blockparser | Simple C bitcoin blockchain parser | Cryptocurrency library

You can't pop an empty stack, so you're only allowed to invoke popMode if you've used pushMode before (and only as often as you've pushed).

In your code, you're invoking popMode when you see a newline in the default mode (and never when you're in block mode, so you can never actually leave block mode), which can only happen when the stack is empty (because you never push the default mode, so you can only be in the default mode if nothing has been pushed yet), so encountering a newline in default mode will always lead to an exception because you're popping an empty stack.