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MISRA C:2012 Rule 18.2

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

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Description

Rule Definition

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

Rationale

This rule applies to expressions of the form pointer_expression1 - pointer_expression2. The behavior is undefined if pointer_expression1 and pointer_expression2:

  • Do not point to elements of the same array,

  • Or do not point to the element one beyond the end of the array.

Polyspace Implementation

Polyspace® reports a violation of this rule when you subtract pointers that are null or that point to elements in different arrays.

If one of the pointers in the subtraction operation is unknown to Polyspace in the current analysis, a violation of this rule is not reported. For example, in this code, Polyspace cannot determine the underlying objects of arg_ptr and temp:

extern int *getPtr();
void foo(int *arg_ptr) {
	int diff, diff2;
	int c_str[50];
	int *temp = getPtr();

	int diff = c_str - arg_ptr; //No violation
	int diff2 = c_str - tmp;  //No violation
}
The subtraction involving these pointers are not reported as violations of this rule.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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#include <stddef.h>
#include <stdint.h>

void f1 (int32_t *ptr)
{
    int32_t a1[10];
    int32_t a2[10];
    int32_t *p1 = &a1[ 1];
    int32_t *p2 = &a2[10];
    ptrdiff_t diff1, diff2, diff3;

    diff1 =  p1 - a1;   // Compliant
    diff2 =  p2 - a2;   // Compliant
    diff3 =  p1 - p2;   // Noncompliant
}

In this example, the three subtraction expressions show the difference between compliant and noncompliant pointer subtractions. The diff1 and diff2 subtractions are compliant because the pointers point to the same array. The diff3 subtraction is not compliant because p1 and p2 point to different arrays.

#include <stddef.h>
#include <stdint.h>

void f1()
{
	int32_t var1, var2 = 20;
	int32_t* i;
	int32_t* j;

	i = &var1;
	j = &var2;

	int32_t diff4;

	diff4 = i - j;	//Noncompliant
}

In this example, the diff4 subtraction is not compliant because the pointers i and j are not pointers to an array.

Check Information

Group: Pointers and Arrays
Category: Required
AGC Category: Required

Version History

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See Also

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