Does sscanf("123456789123456789123456789", "%d", &n) have defined behavior?
From the standard, 7.21.6.2p10 ((f)scanf, applies to the whole family):
… If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.
Looks like another reason to be very cautious with the scanf family. The strtoXX functions have a fully defined behaviour. They return LONG_MAX etc. for too large input and set errno == ERANGE. So if you need exact information, tokenise the input manually and use these functions for conversion. Another benefit: better error handling.