Access violation exception

Question

I have a c# program that calls a C dll which in turn calls java dll( built using jet excelsior a 3rd party program). I am passing an xml string from the c# into the java and the java then returns the string to the the c# to process.

This works on the first iteration, but then on the second iteration I get the following exception...

Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

Here is what I would class as the relevant code but if you require anything else please let me know.

C# Calls to C dll

public static class DllCall
{
    [DllImport("Stubs", CallingConvention = CallingConvention.Cdecl)]
    public static extern int initDll([MarshalAs(UnmanagedType.LPStr)] string userDllName);

    [DllImport("Stubs", CallingConvention = CallingConvention.Cdecl)]
    public static extern void finalizeDll();

    [DllImport("Stubs", CallingConvention = CallingConvention.Cdecl)]
    public static extern UInt32 newClassInstance(String rootPath, String cfgPath, String logPath );

    [DllImport("Stubs", CallingConvention = CallingConvention.Cdecl)]
    public static extern String request(UInt32 hClassInst, [MarshalAs(UnmanagedType.LPStr)] String input);

    [DllImport("Stubs", CallingConvention = CallingConvention.Cdecl)]
    public static extern void close();


}

Method in C dll that is throwing the error

const char* request(jobject obj, char* input )
{
    jstring inputString;
    jstring outputString;
    const char *nativeString;


    jmethodID mID = (*env)->GetMethodID (env, jClass, "request", "(Ljava/lang/String;)Ljava/lang/String;");
    if (!mID){
        printf("\nError: dllClass.request() not found\n");
        return 0;
    }

    inputString = (*env)->NewStringUTF(env, input);


    outputString = (*env)->CallObjectMethod(env, obj, mID, inputString);    

    nativeString = (*env)->GetStringUTFChars(env, outputString, 0); 


    return nativeString;
}

As requested here is the C# code that actually causes the exception.

        public string request(string xmlInput)
{
    LogManager.logMessage("Sending request to Java. Request is - " + xmlInput);
    string rs ="";
    Console.Write("Making request");
    //this works fine
    rs = DllCall.request(hClass, xmlInput); 
    Console.Write("---> request() rs = {0}\n", rs);
    // this throws the error
    rs = DllCall.request(hClass, "<?xml version='1.0' encoding='utf-8'?><moo><request name=\"Panel.Open.GetSelectionTemplate\"/></moo>"); 
    return rs;
}

In Response to Daniel here is where env is declared

#include <jni.h>
#include <windows.h>

JNIEnv  *env;
JavaVM  *jvm;
HANDLE  hUserDll;
jclass  jClass;
char*  dllname;

And here is how it is initialised.

int initDll(char* userDllName) 
{
  jClass = NULL;
  hUserDll = loadDll(userDllName); 
  dllname = userDllName;
  initJavaRT(hUserDll, &jvm, &env); 
  jClass = lookForClass(env, "XActMain/XActGeminiX3/XActGeminiX3IFX");
  return jClass ? 1 : 0;
}

/*
 * Initialize JET run-time.
 */
void initJavaRT(HANDLE myDllHandle, JavaVM** pjvm, JNIEnv** penv)
{
    int            result;
    JavaVMInitArgs args;

    JNI_GetDefaultJavaVMInitArgs_func = 
             (jint (JNICALL *) (void *args))
             GetProcAddress (myDllHandle, "JNI_GetDefaultJavaVMInitArgs");

    JNI_CreateJavaVM_func =
             (jint (JNICALL *) (JavaVM **pvm, void **penv, void *args))
             GetProcAddress (myDllHandle, "JNI_CreateJavaVM");

    if(!JNI_GetDefaultJavaVMInitArgs_func) {
        printf ("%s doesn't contain public JNI_GetDefaultJavaVMInitArgs\n", dllname);
        exit (1);
    }

    if(!JNI_CreateJavaVM_func) {
        printf ("%s doesn't contain public JNI_CreateJavaVM\n", dllname);
        exit (1);
    }

    memset (&args, 0, sizeof(args));
    args.version = JNI_VERSION_1_2;

    result = JNI_GetDefaultJavaVMInitArgs_func(&args);
    if (result != JNI_OK) {
        printf ("JNI_GetDefaultJavaVMInitArgs() failed with result %d\n", result);
        exit(1);
    }

    /*
     * NOTE: no JVM is actually created
     * this call to JNI_CreateJavaVM is intended for JET RT initialization
     */
    result = JNI_CreateJavaVM_func (pjvm, (void **)penv, &args);
    if (result != JNI_OK) {
        printf ("JNI_CreateJavaVM() failed with result %d\n", result);
        exit(1);
    }

    printf ("JET RT initialized\n");
    fflush (stdout);
}

This is in response to Joes comment about initialisation...

public class Test
 {
     public UInt32 hClass;

     public Test()
     {
        initDll();
        newClassInstance(rootConfig, config, logFile);
     }

     ........

     public void newClassInstance(string rootPath, string cfgPath, string logPath)
     {
         hClass = DllCall.newClassInstance(rootPath, cfgPath, logPath);
         Console.Write("---> hClass = {0}\n", hClass);
     }
     public void initDll()
     {
             int rc = DllCall.initDll("dllClass.dll");
             Console.Write("---> initDll() rc = {0}\n", rc);
     }

Hans has pointed out the following link potential answer

I am however not sure how to modify my current code to accomodate this solution.

As I say it works once then crashes on the second iteration.

Answer 1

I'm not sure why it's working the first time, but you're passing a class object (hClass) into request instead of an instance of that class.

rs = DllCall.request(hClass, xmlInput); //The error is thrown on this line

See if this fixes the problem:

public string request(string xmlInput)
{
    LogManager.logMessage("Sending request to Java. Request is - " + xmlInput);
    string rs ="";
    Console.Write("Making request");

    UInt32 hClassInst = DllCall.newClassInstance(rootPath, cfgPath, logPath); // <-- New line, using your own rootPath, cfgPath, logPath variables
    rs = DllCall.request(hClassInst, xmlInput);                               // <-- Modified line, using hClassInst instead of hClass

    Console.Write("---> request() rs = {0}\n", rs);
    return rs;
}

If this fixes the problem, you should probably then refactor it and pull out the newClassInstance call to an earlier initialization method.