Why Does My C Loop Run Forever When Accessing an Array Out of Bounds?

Undefined Behavior in C Compilation

In the given code, the expected behavior is to iterate through the array mc from index 0 to 3 and output the values, resulting in a sequence of numbers "0, 1, 2, 3". However, the observed behavior is an endless loop that outputs an unending series of "0, 1, 2, 3, ....".

The root of the issue lies in the assignment statement delta = mc[di]. This operation attempts to access the array mc beyond its valid indices, specifically at index 4 (out of bounds). In C , such access invokes undefined behavior, allowing the compiler to behave in unpredictable ways.

In this case, the compiler with optimization enabled (e.g., using the -O2 flag) assumes that no undefined behavior occurs. Specifically, it infers that di < 4 is always true, since accessing mc[di] out of bounds would otherwise be undefined.

This assumption leads to aggressive loop optimizations. In the optimized code, the di < 4 check is removed, and an unconditional jump instruction is added. This effectively eliminates the intended loop termination condition and results in the endless loop.

Turning off aggressive loop optimizations using the -fno-aggressive-loop-optimizations flag causes the compiler to behave as expected. It recognizes the potential for undefined behavior and prevents the problematic optimization.

It is important to note that undefined behavior in C can have unpredictable and potentially harmful consequences. It is recommended to avoid relying on undefined behavior and to write code that adheres to the language specifications.

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