Why is Out-of-Bounds Pointer Arithmetic Considered Undefined Behaviour in C ?

Out-of-Bounds Pointer Arithmetic: Why It's Undefined Behaviour

Pointer arithmetic is a feature in programming languages that allows programmers to manipulate memory addresses by adding or subtracting integer values to pointers. While pointer arithmetic can be a useful tool, it's crucial to understand the potential risks associated with it, especially when dealing with out-of-bounds pointers.

The Problem

Consider the following C code snippet:

<code class="cpp">int arr[4] = {0, 1, 2, 3};
int* p = arr + 5;</code>

If the pointer p is never dereferenced (i.e., the value pointed to by p is never accessed), why is the statement arr 5 considered undefined behaviour?

The Answer

Pointers do not behave exactly like integers. While it's true that pointer arithmetic can resemble integer arithmetic, it's important to remember that pointers represent memory addresses, and they are subject to specific rules and constraints.

According to the C language standard, out-of-bounds pointer arithmetic is undefined behaviour because:

• Potential Memory Corruption: Accessing memory beyond the bounds of an array can corrupt data in adjacent memory locations.
• Segmentation Faults: In many operating systems, out-of-bounds pointer arithmetic can lead to segmentation faults, which typically crash the program.
• Incorrect Pointer Comparisons: Using out-of-bounds pointers can lead to incorrect comparisons between pointers, resulting in unpredictable behaviour.

Can It Ever Be Safe?

While accessing memory more than one element past the end of an array is explicitly undefined behaviour, an expression that goes one over the end of the array might be technically correct and won't cause a crash. However, the result of such an expression is unspecified, meaning it's unreliable for use in any meaningful way.

In practice, it's best to avoid out-of-bounds pointer arithmetic altogether and use safe constructs such as array indices or iterators to access memory.

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