As replied on your other comment, when you introduce a custom printf for an embedded platform it makes more sense to just edit in support for your local I/O bac

As replied on your other comment, when you introduce a custom printf for an embedded platform it makes more sense to just edit in support for your local I/O backend rather than having the complexity of a putch() callback function pointer. cf. https://news.ycombinator.com/item?id=43811191 https://news.ycombinator.com/item?id=43811191 for other notes.