Thanks for the answer, I will try to add more information.
I'm using the latest Qt 5.13.1 on Xubuntu 19.04. To create the AppImage I used the command tool linuxdeployqt which is available here:
I'm using the following command.
/opt/linuxdeployqt/linuxdeployqt-continuous-x86_64.AppImage AppDir/usr/share/applications/MyApp.desktop \
I followed what is in the example "sending_arguments" and set the 3rd parameter to true because I want all secondary applications to send a message to the primary application and then quit.
What I wanted to do is to allow the user to click on a file associated to the application and open it up. If the user then clicks on a different file with the same extension, the file will be loaded into the primary application.
As I said, just before packing the application into the AppImage format, it works fine. I can prevent a secondary application to startup and send a message to the primary application. That is why I thought it was an AppImage issue.
Additionally, I tried this snippet of code called RunGuard which is available here:
This small piece of code uses a QSharedMemory and QSystemSemaphore to prevent secondary applications to startup but there is no client-server communication. This piece of code works fine even if the application is packed into an AppImage archive and that is the reason why I reported the issue here.