gdb-frame-number

gdb-frame-number is a variable defined in `gdb-mi.el'.
Its value is

nil

Documentation:
Selected frame level for main current thread.

Updated according to the following rules:

When a thread is selected or current thread stops, set to "0".

When current thread goes running (and possibly exits eventually),
set to nil.

May be manually changed by user with `gdb-select-frame'.