What causes signal 'SIGILL'?
Make sure that all functions with non-void return type have a return statement.
While some compilers automatically provide a default return value, others will send a SIGILL or SIGTRAP at runtime when trying to leave a function without a return value.
It means the CPU attempted to execute an instruction it didn't understand. This could be caused by corruption I guess, or maybe it's been compiled for the wrong architecture (in which case I would have thought the O/S would refuse to run the executable). Not entirely sure what the root issue is.
It could be some un-initialized function pointer, in particular if you have corrupted memory (then the bogus vtable of C++ bad pointers to invalid objects might give that).
BTW gdb watchpoints & tracepoints, and also valgrind might be useful (if available) to debug such issues. Or some address sanitizer.