Question

I want to modify globals declared in java SDK via an NDK C function, eg:

public class NDKTest extends Activity
{
    static int myglobal = 0;

    static { System.loadLibrary("MyCLib"); }
    static public native void incrementmyglobal();
....

and in MyCLib:

#include "NDKTest.h"

JNIEXPORT void JNICALL Java_NDKTest_incrementmyglobal

(JNIEnv * env, jobject jObj)
{
  /*
     this next line is obviously wrong, but illustrates how it would have
     been done had it been a purely C program
  */
  M.myglobal = M.myglobal + 1;
  return;
}

So my question is:

a/ What is the correct syntax of the offending line?

b/ How should the C header file look?

c/ How should the Android.mk file look?

I can compile and run various ‘helloworld’ NDK examples, but I find none that illustrate
how to modify a global declared in java. A supersimple example would be greatly appreciated!

I am programming using Linux command line (ant debug), and simple text editor (emacs).
Im not using Eclipse; have been programming C for years, and am a Java noob.
Destination platform is Android.
Cheers!

Answer 1

The method is static, so its second parameter is a jclass pointer for the current class, not the current object pointer. Also note that the name of the native method must include the package name:

JNIEXPORT void JNICALL Java_com_mypackage_NDKTest_incrementmyglobal 
(JNIEnv * env, jobject jcl) 
{ 
  jfieldID fid = GetStaticFieldID(env, jcl, "myglobal", "I");
  int i = GetStaticIntField(env, jcl, fid);
  SetStaticIntField(env, jcl, fid, i+1);
  return; 
} 

There’s no need for a C header. The Android.mk should look like that of any helloworld example, with LOCAL_SRC_FILES changed to reflect yours.