.
Your error looks very similar to this ==> https://community.toradex.com/t/starting-qt-app-as-a-systemd-service/15954

So I guess you have to change qtapp.service to setup QT_QPA_PLATFORM.
Something like this:

[Unit]
Description=QT Application
After=xserver.service

[Service]
Type=simple
Environment="QT_QPA_PLATFORM=wayland-egl"
ExecStart=/opt/TestQtApp
ExecStop=/bin/bash -c 'pkill TestQtApp'
Restart=always
IgnoreSIGPIPE=no

[Install]
Alias=qtapp.service

[raven-worx]

@vmetodiev said in Starting QT app as a Systemd service:

Jan 07 14:58:57 apalis-imx6 TestQtApp[607]: qt.qpa.screen: QXcbConnection: Could not connect to display
Jan 07 14:58:57 apalis-imx6 TestQtApp[607]: Could not connect to any X display.

i think you are just missing the "DISPLAY=:0" env variable?

try it manually by calling

DISPLAY=:0 /opt/TestQtApp

[vmetodiev]

@KroMignon
Your error looks very similar to this ==> https://community.toradex.com/t/starting-qt-app-as-a-systemd-service/15954
-> Absolutely :D It was posted by me in the Toradex forum, than I switched here...

Environment="QT_QPA_PLATFORM=wayland-egl"
-> As far I understand, in this case the QT app will work with Wayland instead of the X server, correct?

@raven-worx
Well, I added it as an enviroment and achived a certain progress.

root@apalis-imx6:~# cat /lib/systemd/system/qtapp.service
[Unit]
Description=QT Application
After=xserver.service

[Service]
Type=simple
Environment="DISPLAY=:0"
ExecStart=/opt/TestQtApp
ExecStop=/bin/bash -c 'pkill TestQtApp'
Restart=always
IgnoreSIGPIPE=no

[Install]
Alias=qtapp.service

The service now starts when I manually invoke it by "systemctl start qtapp".
Nevertheless, it won't start automatically after reboot. The journal log is empty. Maybe the "After=xserver.service" is not quite correct?

[raven-worx]

@vmetodiev
do you start the xserver service manually?
if not you are also missing Require=xserver.service so it is ensured that its running when your qt app gets started

also to start it after reboot, you have to call systemctl enable qtapp

[vmetodiev]

@raven-worx

Yes, the service is enabled. Well, I added "Requires=xserver.service", but still the qtapp.service would not start.

The xserver is fine upon reboot, visible inside "ps -auxf".

Invoking " systemctl start qtapp" start the Qt app...

[SGaist]

Maybe this service example might help.

[KroMignon]

@vmetodiev said in Starting QT app as a Systemd service:

Invoking " systemctl start qtapp" start the Qt app...

I am far a way to be a systemd expert, so I may be wrong.
But I guess you have to add WantedBy=multi-user.target in the [Install] section to start the service automatically.

[raven-worx]

@KroMignon said in Starting QT app as a Systemd service:

But I guess you have to add WantedBy=multi-user.target in the [Install] section to start the service automatically.

yep, thats true, good catch

[vmetodiev]

@SGaist @KroMignon
Yes, your suggestions helped. Now it works perfectly. Thank you so much for your support!!!

I am providing the service files below:

xserver.service

[Unit]
Description=X Server
Conflicts=getty@tty1.service plymouth-quit.service
After=systemd-user-sessions.service getty@tty1.service plymouth-quit.service

[Service]
ExecStart=/usr/bin/X
ExecStop=/usr/bin/killall -9 X
Restart=always
IgnoreSIGPIPE=no

[Install]
Alias=display-manager.service

qtapp.service

[Unit]
Description=QT Application
Requires=xserver.service
After=xserver.service

[Service]
Type=simple
Environment="DISPLAY=:0"
ExecStart=/opt/TestQtApp
ExecStop=/bin/bash -c 'pkill TestQtApp'
Restart=always
IgnoreSIGPIPE=no

[Install]
WantedBy=multi-user.target