Pitfalls and caveats in pointer arithmetic?

There are pitfalls and precautions in pointer arithmetic, including out-of-bounds reading/writing, wild pointers, pointer division operations, and pointer type conversions. Ways to avoid these pitfalls include verifying that pointers point to valid memory, using pointer offsets carefully, avoiding wild pointers, and handling pointer type conversions carefully. For example, the str_len() function calculates the string length by incrementing the pointer using the pointer offset str to ensure it does not go out of bounds.

Traps and Precautions of Pointer Arithmetic

Pointer arithmetic is a powerful technique that allows programmers to work more efficiently Process memory. However, there are some pitfalls and caveats to be aware of to avoid undefined behavior or crashes.

Out-of-bounds reading or writing

The most common pitfall is out-of-bounds reading or writing. This happens when the pointer points to memory outside the array or structure. For example:

int array[] = {1, 2, 3};
int *ptr = &array[2];
*(ptr + 1) = 4; // 越界写入

Wild pointer

Wild pointer is a pointer to an unallocated memory area. Undefined behavior occurs when data from wild pointers is referenced. For example:

int *ptr;
*ptr = 5; // 野指针

Division operation

Pointer division operation may lead to unexpected results. For example:

int *ptr1, *ptr2;
// ptr1 和 ptr2 指向同一数组的不同元素
ptr2 = ptr1 + 5;
// 现在 ptr2 指向 ptr1 指向的元素之外

Pointer type conversion

Pointer type conversion (such as (char *)ptr) may cause strange behavior. For example:

int *ptr = ...;
char *char_ptr = (char *)ptr;
char_ptr[0] = 'a'; // 可能覆盖 int 数据

Avoid Pitfalls

Best practices to avoid these pitfalls include:

• Always verify that pointers point to valid memory areas , for example by using bounds checking.
• Be careful when using pointer offsets to make sure they don't cause out-of-bounds.
• Avoid using wild pointers.
• Handle pointer type conversions with caution.

Practical case

The following function uses pointer arithmetic to iterate over a string and calculate its length:

int str_len(char *str)
{
    int len = 0;
    while (*str++) {
        len++;
    }
    return len;
}

In this function, Pointer str is incremented by pointer offset str . This offset makes the pointer point to the next character of the string and ensures that it does not go out of bounds because str always points to a valid character or terminator '\0'.

Conclusion

Pointer arithmetic is a powerful technique, but one needs to be aware of its pitfalls and caveats. By following these best practices, programmers can avoid undefined behavior, crashes, and unexpected results, and write more reliable and efficient code.

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