JNI Returning value from a DWord

Question

I have a function with this signature from the header file

SIMAPI_DECL DWORD WINAPI SimReadDwordBuffer (DWORD* pBuffer, 
                                             DWORD dwDwordsToRead, 
                                             DWORD* pdwDwordsRead, 
                                             DWORD dwBlockDwords, 
                                             DWORD dwNoWait);

With the following native call defined

protected native int  SimReadDwordBuffer (int[] pBuffer, 
                                          int dwDwordsToRead, 
                                          int pdwDwordsRead, 
                                          int dwBlockDwords, 
                                          int dwNoWait);

I use javah.exe to create the jni header and it looks like this

protected native int  SimReadDwordBuffer (int[] pBuffer, 
                                          int dwDwordsToRead, 
                                          int pdwDwordsRead, 
                                          int dwBlockDwords, 
                                          int dwNoWait);

And the implementation is this

JNIEXPORT jint JNICALL Java_com_sig_ccm_CcmBase_SimReadDwordBuffer
  (JNIEnv *env, jobject obj, jintArray pBuffer, jint dwWordsToRead, 
                             jint pdwWordsRead, 
                             jint dwBlockWords, jint dwNoWait){
jint *body = env->GetIntArrayElements(pBuffer, 0);
//DWORD foo = 0;
jint value = SimReadDwordBuffer((unsigned long int *)body,
                         dwWordsToRead,
                                     //&foo, 
                         (unsigned long int *)&pdwWordsRead,
                         dwBlockWords,
                         dwNoWait );
//cout << foo;
env->ReleaseIntArrayElements(pBuffer, body, 0);
return value;
}

The problem is no matter what I have tried I cannot get the value pdwWordsRead copied to the parameter I passed from java to jni. If I use a local variable I can write out the value so the c++ function is passing it back. Any suggestions would be appreciated.

Answer 1

Java passes primitives by value, not by reference. Also doing things like address of jint and passing them to functions for population isn't a good idea.

JNIEXPORT jint JNICALL Java_com_sig_ccm_CcmBase_SimReadDwordBuffer
  (JNIEnv *env, jobject obj, jintArray pBuffer, jint dwWordsToRead, 
                             jint pdwWordsRead, 
                             jint dwBlockWords, jint dwNoWait){
jint *body = env->GetIntArrayElements(pBuffer, 0);

// I would consider copying the jint[] to a dword[] here... 
// I would also look into jlong instead of jint because jint 
// is 32-bit signed, and dword is 32-bit unsigned.  Then do 
// something like dword == jlong & 0xFFFFFFFF 

DWORD foo = 0;
jint value = SimReadDwordBuffer((DWORD *) body, // Potentially Bad!
                         (DWORD) dwWordsToRead,
                         &foo, 
                         //(DWORD *) &pdwWordsRead, Bad!
                         dwBlockWords,
                         dwNoWait );
// You need to return an object or something else to get both values out!!! 
env->ReleaseIntArrayElements(pBuffer, body, 0);
return value;
}

Again, I have no idea what value is but if its a success indicator, check it and throw an exception if a failure occurred and return the foo variable instead.