ARR36-C. Do not subtract or compare two pointers that do not refer to the same array

According to section 6.5.6 of C99, when two pointers are subtracted, both must point to elements of the same array object or to one past the last element of the array object; the result is the difference of the subscripts of the two array elements. Otherwise the operation results in undefined behavior (see undefined behavior 45 of Appendix J). This restriction exists because pointer subtraction in C produces the number of objects between the two pointers, not the number of bytes.

Similarly, comparing pointers using the relational operators <, <=, >=, and > gives the positions of the pointers relative to each other. Subtracting or comparing pointers that do not refer to the same array results in undefined behavior (see undefined behavior 46 of Appendix J).

Comparing pointers using the equality operators == and != has well-defined semantics regardless of whether or not either of the pointers is null, points into the same object, one past the last element of an array object, or function.

It is acceptable to subtract or compare two member pointers within a single struct object, suitably cast, because any object can be treated as an array of unsigned char. However, when doing so remember to account for the effects of alignment and padding on the structure.

Noncompliant Code Example

In this noncompliant code example pointer subtraction is used to determine how many free elements are left in the nums array.

int nums[SIZE];
char *strings[SIZE];
int *next_num_ptr = nums;
int free_bytes;

/* increment next_num_ptr as array fills */

free_bytes = strings - (char **)next_num_ptr;

The first incorrect assumption is that the nums and strings arrays are necessarily contiguous in memory. The second is that free_bytes is the number of bytes available. The subtraction returns the number of elements between next_num_ptr and strings.

Compliant Solution

In this compliant solution, the number of free elements is kept as a counter and adjusted on every array operation. It is also calculated in terms of free elements instead of bytes. This prevents further mathematical errors.

int nums[SIZE];
char *strings[SIZE];
int *next_num_ptr = nums;
int free_bytes;

/* increment next_num_ptr as array fills */

free_bytes = (&(nums[SIZE]) - next_num_ptr) * sizeof(int);

Risk Assessment

Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as ARR36-CPP. Do not subtract or compare two pointers or iterators that do not refer to the same array or container.

References

[[Banahan 03]] Section 5.3, "Pointers," and Section 5.7, "Expressions involving pointers"
[[ISO/IEC 9899:1999]] Section 6.5.6, "Additive operators"
[[MITRE 07]] CWE ID 469, "Use of Pointer Subtraction to Determine Size"

ARR35-C. Do not allow loops to iterate beyond the end of an array      06. Arrays (ARR)