Please beware that attaching to a running app is at the moment still an experimental feature of snapcraft. At the moment there is a problem when debugging an app that is running on a ctrlX COREvirtual. So I guess it is the same in your case. Please follow this steps:
Just ignore the raise.c error(s) at the beginning. If they rise.
Make sure that no breakpoints are set (-exec info b). See GDB cheat sheet for further information.
Step until all libraries are loaded.
Add breakpoints and you are good to go
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