How to access values of a jobject in JNI

Question

JNIEXPORT jbyteArray JNICALL Java_ramRead_MainClass_readRam
        (JNIEnv *env, jobject, jobject len, jobject addr, jint pid)
{
        jclass c = (*env).GetObjectClass(len);
        jfieldID fid = (*env).GetFieldID(c, "len", "Ljava/math/BigInteger;");
        std::cout << "Object Field: " << (*env).GetObjectField(len, fid);

        struct iovec local[1];
        struct iovec remote[1];
        unsigned long long len1 = (*env).GetObjectField(len, fid) //This did not work out. the conversion to unsigned long long
        jbyte buf[len];
        jbyteArray buf1 = (*env).NewByteArray(len);

        local[0].iov_base = buf;
        local[0].iov_len = len;

        remote[0].iov_base = (void *) addr;
        remote[0].iov_len = len;

        nread = process_vm_readv(pid, local, 1, remote, 1, 0);

        (*env).SetByteArrayRegion(buf1, 0, len, buf);
        return buf1;
}

This is my native method and as you can see, it receives 3 parameters as arguments two jobject and one int . The two jobjects are big integers, Originally it revived as parameters two int and one long but as I need to pass big integers now I have a problem or should I say several of them.

Goal:

I need to get the BigInteger value from jobject len and then I need to use it to initialize the size of jbyte buf[len] , jbyteArray buf1 = (*env).NewByteArray(len) and local[0].iov_len = len; I cannot of course just initialize them with len I need to get the big integer value from len and then initialize the size with that. And then I need to do the same, with jobject addr that is extract the big integer value from it.

Any help would be appreciated.

Answer 1

The problem with your approach is that you're trying to access a field of BigInteger call len :

GetFieldID(c, "len", "Ljava/math/BigInteger;");

But BigInteger doesn't have such a field.

There is no way to access the "value" of a BigInteger directly. The best you can do from JNI is convert the BigInteger to a jlong (by calling the Java method longValue , on the BigInteger ) and use that.

But, there is no point in using a BigInteger if you're converting it to an jlong any ways. You would lose any precision you'd gain from using a BigInteger . So might as well use a long in Java instead (which will still fit the value 0xffffffffff601000 that you mentioned).

Furthermore, a byte[] can only be indexed with an int , so even if you make buf bigger than that, it won't be accessible from Java.

I would recommand using a direct ByteBuffer instead of a byte[] , since you won't have to copy data across buffers.

You'd get something like this:

private static native void readRam(ByteBuffer buff, long address, int pid);

And then in C++:

JNIEXPORT void JNICALL Java_Main_readRam
    (JNIEnv *env, jclass, jobject byteBuffer, jlong addr jint pid) {

    struct iovec local[1];
    struct iovec remote[1];

    void* buf = env->GetDirectBufferAddress(byteBuffer);
    jlong len = env->GetDirectBufferCapacity(byteBuffer);

    local[0].iov_base = buf;
    local[0].iov_len = len;

    remote[0].iov_base = (void *) addr;
    remote[0].iov_len = len;

    process_vm_readv(pid, local, 1, remote, 1, 0);
}

And then calling:

int bufferLength = ...;
long address = 0xffffffffff601000L;
int pid = ...;

ByteBuffer bb = ByteBuffer.allocateDirect(bufferLength);

readRam(bb, address, pid);

// use bb...

---

In regards to your comment, you could do this:

StringBuilder sb = new StringBuilder();
while(bb.hasRemaining()) {
    byte b = bb.get();
    if((b >= 32 && b < 127) || b == 10) {
        sb.append((char) b);
    }
}
String result = sb.toString();