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, which creates a problem when these types are used with formatted output functions, such as printf(), and formatted input functions, such as scanf(). (See FIO00-C. Take care when creating format strings.)

The C intmax_t and uintmax_t types can represent 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 capability allows conversion between programmer-defined integer types (of the same signedness) and intmax_t and uintmax_t:

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. C 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 provide 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 still must 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); 

There is no guarantee that this code prints the correct value of x, as x may be too large to represent as an unsigned long long.

Compliant Solution (printf())

The C intmax_t and uintmax_t can be safely used to perform formatted I/O with programmer-defined integer types 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);

Compliant Solution (Microsoft printf())

Visual Studio 2012 and earlier versions do not support the standard j length modifier and do not have a nonstandard analog. Consequently, the programmer must hard code the knowledge that intmax_t is int64_t and uintmax_t is uint64_t for Microsoft Visual Studio versions.

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

mytypedef_t x;

/* ... */

#ifdef _MSC_VER
  printf("%llu", (uintmax_t) x);
#else
  printf("%ju", (uintmax_t) x);
#endif  

Microsoft has submitted a feature request to add support for the j length modifier to a future release of Microsoft Visual Studio.

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;
} 

Compliant Solution (Microsoft scanf())

Visual Studio 2012 and earlier versions do not support the standard j length modifier and do not have a nonstandard analog. Consequently, the programmer must hard code the knowledge that intmax_t is int64_t and uintmax_t is uint64_t for Microsoft Visual Studio versions.

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

mytypedef_t x;
uintmax_t temp;

/* ... */
#ifdef _MSC_VER
#  define UINTMAX_CS "%llu"
#else
#  define UINTMAX_CS "%ju"
#endif

if (scanf(UINTMAX_CS, &temp) != 1) {
 /* Handle error */
}
if (temp > MYTYPEDEF_MAX) {
  /* Handle error */
}
else {
  x = temp;
} 

Microsoft has submitted a feature request to add support for the j length modifier to a future release of Microsoft Visual Studio.

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

Related Vulnerabilities

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

Related Guidelines

Bibliography