function::ucallers - Return first n elements of user stack backtrace
number of levels to descend in the stack (not counting
the top level). If n is -1, print the entire stack.
This function returns a string of the first n hex addresses from the backtrace
of the user stack. Output may be truncated as per maximum string length
To get (full) backtraces for user space applications and shared shared libraries
not mentioned in the current script run stap with -d /path/to/exe-or-so and/or
add --ldd to load all needed unwind data.