Each time the program performs a function application, it saves the location of the application (the return address) in a block of data called a stack frame. The frame also contains the local variables of the caller function. All the frames are allocated in a region of memory called the call stack. The command backtrace (or bt) displays parts of the call stack.
At any time, one of the stack frames is "selected" by the debugger; several debugger commands refer implicitly to the selected frame. In particular, whenever you ask the debugger for the value of a local variable, the value is found in the selected frame. The commands frame, up and down select whichever frame you are interested in.
When the program stops, the debugger automatically selects the currently executing frame and describes it briefly as the frame command does.
Describe the currently selected stack frame.
Select a stack frame by number and describe it. The frame currently executing when the program stopped has number 0; its caller has number 1; and so on up the call stack.
Print the call stack. This is useful to see which sequence of function calls led to the currently executing frame. With a positive argument, print only the innermost count frames. With a negative argument, print only the outermost -count frames.
Select and display the stack frame just "above" the selected frame, that is, the frame that called the selected frame. An argument says how many frames to go up.
Select and display the stack frame just "below" the selected frame, that is, the frame that was called by the selected frame. An argument says how many frames to go down.