Signal emitted by “Force Quit” option in Ubuntu when killing a non-responsive GUI task

I have a GTK application written in C that sometimes becomes unresponsive in Ubuntu. It spawns a child process that is not killed when the “Force Quit” option is selected to kill the GUI, as seen below:


I would like to know what signal is emitted by the “Force Quit” option as I am hoping to capture it (if possible) to close the child process. Sometimes the GUI process also remains running when choosing this option, despite the window closing, and I would like to ensure this process is killed as well. I tried to capture SIGTERM and it didn’t work, so I am thinking that the signal being emitted here is probably SIGKILL, which can’t be captured. I would like to know, regardless, what is happening behind the scenes when this option is used.

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