Is Storing an Invalid Address in a Pointer Variable Always Undefined Behavior in C and C ?

Is Pointer Arithmetic with Invalid Addresses Always Undefined Behavior?

Pointer arithmetic is a fundamental operation in C and C , allowing programmers to manipulate memory addresses and navigate data structures. However, the behavior of pointer arithmetic with invalid addresses is not always clear.

Storing Invalid Addresses in Pointers: Undefined or Not?

Consider the following snippet, which attempts to test the validity of a pointer by subtracting a constant:

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

In this example, the expression begin - 1 results in an invalid memory address. The question is, does the mere act of storing this address in the pointer variable begin constitute undefined behavior?

The Standard's Ambiguity

The C and C standards do not explicitly address this scenario. Section 6.5.6/8 defines pointer arithmetic operations, but does not cover the case of invalid addresses.

However, it does define the behavior of pointer arithmetic within valid array boundaries and for pointers that point one element past the end of an array. Neither of these conditions applies to the above example.

Possible Architectures-Specific Undefined Behavior

Some architectures implement memory protection mechanisms that may raise an exception (e.g., bus error) when an invalid memory address is accessed. In such cases, storing an invalid pointer in a register could indeed be considered undefined behavior.

Conclusion

The C and C standards do not definitively answer whether storing an invalid address in a pointer variable constitutes undefined behavior. However, it is possible that certain architectures may exhibit undefined behavior in such scenarios due to memory protection mechanisms. It is best to avoid such practices and rely on proper pointer validation techniques to prevent undefined behavior.

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