CWE-469ΒΆ

Use of Pointer Subtraction to Determine Size. [Pointer-Issues, Incorrect-Calculation]

Required inputs: IR

The product subtracts one pointer from another in order to determine size, but this calculation can be incorrect if the pointers do not exist in the same memory chunk.
Demonstrative Examples
Example 1

The following example contains the method size that is used to determine the number of nodes in a linked list. The method is passed a pointer to the head of the linked list.

Example Language:C
    struct node {
        int data;
        struct node* next;
    };

    // Returns the number of nodes in a linked list from

    // the given pointer to the head of the list.
    int size(struct node* head) {
        struct node* current = head;
        struct node* tail;
        while (current != NULL) {
            tail = current;
            current = current->next;
        }
        return tail - head;
    }

    // other methods for manipulating the list
    ...

However, the method creates a pointer that points to the end of the list and uses pointer subtraction to determine the number of nodes in the list by subtracting the tail pointer from the head pointer. There no guarantee that the pointers exist in the same memory area, therefore using pointer subtraction in this way could return incorrect results and allow other unintended behavior. In this example a counter should be used to determine the number of nodes in the list, as shown in the following code.

Example Language:C
    ...

    int size(struct node* head) {
        struct node* current = head;
        int count = 0;
        while (current != NULL) {
            count++;
            current = current->next;
        }
        return count;
    }
Excerpts from CWE [https://cwe.mitre.org], Copyright (C) 2006-2026, the MITRE Corporation. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

pointer_difference

Do not use pointer subtraction.

None

False

Options