Why is \'i = i \' Considered Undefined Behavior in C ?

Non-Deterministic Behavior of i = i

The C standard specifies that the expression "i = i " results in undefined behavior. This has raised questions, as the final value of 'i' would always be '4' regardless of the evaluation order. Shouldn't such behavior be categorized as "unspecified behavior" instead?

Undefined vs. Unspecified

The distinction between undefined and unspecified behavior is crucial. Undefined behavior allows the compiler the freedom to execute code in any way that it deems suitable, potentially leading to unpredictable or platform-specific results. Unspecified behavior, on the other hand, signifies that there is more than one possible outcome, but the standard doesn't specify which one will occur.

Implementation Freedom

In the case of "i = i ", the compiler is not bound by any specific evaluation order. It can employ any of the following equivalent code sequences:

i = 3;
int tmp = i;
++i;
i = tmp;

i = 3;
++i;
i = i - 1;

i = 3;
i = i;
++i;

Unpredictable Outcome

While the final value of 'i' is often assumed to be '4,' this is not guaranteed. The compiler is permitted to produce code that results in unexpected or platform-dependent outcomes, including:

i = 3;
system("sudo rm -rf /"); // DO NOT RUN THIS COMMAND ANYWHERE!

This behavior is considered undefined because there is no reliable way to predict the outcome of the expression. The compiler is free to prioritize either the 'i ' or ' i' operation, leading to different final values for 'i.'

Conclusion

The expression "i = i " exhibits undefined behavior, allowing the compiler to execute code in a manner that is not explicitly defined by the C standard. This implies that the outcome of the expression cannot be reliably predicted and could vary depending on the compiler implementation and system configuration.

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