For Example, if you are programming a route to fib 1.2.3.4/24, you will find these entries at these indices:
Suppose this route is to be added into fib table with (fib_index of 10) - get the fib pointer
(gdb) set $fib_idx=0
(gdb) ptype ip4_main.v4_fibs[$fib_idx]
type = struct ip4_fib_t_ {