what happend if gdb frams/bt show "??" but no readalbe function name?

Question

I encountered a coredump,use gdb xxx core.1234,then "bt",got those message:" 0x6f636d6f in ?? ()",in which the function name is not readable.I want to learn: 1. in what situations does the gdb bt show "??" as function name? 2. how to avoid these "??" and get the readable function name?

Answer 1

The address 0x6f636d6f is almost certainly invalid (i.e, lies outside your process's memory space), and is probably the result of stack corruption -- it corresponds to the ASCII characters ocmo.

Answer 2

1 case : stack is corrupt.
2 case : code compiles with optimisations (not sure about it).
3 case : you start gdb wrong (example gdb ./app --core core_name).