INT00-C. Understand the data model used by your implementation(s)

A data model defines the sizes assigned to standard data types. It is important to understand the data models used by your implementation. However, if your code depends on any assumptions not guaranteed by the standard, you should provide static assertions to ensure that your assumptions are valid. (See DCL03-C. Use a static assertion to test the value of a constant expression.) Assumptions concerning integer sizes may become invalid, for example, when porting from a 32-bit architecture to a 64-bit architecture.

Common Data Models

Code frequently embeds assumptions about data models. For example, some code bases require pointer and long to have the same size, whereas other large code bases require int and long to be the same size [van de Voort 2007]. These types of assumptions, while common, make the code difficult to port and make the ports error prone. One solution is to avoid any implementation-defined behavior. However, this practice can result in inefficient code. Another solution is to include either static or runtime assertions near any platform-specific assumptions, so they can be easily detected and corrected during porting.

<limits.h>

Possibly more important than knowing the number of bits for a given type is knowing that limits.h defines macros that can be used to determine the integral ranges of the standard integer types for any conforming implementation. For example, UINT_MAX is the largest possible value of an unsigned int, and LONG_MIN is the smallest possible value of a long int.

<stdint.h>

The stdint.h header introduces types with specific size restrictions that can be used to avoid dependence on a particular data model. For example, int_least32_t is the smallest signed integer type supported by the implementation that contains at least 32 bits. The type uint_fast16_t is the fastest unsigned integer type supported by the implementation that contains at least 16 bits. The type intmax_t is the largest signed integer, and uintmax_t is the largest unsigned type, supported by the implementation. The following types are required to be available on all implementations:

Additional types may be supported by an implementation, such as int8_t, a type of exactly 8 bits, and uintptr_t, a type large enough to hold a converted void * if such an integer exists in the implementation.

<inttypes.h>

The inttypes.h header declares functions for manipulating greatest-width integers and converting numeric character strings to greatest-width integers.

Noncompliant Code Example

This noncompliant example attempts to read a long into an int. This code works for models in which sizeof(int) == sizeof(long). For others, it causes an unexpected memory write similar to a buffer overflow.

int f(void) {
  FILE *fp;
  int x;
/* ... */
  if (fscanf(fp, "%ld", &x) < 1) {
    return -1; /* Indicate failure */
  }

/* ... */
  return 0;
}

Some compilers can generate warnings if a constant format string does not match the argument types.

Compliant Solution

This compliant solution uses the correct format for the type being used:

int f(void) {
  FILE *fp;
  int x;
/* Initialize fp */
  if (fscanf(fp, "%d", &x) < 1) {
    return -1; /* Indicate failure */
  }

/* ... */
  return 0;
}

Noncompliant Code Example

This noncompliant code attempts to guarantee that all bits of a multiplication of two unsigned int values are retained by performing arithmetic in the type unsigned long. This practice works for some platforms, such as 64-bit Linux, but fails for others, such as 64-bit Microsoft Windows.

unsigned int a, b;
unsigned long c;
/* Initialize a and b */
c = (unsigned long)a * b; /* Not guaranteed to fit */

Compliant Solution

This compliant solution uses the largest unsigned integer type available if it is guaranteed to hold the result. If it is not, another solution must be found, as discussed in INT32-C. Ensure that operations on signed integers do not result in overflow.

#if UINT_MAX > UINTMAX_MAX/UINT_MAX
#error No safe type is available.
#endif
/* ... */
unsigned int a, b;
uintmax_t c;
/* Initialize a and b */
c = (uintmax_t)a * b; /* Guaranteed to fit, verified above */

Risk Assessment

Understanding the data model used by your implementation is necessary to avoid making errors about the sizes of integer types and the range of values they can represent. Making assumptions about the sizes of data types may lead to buffer-overflow-style attacks.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography