I am developing an Android application for which I am using one native C library. This library is used for file decompression.

The definition of the native method is as follows:

public static native Long decompress(ByteBuffer src, ByteBuffer dst); 

My application code to use this function is as follows:

try {             File inFile = new File(Environment.getExternalStorageDirectory().getPath() +"/1"); // source file              ByteBuffer buf = ByteBuffer.allocateDirect((int)inFile.length());              ByteBuffer buf_out = ByteBuffer.allocateDirect((int)inFile.length()*20);              InputStream is = new FileInputStream(inFile);              int b;              while ((b=is.read())!=-1) {                 buf.put((byte)b);             }             Log.d("nims","source buffer zie"+buf.position());             File file = new File(Environment.getExternalStorageDirectory().getPath() +"/2");              // append or overwrite the file             boolean append = false;              FileChannel channel = new FileOutputStream(file, append).getChannel();              Long t = decompress(buf,buf_out); // t is some XYZ number              Log.d("nims","dest buffer zie"+buf_out.position()); // buf_out.position() returns 0             buf_out.flip();              // Writes a sequence of bytes to this channel from the given buffer.             channel.write(buf_out);              // close the channel             channel.close();          }         catch (IOException e) {             System.out.println("I/O Error: " + e.getMessage());         } 

JNI code:

JNIEXPORT jlong JNICALL       Java_com_company_appName_MainActivity_decompress(JNIEnv* env, jclass cls, jobject src, jobject dst) {          uint8_t* src_buffer = (*env)->GetDirectBufferAddress(env,src);         const size_t src_size = (*env)->GetDirectBufferCapacity(env, src);          uint8_t* dst_buffer = (*env)->GetDirectBufferAddress(env,dst);          size_t dst_size = (*env)->GetDirectBufferCapacity(env, dst);         jlong test = _decode_buffer(dst_buffer, dst_size, src_buffer, src_size, NULL);          return test;     } 

The destination buffer doesn’t contain any data.

I have following questions:

1. How can I read the destination bytebuffer back from JNI code to application’s code?
2. How can I write the destination bytebuffer data to a file?

I would appreciate any suggestion and thoughts on this topic.