'up' (gdb command) takes you closer to main(…)
'down' (gdb command) takes you closer to the function that has control right now.
main(…) is considered the "top" stack frame.

Every time a function is called (="invoked"), it creates a stack frame containing:
∙ args
∙ local variables (="locals")
∙ return address


GDB
∙ Every command has a short version.
    list ↔ l
    break ↔ b
  See the underlined letters in the reference sheet.
∙ Press Enter to continue/repeat the previous command.


FUNCTION DECLARATION declares what parameters are accepted/required
and what data type (if any) is returned.
    void print_integer(int n, int radix, char* prefix);

FUNCTION DEFINITION is the whole thing including the body.
    void print_integer(int n, int radix, char* prefix) {
        // …
    }