This is a very simple debugger that shows how ptrace can be used to set breakpoints and generate stack traces in a target process. You can choose where to breakpoint the target process, and single step or continue to the next breakpoint (or target exit).
Big caveat: it assumes there is only one file in the target!
I fully expect this only to work on Linux.
This is a more detailed version of the code I wrote in my Debuggers From Scratch talk at dotGo Paris.
You may find it helpful to compile your target excutable with gcflags="-N -l" to turn off compiler optimization and in-lining (otherwise you might find that the symbol table doesn't appear to know about code where you expect it to be!)