Invoking native functions of ported library

Question

i have been following this guide on how to port the LAME library with and Android NDK. http://blog.libertadtech.com/2011/02/porting-lame-encoder-to-android-arm.html

Ever

Answer 1

Sadly you can't just call any old C function from Java. You have to use a function with a "mangled" name or use the JNI_OnLoad mechanism. Either way, the Dalvik VM will add extra arguments to the function call that the regular lame get_version_number function is not expecting.

This example of using JNI explains the name mangling. For your Java class, you would need to have a function with this prototype:

void Java_de_goddchen_android_youtubeconverter_LameWrapper_get_11lame_11version
  (JNIEnv *, jclass, jcharArray, jint, jcharArray);

The JNIEnv and jclass arguments are part of the calling interface. Remember to specify extern "C" if you're using C++. The javah tool will produce the right header file for you.

If I were doing this, I'd probably write JNI C wrapper functions for the lame calls I needed. In this particular case instead of using a char * with a length argument, and passing an empty char * as the last argument, I'd create a simpler Java to C interface that just returned a Java String. So in Java:

package de.goddchen.android.youtubeconverter;

public class LameWrapper {
    public static native String getVersion();
}

Then in C, which calls the actual lame API:

JNIEXPORT jstring JNICALL Java_de_goddchen_android_youtubeconverter_LameWrapper_getVersion
  (JNIEnv * env, jclass c)
{
    char buffer[80];
    get_lame_version(buffer, sizeof(buffer) - 1, "");
    return (*env)->NewStringUTF(env, buffer));
}

The life cycle of the new String created here is then controlled on the Java/Dalvik side.