How call java vararg method from C with valist

Question

I have C method with varargs

void message(int id,  ...) 

And it should call Java static method with vararg

public static void message(String messageName, String ... args)

How can I do it using C valist and JNI API?

Thank you!

Answer 1

Your not likely to get any use out of passing the address of a va_list up to Java. Here are a few snippets showing how to pass up an array using "..." and va_list. Add error/exception/return checking as necessary.

In Java:

static private void javaDefineArray(Object ... args) { 
    for (Object o : args) {
        print("javaDefineArray " + o);
    }
}

In C:

// Lookup the method using GetMethodID or GetStaticMethodID
mid = (*env)->GetStaticMethodID(env, class, "javaDefineArray", "([Ljava/lang/Object;)V");

// C function taking variable args ( all String in this example )
static void javaDefineArray(JNIEnv * env, jobject o, ...) {
  int i = 0;
  int count = 0;
  char * msg[100+1] = {0};  // 100 args max
  jobjectArray ja;
  va_list list;
  va_start(list, o);
  do {msg[count] = va_arg(list,char *);
  } while(msg[count] !=0 && ++count < 100); // end/saftey check
  va_end(list);
  ja = (*env)->NewObjectArray(env, count, (*env)->FindClass(env, "java/lang/String"), (*env)->NewStringUTF(env, ""));
  for(i=0;i!=count;i++) {
    (*env)->SetObjectArrayElement(env,ja,i,(*env)->NewStringUTF(env, msg[i]));
  }
  (*env)->CallVoidMethod(env, o, mid, ja);
}

C Usage:

javaDefineArray(env, o, "0", "1", "2", "3", "4", NULL);

Java Output:

javaDefineArray 0
javaDefineArray 1
javaDefineArray 2
javaDefineArray 3
javaDefineArray 4