Fault, Error & Failure

• 在軟體與硬體中都有Fault, Error & Failure，此處我們只討論軟體

Software Fault

• A static defect in the software(i.e., defect)
• 靜態存在於code中的錯誤

Software Error

• An incorrect internal state that is the manifestation of some faults
• 由fault觸發的錯誤中間狀態

Software Failure

• External, incorrect behavior with respect to the requirements of othere description of the expected behavior
• 由Error所傳播出去，使測試人員或使用者所觀測到的失效行為

實例

• 說明 - 給定input numbers, 算出平均值
• Fault - i的初始值誤設為1(應為0)
• Error - sum誤算為4+5 (應為3+4+5)
• Failure - mean誤算為3 (應為3)

PIE Model

• 首字母P、I、E所組成
• 只有達成下面三項，才會得到failure

Execution/Reachability -> Fault

• The location or lcations in the program that contain the fault must be reached
• 測試必須執行到fault的位置

Infection -> Error

• The state of the program must be incorrect
• 即使執行到fault的位置，也不一定會觸發Error
• 
• 此例中，即使執行到了fault，但碰巧地沒有觸發error，最終的結果也是正確

Propagation -> Failure

• The infected state must propagate to cause some output of the program to be incorrect
• 即使執行到了fault，觸發了error，也可能沒有failure
• 
• Fault - length長度錯誤值 number.length-1 (應為number.length)
• Error - sum值計算錯誤3+5(應為3+5+4)
• Failure - 然後最終碰巧地沒有Failure

Quiz

• Is there is a fault, which cannot be revealed by any test?
• Is it a fault or not a fault?
• 我的見解是，這不算fault，因為唯有被測試出來，才能稱之為fault，雞生蛋、蛋生雞的概念

Exercise

• Please constrct a simple program P(with a fault) and 3 tests(t1, t2, t3), s.t.
  • t1 executes the fault, but no error
  • t2 (executes the fault and) produces an error, but no failure
  • t3 produces a failure

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string string_reverse(string input){
    int length = input.length() - 2;
    int i = 1;
    int j = length;
    while(i < j) {
        swap(input[i], input[j]);
        i++;
        j--;
    }
    return input;
}

• tests(t1, t2, t3)
  1. t1 =
  2. t2 =
  3. t3 =