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

A data model defines the sizes assigned to standard data types. These data models are typically named using a XXXn pattern where X referes to a C type and n refers to a size (typically 32 or 64). ILP64, for example, means that int, long and pointer types are 64 bits wide, LP32 means that long and pointer are 32 bits wide, and LLP64 means that long long and pointer are 64 bits wide.

Common data models

Data Type	LP32	ILP32	ILP64	LLP64	LP64
char	8	8	8	8	8
short	16	16	16	16	16
int	16	32	64	32	32
long	32	32	64	32	64
long long				64
pointer	32	32	64	64	64

The following observations are derived from the Development Tutorial by Marco van de Voort [[van de Voort 07]]:

The usual programming model for current (Intel family) PC processors is ILP32.
One issue with long in C was that there are both code bases that expect pointer and long to have the same size, while there are also large code bases that expect int and long to be the same size. The compatibility model LLP64 was designed to preserve long and int compatibility by introducing a new type to remain compatible with pointer (long long).
LLP64 is the only data model that defines a size for the long long type.
LP32 is used as model for the win-16 APIs of Windows 3.1.
Most Unix versions use LP64, primarily to conserve memory space compared to ILP64, including: 64-bit Linux, FreeBSD, NetBSD, and OpenBSD.
Win64 uses the LLP64 model (also known as P64). This model conserves type compatibility between long and int, but loses type compatibility between long and pointer types. Any cast between a pointer and an existing type requires modification.
ILP64 is the easiest model to work with, because it retains compatibility with the ubiquitous ILP32 model, except specific assumptions that the core types are 32-bit. However this model requires significant memory, and both code and data size significantly increase.

`<limits.h>`

Possibly more important than knowing the number of bits for a given type, one can use macros defined in <limits.h> to determine the integral ranges of the standard integer types. 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 the implementation supports that contains at least 32 bits. The type uint_fast16_t is the fastest unsigned integer type the implementation supports that contains at least 16 bits. The type intmax_t is the largest signed integer type supported by the implementation. The following types are required to be available on all implementations.

Smallest Types	signed	unsigned
8 bits	int_least8_t	uint_least8_t
16 bits	int_least16_t	uint_least16_t
32 bits	int_least32_t	uint_least32_t
64 bits	int_least64_t	uint_least64_t
Fastest Types	signed	unsigned
8 bits	int_fast8_t	uint_fast8_t
16 bits	int_fast16_t	uint_fast16_t
32 bits	int_fast32_t	uint_fast32_t
64 bits	int_fast64_t	uint_fast64_t
Largest Types	signed	unsigned
maximum	intmax_t	uintmax_t

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.

Non-Compliant Code Example

The following non-compliant code attempts to guarantee that all bits of a multiplication of two unsigned values will be retained by performing arithmetic in the type unsigned long. This works for the LP32 and LP64 models, but not for others.

unsigned a, b;
unsigned long c;
/* ... */
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 integer operations do not result in an overflow.

#if UINTMAX_MAX < (UINT_MAX*UINT_MAX)
#error No safe type is available.
#endif
/* ... */
unsigned a, b;
uintmax_t c;
/* ... */
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 range of values that can be represented using integer types.

Recommendation	Severity	Likelihood	Remediation Cost	Priority	Level
INT00-A	1 (low)	1 (unlikely)	1 (high)	P1	L3

Related Vulnerabilities

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

References

[[van de Voort 07]]
[[Open Group 97]]

04. Integers (INT) 04. Integers (INT) INT01-A. Use rsize_t or size_t for all integer values representing the size of an object

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