MISRA C++:2008 Rule 5-0-17

Subtraction between pointers shall only be applied to pointers that address elements of the same array

Description

Rule Definition

Subtraction between pointers shall only be applied to pointers that address elements of the same array.¹

Rationale

When you subtract between two pointers to elements in the same array, the result is the distance between the two array elements. If the pointers are null or point to different arrays, a subtraction operation is undefined. If you use the subtraction result as a buffer index, it can cause a buffer overflow.

Before you subtract between pointers to array elements, make sure that they are non-null and that they point to the same array.

Polyspace Implementation

Polyspace^® flags subtraction between pointers to elements of different arrays.

Troubleshooting

If you expect a rule violation but Polyspace does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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Avoid Subtracting Pointers to Elements of Different Arrays

void foo(){
	int a[10];
	int b[10];
	int distance = a-b;//Noncompliant
	
}

In this example, Polyspace flags the subtraction between a and b, which are elements of different arrays.

Check Information

Group: Expressions

Category: Required

Version History

Introduced in R2013b

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R2023a: Pointer subtraction involving outputs of stubbed function no longer ignored

Polyspace reports violations of this rule even if you use the output of a stubbed function to perform a pointer subtraction or pointer comparison operation. For instance, in this code, the pointer ptr is returned by the function get_ptr():

int32_t *ptr = get_ptr ( );
int32_t a1[ 10 ];
int32_t *p1 = &a1[ 1 ];
int32_t diff = ptr - p1;   //Noncompliant

Because the function get_ptr() lacks a body, Polyspace treats it as a stubbed function. Polyspace no longer ignores pointer arithmetic operations when one of the operands is the output of the stubbed function. In this case, because get_ptr() is called before p1 is declared, the pointers ptr and p1 cannot point to the same array. Polyspace reports a violation.

MISRA C++:2008 Rule 5-0-17

Description

Rule Definition

Rationale

Polyspace Implementation

Troubleshooting

Examples

Avoid Subtracting Pointers to Elements of Different Arrays

Check Information

Version History

R2023a: Pointer subtraction involving outputs of stubbed function no longer ignored

See Also

Topics