INT15-A. Use intmax_t or uintmax_t for for formatted IO on programmer-defined integer types

Few programmers consider the issues around formatted I/O and typedefs. A programmer-defined integer type might be any type supported by the implementation, even a type larger than unsigned long long.

For example, an implementation that supports 128-bit unsigned integers and provides a uint_fast128_t type for them would allow a programmar to use them:

typedef uint_fast128_t mytypedef_t;

Furthermore, the definition of user defined types may change (which may be one of the reasons the programmer-defined type was created to begin with). 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 defined integer types (of the same signedness) and intmax_t and uintmax_t. For example:

mytypedef_t x;
uintmax_t temp;
/* ... */
temp = x; /* always safe */
if (temp <= MYTYPEDEF_MAX) {
  x = temp;
}

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.

Similarly, there is no requirement that an implementation provide format length modifiers for implementation-defined integer types. For example, a machine with a 16-bit word size 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, and intmax_t would probably be that type. So, this solution can be applied even if there were no format length modifiers for the 48-bit integers.

Non-Compliant Code Example (printf())

There is no guarantee that this non-compliant code example prints the correct value of x.

#include <stdio.h>
/* ... */
mytypedef_t x;
/* ... */
printf("%llu", (unsigned long long) x); 

This code example is non-compliant for two reasons: it assumes that x has type unsigned long long and that unsigned long long is large enough to represent x.

Compliant Solution (printf())

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);

Non-Compliant Code Example (scanf())

This non-compliant code example will result in a "buffer overflow", if the size of mytypedef_t is smaller than unsigned long long or it may result in an incorrect value if the size of mytypedef_t is larger than unsigned long long.

#include <stdio.h>
/* ... */
mytypedef_t x;
/* ... */
scanf("%llu", &x);

Compliant Solution (scanf())

This compliant solution guarantees that a correct value in the range of mytypedef_t is read, or an error condition is detected.

#include <stdio.h>
#include <inttypes.h>
/* ... */
mytypedef_t x;
uintmax_t temp;
/* ... */
scanf("%ju", &temp);
if (temp > MYTYPEDEF_MAX) {
  /* handle error */
}
x = temp;

Risk Assessment

Failure to use an appropriate conversion specifier when inputting or outputting programmer-defined integer types can result in buffer overflow and lost or misinterpreted data.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899-1999]] Section 7.18.1.5, "Greatest-width integer types," and Section 7.19.6, "Formatted input/output functions"

INT14-A. Avoid performing bitwise and arithmetic operations on the same data      04. Integers (INT)