INT34-C. Use intmax_t for signed integer types or uintmax_t for unsigned integer types where the underlying type is unknown.

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.

C99 Section 7.19.6.2, "The fscanf function" provides the following definitions for the j and t length modifiers.

j specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.

t Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned integer type.

C99 Section 7.18.1.5, "Greatest-width integer types"
The following type designates a signed integer type capable of representing any value of any signed integer type:
intmax_t
The following type designates an unsigned integer type capable of representing any value of any unsigned integer type:
uintmax_t

Use the j and t length modifier in calls to scanf() and printf() for integer types where the underlying type is unknown but the signedness of the type is known. This includes types such as size_t and ptrdiff_t.