If main(…) calls round_down(…), then
∙ main(…) is the “caller”.
∙ round_down(…) is the “callee”.

'up' gdb command lets us look at the stack closer to main(…).  This lets
       us see what was going on in the functions that got us to whatever
       is running now.

'down' gdb command lets us look at the stack closer to the one that
       actually has control right now.

'next' executes the current statement, including any function calls,
       without stepping into the function calls.  It goes to the next
       statement in this function (unless we have breakpoints in a
       callee).

'step' steps in it.