{"id":7352,"date":"2019-07-23T09:51:27","date_gmt":"2019-07-23T16:51:27","guid":{"rendered":"https:\/\/www.coretechnologies.com\/blog\/?p=7352"},"modified":"2019-07-23T09:51:27","modified_gmt":"2019-07-23T16:51:27","slug":"detect-endless-loop","status":"publish","type":"post","link":"https:\/\/www.coretechnologies.com\/blog\/alwaysup\/detect-endless-loop\/","title":{"rendered":"Q&A: Why Doesn’t AlwaysUp Catch my Endless Loop?"},"content":{"rendered":"
\"Q<\/div>\n
\n  Following your AlwaysUp documentation<\/a>, I am using the “Monitor the application and stop it whenever it hangs” option. When I set its value to 1 minute, this feature does not work as I expect.<\/p>\n

I wrote a simple application that goes into an endless loop after one minute of execution time. But the application is not restarted by AlwaysUp. Why not?<\/p>\n

— Beatrix<\/p>\n<\/div>\n

Hi Beatrix. Not all infinite loops can be detected by AlwaysUp. Let me explain…<\/p>\n

AlwaysUp catches non-responsive GUI applications<\/h2>\n

AlwaysUp’s hang detection feature is designed to identify a specific situation — where a Windows GUI application is not responding to the operating system.<\/p>\n

We have all encountered this type of hang before. It happens when Firefox suddenly locks up<\/a>, or when Excel goes gray<\/a> and the cursor turns into a spinning wheel. The application is frozen and all input is ignored.<\/p>\n

If you are lucky, the disruption only lasts a few seconds; if not, you may have to forcibly terminate the application from the Task Manager. Very annoying!<\/p>\n

Beyond the gray window and spinning cursor, there are a couple of telltale signs when an application is hung\/non-responsive:<\/p>\n

    \n
  1. \n

    You will see “Not responding” in the application’s title bar.<\/p>\n

  2. \n

    The application’s process will be marked as “Not responding” in the Task Manager:<\/p>\n

    \"Task<\/a><\/div>\n<\/li>\n<\/ol>\n

    Furthermore, Windows may throw up a warning outlining your very limited options:<\/p>\n

    \"Excel<\/a><\/div>\n

    The good news is that AlwaysUp has you covered in all of those situations!<\/p>\n

    How AlwaysUp checks if your application is hung<\/h2>\n

    When you enable hang-checking, the AlwaysUp Windows Service component executes the following procedure every few seconds:<\/p>\n

      \n
    1. \n

      Scans the desktop to identify all top-level windows<\/a> belonging to the application being monitored.<\/p>\n<\/li>\n

    2. \n

      Sends each of those top-level windows a WM_NULL (no-op)<\/a> message.<\/p>\n<\/li>\n

    3. \n

      Waits a few seconds for each of the windows to acknowledge the message.<\/p>\n<\/li>\n

    4. \n

      Declares the application a hang if no windows respond.<\/p>\n<\/li>\n<\/ol>\n

      An application that remains hung over the user-supplied duration (1 minute, in your case) will be terminated and restarted.<\/p>\n

      Not all endless loops result in a frozen\/non-responsive application<\/h2>\n

      There are a couple of consequences stemming from the way that AlwaysUp checks for a hang.<\/p>\n

      First, an application without a top-level window can never be classified as a hang<\/b>.<\/p>\n

      For example, this simple, non-GUI C# program that loops forever will escape detection:<\/p>\n

      \n
      \nusing System;\nnamespace InfiniteLoop {\n   class Program {\n      static void Main(string[] args) {\n         while (true) {\n            Thread.Sleep(1000);\n         }\n      }\n   }\n}\n<\/pre>\n<\/div>\n
      Second, if an application creates multiple windows, all windows must be non-responsive for AlwaysUp to declare a hang<\/b>. If even a single window processes the WM_NULL message, the application will be classified as responsive.<\/p>\n
      My guess is that your endless loop test fits into one of the scenarios above. Unfortunately AlwaysUp’s hang detection feature will not be able to help there. Sorry about that!<\/p>\n
      But all is not lost…<\/p>\n
      Use the Sanity Check feature to detect endless loops and other failures<\/h2>\n
      There can be many indications that a program is stuck. For example, an application might:<\/p>\n
        \n
      • \n
        stop responding to requests over the network;<\/p>\n<\/li>\n
      • \n
        stop writing entries to a status file;<\/p>\n<\/li>\n
      • \n
        report an “I have failed” message to a log file;<\/p>\n<\/li>\n
      • \n
        or leave files collecting dust in a specific “inbox” folder<\/p>\n<\/li>\n<\/ul>\n
        In situations like these — where the failure is detectable by a script or simple utility — you should look to AlwaysUp’s powerful Sanity Check feature<\/a>. It allows you to extend failure detection to encompass anything you like, including the issues listed above.<\/p>\n
        The Sanity Check page<\/a> digs into the technical details, which we will not repeat here. Please review the documentation and get in touch<\/a> if you need help to use that flexible functionality.<\/p>\n