Why Does This C Code Result in an Infinite Loop Instead of a Simple Output?

Unexpected Endless Loop Output: A Gotcha in C Compilation

In C , certain seemingly innocuous operations can lead to unexpected behavior, as exemplified by the following code snippet:

<code class="cpp">#include <iostream>
#include <complex>

using namespace std;

int main() {
    complex<int> delta;
    complex<int> mc[4] = {0};

    for (int di = 0; di < 4; di++, delta = mc[di]) {
        cout << di << endl;
    }

    return 0;
}</code>

Contrary to the expected output of "0, 1, 2, 3," the code mistakenly produces an endless series of numbers. This baffling behavior stems from a subtle yet significant issue related to undefined behavior.

The Undefined Behavior Conundrum

The assignment statement delta = mc[di] accesses the mc array beyond its valid indices on the last iteration of the loop. In the realm of C , accessing memory out of bounds constitutes undefined behavior, a realm where unpredictable outcomes reign supreme.

Aggressive Loop Optimization: A Double-Edged Sword

Compilers often employ aggressive loop optimizations to enhance performance. These optimizations leverage assumptions about the absence of undefined behavior. In the case of the given code, the compiler may deduce that di < 4 always holds true, regardless of the actual array bounds, based on the presumption that undefined behavior will not occur. This assumption, in turn, removes the boundary check, allowing the loop to iterate indefinitely.

Unraveling the Mystery

GCC with optimization enabled and without the -fno-aggressive-loop-optimizations flag exhibits the infinite loop behavior. However, with that flag enabled, the erroneous behavior disappears. Inspecting the assembly code revelaes that the di < 4 check has been removed and replaced with an unconditional jump.

Avoiding the Pitfall

To guard against such pitfalls, it is crucial to avoid undefined behavior and explicitly check for array bounds. Additionally, ensuring appropriate diagnostic messages through warning flags can provide valuable insights into potential issues.

Conclusion

Undefined behavior can lead to unexpected and unpredictable behavior in C code. It is essential to be aware of such potential pitfalls and diligently adhere to sound programming practices to prevent unexpected outcomes.

The above is the detailed content of Why Does This C Code Result in an Infinite Loop Instead of a Simple Output?. For more information, please follow other related articles on the PHP Chinese website!