GDB, first use

Thanks to humph.
Find the process id you're looking for:

ps aux | grep appname

Now we have the Process ID, and the path to the app.

gdb /path/to/your/app -p pidForApp

Press enter alot. The app is now frozen. Type c then press enter to continue. Hit CTRL+C to lock it again. b somefile.cpp:567 to set a break point.