QProcess.kill or terminate stops the process but leave it in memory if the app is not run as root and from conky you still see the outgoing process. To solve it I get the PID from the started process and run QProcess::execute("sudo kill PID") when you stop the process or close the app. For the improperly exited app situation, you may try a sudo killall for gst-launch at mainwindow constructor to clean from previous process, if any.