## Does Storing an Invalid Memory Address in a Pointer Variable Lead to Undefined Behavior?

Pointer Manipulation and Undefined Behavior

The behavior of pointers is a fundamental aspect of C and C programming. Undefined behavior (UB), a language-implementation-defined behavior that results in undefined results or crashes, arises from improper pointer handling. While dereferencing an invalid pointer is a well-known source of UB, the question remains: does merely storing an invalid memory address in a pointer variable also trigger UB?

This question stems from pointer arithmetic commonly used in code, relying on the seemingly innocuous storage of invalid memory addresses. For instance, the code snippet provided compares the validity of a pointer value to a previously determined address:

const char* str = "abcdef";
const char* begin = str;
if (begin - 1 < str) { /* ... do something ... */ }

The expression begin - 1 evaluates to an invalid memory address, potentially raising concerns about UB. It's important to note that while the address is not dereferenced, it is loaded into a register, potentially triggering an error on certain architectures.

To address this ambiguity, the C Draft Standard defines the behavior of pointer addition (ptr I) in 6.5.6/8. This section explicitly defines the actions taken when the pointer operand points to an array element or when it points one past the end of an array. However, it omits the case of storing an invalid memory address in a pointer variable.

By omission, the C Draft Standard leaves the behavior undefined, rendering the storage of invalid memory addresses in pointer variables a potential source of UB. This implies that code relying on such operations may behave unpredictably or terminate unexpectedly, depending on the implementation.

Therefore, it's essential to strictly adhere to pointer manipulation behaviors defined in the language standard to avoid UB and ensure consistent program behavior.

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