Comment by dfox

For me the moral of the story is do not use (whatever)scanf() for anything other than toy programs. In most cases implementing your own tokenizer (for both of these cases of reading numbers that involves str(c)spn() to get length of candidate token and then strtosomething()) is significantly easier than reasoning about what scanf() really does (even ignoring accidentally quadratic implementation details) and whether that can be adapted to your usecase.