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Furthermore, the definition of user programmer-defined types may change. This creates a problem using these types with formatted output functions (such as printf()) and formatted input functions (such as scanf()) (see FIO00-A. Take care when creating format strings).

The C99 intmax_t and uintmax_t types are capable of representing any value representable by any other integer types of the same signedness. This allows conversion between user programmer-defined integer types (of the same signedness) and intmax_t and uintmax_t. For example:

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Formatted input and output functions contain a length modifier which provides the above facilities for input/output. The j length modifier 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 intmax_t or uintmax_t. C99 also specifies the z length modifier for use with arguments of type size_t, and the t length modifier for arguments of type ptrdiff_t.

The C99 intmax_t and uintmax_t can safely be used to perform formatted I/O with programmer-defined integer types. Convert signed programmer-defined integer types to intmax_t and unsigned programmer-defined integer types to uintmax_t, then output using the j length modifier. Similarly, input programmer-defined integer types into variables of intmax_t or uintmax_t (whichever matches the signedness of the programmer-defined integer type) and then convert to the programmer-defined integer types using appropriate range checks.

There is no requirement that an implementation provide format length modifiers for implementation-defined integer types. For example, a machine with an implementation-defined 48-bit integer type might not bother to provide format length modifiers for the type. Such a machine would still have to have a 64-bit long long, with intmax_t being at least that large. So, this solution can be applied even if there are no format length modifiers for the 48-bit integers.

This rule is closely related to INT00-A. Understand the data model used by your implementation(s).

Non-Compliant Code Example (printf())

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Compliant Solution (printf())

The C99 intmax_t and uintmax_t can safely be used to perform formatted I/O with programmer-defined integer types. Convert signed programmer-defined integer types to intmax_t and unsigned programmer-defined integer types to uintmax_t, then output using the j length modifier. Similarly, input programmer-defined integer types into variables of intmax_t or uintmax_t (whichever matches the signedness of the programmer-defined integer type) and then convert to the programmer-defined integer types using appropriate range checks.

This compliant solution guarantees that the correct value of x is printed, regardless of its length, provided that mytypedef_t is an unsigned type.

#include <stdio.h>
#include <inttypes.h>
/* ... */
mytypedef_t x;
/* ... */
printf("%ju", (uintmax_t) x);

There is no requirement that an implementation provides format length modifiers for implementation-defined integer types. For example, a machine with an implementation-defined 48-bit integer type might not bother to provide format length modifiers for the type. Such a machine would still have to have a 64-bit long long, with intmax_t being at least that large. So, this solution can be applied even if there are no format length modifiers for the 48-bit integers.

Non-Compliant Code Example (scanf())

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