Why is Pointer Arithmetic Beyond Array Bounds Considered Undefined Behaviour in C ?

Pointer Arithmetic and Undefined Behaviour: Exploring the Boundaries

Pointer arithmetic is a powerful tool in programming, allowing efficient memory manipulation. However, when it ventures beyond the bounds of an array, it enters the realm of undefined behaviour.

In the example provided, an integer array arr is assigned with four elements. A pointer p is then set to point five elements beyond arr, i.e., arr 5. This operation is flagged as undefined behaviour by the C standard.

The intuition behind this restriction may not be immediately apparent, especially if the pointer is never dereferenced (i.e., attempts to access the memory it points to). One might assume that it should behave like a standard integer operation, but this is not the case.

The crux of the matter lies in the fact that pointers and integers are not inherently equivalent. While they can share similar characteristics, such as being incremented, decremented, and compared, there are fundamental differences. Pointers reference memory locations, while integers represent numeric values.

The C standard explicitly defines that exceeding array bounds using pointer arithmetic is undefined behaviour. This means that the compiler has no obligation to handle the situation consistently across different platforms or even different executions of the same code.

Why does this matter? Even if no explicit dereference is performed, the result of the pointer arithmetic may affect other code paths. For example, if the out-of-bounds pointer is used in a pointer comparison or passed as an argument to a function, it can potentially lead to unexpected results.

In summary, pointer arithmetic beyond array bounds is deemed undefined behaviour to ensure consistent compiler behaviour and safeguard against potential hazards. While it may seem harmless to add one element past the end of an array, the repercussions can be unpredictable and should be avoided.

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