Question

I have an application server (JBoss, but this also happens in Tomcat) running as a service in Windows Server 2003. It is running with the -Xrs flag.

The Java application running under the application server calls a customizable interface written in C++ via JNI (meaning we can alter this code), referencing a third-party DLL file for processing images (Lincoln for converting PostScript).

When we log into the server via Remote Desktop Connection in either console (mstsc /console) or administrator (mstsc /admin) mode, when we logout, if the Lincoln DLL file has been loaded, the application server will acknowledge the logoff signal and the service process will immediately terminate without prejudice.

I believe the signal is CTRL_LOGOFF, but I could be incorrect.

After JavaJiggle Article on Signal Handling, apparently signal handlers are passed to the DLL file when the DLL file is processing. This means the third-party DLL file (Lincoln in this case) listens to and responds to the CTRL_LOGOFF signal by logging off.

I believe, I should be able to code a signal catcher in my C++ interface to the DLL to intercept the CTRL_LOGOFF before it reaches the DLL and if so, then we won’t constantly die when someone logs off of a console/admin RDP.

Here is what I need:

1. Am I correct that the signal that I am getting on console/admin logoff/logout is CTRL_LOGOFF?

2. Can I write a signal interceptor in the C++ interface?

3. How do I code that signal interceptor, or is there pre-existing code? I am using a 32-bit DLL.

I have found Microsoft article Registering a Control Handler Function, which may help answer this question.

Answer 1

I seem to have resolved it by adding an ignore handler to the stack every time the third-party DLL is called, but I fear each time I call my method, we keep adding handlers to the stack and obviously the third-party DLL does not remove its handlers. I do not know whether this is creating a memory leak.

Is there is a way to prevent a third-party handler from being placed in the first place? I have asked a follow-up question to answer this: How can I prevent my Console Control Handler from being overridden?.

Here is my custom JNI class method, which calls the third-party DLL file:

JNIEXPORT jint JNICALL Java_com_company_ConvertProxy_convertToImageType(JNIEnv *env, jclass cls, jstring input, jstring output) {

    jboolean isCopy;
    inFilename = env->GetStringUTFChars(input, &isCopy);
    outFilename = env->GetStringUTFChars(output, &isCopy);

    // I tried to call SetConsoleCtrlHandler() here, but failed;
    // it turns out third-party code in ConvertImage() also
    // calls SetConsoleCtrlHandler and overrides it if placed here.

    int value = ConvertImage();

    // Deafen Control Logoffs set by third-party ConvertImage.
    // SetConsoleCtrlHandler( NULL, TRUE ); // DOES NOT WORK, must use custom CtrlHandler.
    SetConsoleCtrlHandler( (PHANDLER_ROUTINE) CtrlHandler, TRUE );

    return value;
}

BOOL CtrlHandler( DWORD fdwCtrlType ) {
    switch( fdwCtrlType )
    {
        // Handle the CTRL-C signal.
        case CTRL_C_EVENT:
          return( TRUE );

        // CTRL-CLOSE: confirm that the user wants to exit.
        case CTRL_CLOSE_EVENT:
          return( TRUE );

        case CTRL_BREAK_EVENT:
          return( TRUE );

        case CTRL_LOGOFF_EVENT:
          return( TRUE );

        case CTRL_SHUTDOWN_EVENT:
          return( TRUE );

        default:
          return FALSE;
      }
}