By definition, the types intmax_t and uintmax_t are capable of representing any value representable by any other types of the same signedness. By using these types, the danger of truncation associated with the ambiguities of a change in platform or architecture, e.g. IA-32 to x86-64, can be mitigated.
Formatted input and output functions contain a length modifier which provides the above facilities for input/output. The j specifier in a format string indicates that the following d, i, o, u, x, X, or n conversion specifier will apply to an argument with type pointer to intmax_t or uintmax_t.
[[ISO/IEC 9899-1999:TC2]] Section 7.19.6, "Formatted input/output functions," and Section 7.18.1.5, "Greatest-width integer types"