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

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

Unlike integers, pointers in C do not behave identically. The C standard explicitly defines out-of-bounds pointer arithmetic as undefined behaviour. This means that any manipulation of a pointer that results in it pointing beyond the valid memory range is considered a severe error.

Explanation of Undefined Behaviour

Even if the pointer is not dereferenced (i.e., accessed for its value), adding an index that exceeds the array's bounds is still undefined behaviour. This is because pointer arithmetic involves more than just accessing memory; it also affects the validity of the pointer itself.

Potential Consequences

While out-of-bounds pointer arithmetic may not always crash a program, it can lead to:

• Editing data outside the allocated array, potentially causing memory corruption
• Comparing pointers that do not point to the same array, resulting in unexpected behaviour
• Carrying out operations on invalid memory addresses

Exception to Undefined Behaviour

The C 11 standard does allow one exception to undefined behaviour: accessing an element that is one past the end of an array. While this expression is technically "correct" and won't cause an overflow exception, the result is unspecified and should not be relied upon.

Reason for Undefinition

The reason for prohibiting out-of-bounds pointer arithmetic is to ensure program correctness and memory safety. By declaring such behaviour undefined, the compiler is not required to enforce it, allowing for more efficient implementations. It also serves as a warning to programmers to be cautious when dealing with pointers.

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