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Why Does Signed Integer Overflow Cause an Infinite Loop in This C Code?

Patricia Arquette

Dec 02, 2024 pm 10:12 PM

Why Does Signed Integer Overflow Cause an Infinite Loop in This C Code?

While seemingly inconsequential, the integer overflow in the provided code leads to a significant consequence: the broken value of the loop variable.

The behavior observed here stems from the undefined behavior resulting from signed integer overflow, as noted in C 11 draft N3337:

"If during the evaluation of an expression, the result is not mathematically defined or not in the range of
representable values for its type, the behavior is undefined."

With this undefined behavior, anything can happen, rendering analysis of why specifically this behavior occurs meaningless under C rules.

The assembly listing of the code reveals the explicit addition that causes the overflow, effectively turning the loop into an infinite one. This behavior is due to the assumption that i is less than or equal to 2 for optimization purposes, based on the presence of undefined behavior for values larger than 2. As a result, the loop condition is considered always true.

To rectify this issue and ensure correct execution, it is crucial to eliminate the undefined behavior by revising the condition to ensure it termin

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