How do I send , pointer to array from JNI C code, to Java code?

Question

How do I send a pointer to array from JNI C code to a Java code ? For example :

JNI C Code :

jclass cls = (*env)->GetObjectClass(env,obj);
jmethodID mid = (*env)->GetMethodID(env,cls,"print"," ?? "); // What should be the signature here ?
jvalue a1,*arr;
a1.i = 2002;
a1.f = 12.90;
a1.c = 's';
a1.j = 344554;
a1.b = TRUE;
arr = &a1;
(*env)->CallVoidMethodA(env,obj,mid,arr);

Java Func :

public void print(?????) { // what should be the argument here ?
  // add code here
}

How should the java function look like to receive a pointer to an array ?

@auselen leave this comment under all the questions on SO !(This homework is a preparatory work for some module I want to write) Next time try to give a better answer,so that you don't get 2 downvotes.. — Y.E.P
– Y.E.P, Commented Oct 6, 2012 at 7:21
Are there any special reasons to why you are using CallVoidMethodA() instead of the more easy CallVoidMethod()? — maba
– maba, Commented Oct 7, 2012 at 18:28

nneonneo · Accepted Answer · 2012-10-06 08:35:44Z

1

First off, two problems with your code:

arr is not actually an array (it points to a single jvalue), so therefore there's no reason to pass it by pointer instead of just by value.
All those a1.X assignments are useless, except the last one, because jvalue is a union of all the possible primitive and reference types.

Next: It's impossible to pass raw C pointers to JNI (well, you can, but you can only get Java to see the pointer as a number and not as an object). You have to create a Java array or buffer object. Arrays are good if you will be creating an array of references, though the memory will have to be copied; buffers are good if you want the C array to share memory with the Java buffer.

answered Oct 6, 2012 at 8:35

nneonneo

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