Using glMapBufferRange to read data to a buffer

Issue

I have a question, I want to use glMapBufferRange to read out data. The way I use it is the following:

 bindBuffer(vboIdx);
 void *ptr = glMapBufferRange(GL_ARRAY_BUFFER, 0, size, GL_MAP_READ_BIT);
 int err = glGetError();
 assert(ptr);
 memcpy(dst, ptr, size);
 glUnmapBuffer(GL_ARRAY_BUFFER);
 unbindBuffer();

However, ptr seems to be NULL. As stated in the documentation here: https://www.khronos.org/registry/OpenGL-Refpages/es3.0/html/glMapBufferRange.xhtml the function returns NULL if an error occured. Therefore, I checked for the error by using glGetError(). But the error I get is 0, which is the same as GL_NO_ERROR. So what am I missing, what am I doing wrong?

Thanks in advance!

Solution

Honestly, I haven't heard about the openGL context so far (just read about it), I started writing my code based on android's sensor graph sample project: https://github.com/android/ndk-samples/tree/master/sensor-graph. To answer your question, I haven't had a problem so far and I guess yes, but I'm not 100% sure. Before requesting a data read out I write data to the buffer:

   if(dataSize == 0) {
       return;
   }

    bindBuffer(vboIdx);
    if(dataSize > vertexBufSize[vboIdx] - offset) {
        glBufferData(GL_ARRAY_BUFFER, dataSize, data, GL_STREAM_DRAW);
        vertexBufSize[vboIdx] = dataSize;
    }

    void *ptr = glMapBufferRange(GL_ARRAY_BUFFER, offset, dataSize, GL_MAP_WRITE_BIT | GL_MAP_UNSYNCHRONIZED_BIT);
    int err = glGetError();
    assert(ptr);

    if(data != nullptr) {
        memcpy(ptr, data, dataSize);
    }

    glUnmapBuffer(GL_ARRAY_BUFFER);
        unbindBuffer();

This is where I call glBufferData()

Update: Java Side:

mView = (GLSurfaceView) frameLayout.findViewById(R.id.plot_view);
mView.setEGLContextClientVersion(3);
mView.setEGLConfigChooser(true);
if(LOG) Log.d(TAG, "onCreateView(): get ID: " + mView.getId());

isStreaming = true;
mView.setRenderer(new GLSurfaceView.Renderer() {
@Override
public void onSurfaceCreated(GL10 gl, EGLConfig config) {
  mLiveViewJNI.surfaceCreated();
}

@Override
public void onSurfaceChanged(GL10 gl, int width, int height) {
  mLiveViewJNI.surfaceChanged(width, height);
}

@Override
public void onDrawFrame(GL10 gl) {
  mLiveViewJNI.drawFrame();
}

});      

Update: @Matic Oblak You were right, the context was actually a problem. As long as I only touched the buffers via JNI within onSurfaceCreated, onSurfaceChanged or onDrawFrame everything was fine. But as soon as I tried to access them outside for instance via mLiveViewJNI.dummyCall I ran into that problem and everything returned 0 or NULL. Thanks :)!

Answered By - Grimfandango