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

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

For example, given an implementation that supports 128-bit unsigned integers and provides a uint_fast128_t type, a programmer may define the following type:

typedef uint_fast128_t mytypedef_t;

Furthermore, the definition of 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-C. 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 (see INT00-C. Understand the data model used by your implementation(s)). This allows conversion between programmer-defined integer types (of the same signedness) and intmax_t and uintmax_t. For example:

mytypedef_t x;
uintmax_t temp;
/* ... */
temp = x; /* always secure*/

/* ... change the value of temp ... */

if (temp <= MYTYPEDEF_MAX) {
  x = temp;
}

Formatted I/O functions can be used to input and output greatest-width integer typed values. 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.

In addition to programmer-defined types, 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 may not 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.

Noncompliant Code Example (printf())

This noncompliant code example prints the value of x as an unsigned long long value, even though the value is of a programmer-defined integer type.

#include <stdio.h>

mytypedef_t x;

/* ... */

printf("%llu", (unsigned long long) x); 

Consequently, there is no guarantee that this code prints the correct value of x, as x is not necessarily an unsigned long long value and may be too large to represent as an unsigned long long.

Compliant Solution (printf())

The C99 intmax_t and uintmax_t can be safely used to perform formatted I/O with programmer-defined integer types. This is accomplished by converting signed programmer-defined integer types to intmax_t and unsigned programmer-defined integer types to uintmax_t, then outputting these values using the j length modifier. Similarly, programmer-defined integer types can be input to variables of intmax_t or uintmax_t (whichever matches the signedness of the programmer-defined integer type) and then converted to 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);

Noncompliant Code Example (scanf())

The following noncompliant code example reads an unsigned long long value from standard input and stores the result in x, which is of a programmer-defined integer type.

#include <stdio.h>

mytypedef_t x;
/* ... */
if (scanf("%llu", &x) != 1) {
  /* handle error */
}

This noncompliant code example can result in a buffer overflow, if the size of mytypedef_t is smaller than unsigned long long, or it might result in an incorrect value if the size of mytypedef_t is larger than unsigned long long.

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, assuming the value of MYTYPEDEF_MAX is correct as the largest value representable by mytypedef_t.

#include <stdio.h>
#include <inttypes.h>

mytypedef_t x;
uintmax_t temp;

/* ... */

if (scanf("%ju", &temp) != 1) {
  /* handle error */
}
if (temp > MYTYPEDEF_MAX) {
  /* handle error */
}
else {
  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.

Automated Detection

Compass/ROSE can catch violations of this rule by scanning the printf() and scanf() family of functions. For each such function, any variable that corresponds to a "%d" qualifier (or any qualifier besides "%j"), and that variable is not one of the built-in types (char, short, int, long, long long) indicates a violation of this rule.

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"
[[MITRE 07]] CWE ID 681, "Incorrect Conversion between Numeric Types"

      04. Integers (INT)