Why Does Signed Integer Overflow Cause a Loop to Produce Undefined Behavior and Unexpected Output?

Why does this loop produce "warning: iteration 3u invokes undefined behavior" and output more than 4 lines?

GCC produces the warning "warning: iteration 3u invokes undefined behavior" because it detects that the signed integer overflow operation i * 1000000000 in the loop condition results in undefined behavior. According to the C standard, undefined behavior can lead to anything, including unexpected output.

Why is the i value broken by the overflow operation?

Integer overflow occurs when the result of an arithmetic operation exceeds the maximum representable value for the data type. In this case, the data type is int, which has a maximum value of 2,147,483,647. When the integer overflow occurs, the result is truncated and wraps around to the minimum value of the data type, which is -2,147,483,648. This means that the value of i is broken by the overflow operation and becomes equal to -2,147,483,648.

Why does the loop output more than 4 lines?

The loop condition is evaluated to true while i is less than 4. However, due to the integer overflow, the value of i is broken and becomes equal to -2,147,483,648. This value is not less than 4, but it is still a valid integer value and the loop will continue to execute until it reaches a value that is greater than 4 or when a termination condition is met.

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