Blender cycles, samples color

Question

I am trying to collect from Cycles source code, the color of each sample during the rendering algorithm. However, after 2 months of research (...), I still don't understand how to get them.

I've been looking at the device_cpu.cpp file and in particular at the path_trace() function.

When looking at the render_buffer variable, and printing its content, it doesn't seem to store the samples' color: I write the value in a .txt file (samplesOutput) writing the following line

 void path_trace(DeviceTask &task, RenderTile &tile, KernelGlobals *kg)
 {
            float *render_buffer = (float*)tile.buffer;
            int start_sample = tile.start_sample;
            int end_sample = tile.start_sample + tile.num_samples;

            for(int sample = start_sample; sample < end_sample; sample++) {
                if(task.get_cancel() || task_pool.canceled()) {
                    if(task.need_finish_queue == false)
                        break;
                }

                for(int y = tile.y; y < tile.y + tile.h; y++) {
                    for(int x = tile.x; x < tile.x + tile.w; x++) {
                        path_trace_kernel()(kg, render_buffer,
                                            sample, x, y, tile.offset,
                                            tile.stride);
                        int step = tile.offset + x + y*tile.stride;
                        step *= kernel_data.film.pass_stride;
                        samplesOutput << x << " " << y << " " << *(render_buffer + step) << " "
                                << *(render_buffer + step + 1) << " " << *(render_buffer + step + 2) << " "
                                << *(render_buffer + step + 3) << " " << std::endl;

            task.update_progress(&tile, tile.w*tile.h);
        }
}

Here is an extract of what I get for this image

25 2 0.0508761 0.0508761 0.0508761 1 
26 2 0.0508761 0.0508761 0.0508761 1 
27 2 0.0508761 0.0508761 0.0508761 1 

28 2 0.0508761 0.0508761 0.0508761 1 
1 164 0.661389 0.661389 0.661389 13 
1 164 0.661389 0.661389 0.661389 13 
29 2 0.0508761 0.0508761 0.0508761 1 
2 164 0.661389 0.661389 0.661389 13 
2 164 0.661389 0.661389 0.661389 13 
30 2 0.0508761 0.0508761 0.0508761 1 
3 164 0.661389 0.661389 0.661389 13 
3 164 0.661389 0.661389 0.661389 13 
31 2 0.0508761 0.0508761 0.0508761 1 
4 164 0.661389 0.661389 0.661389 13 
4 164 0.661389 0.661389 0.661389 13
...

The color of the cube is rgba(0.8, 0, 0.8, 1.0).

I figured out that (0.0508761 0.0508761 0.0508761 1) is the color of the background (gray).

What is weird, is the 4th value which I expected to be alpha but I get 13 which greater than 1.

Any clue how where to look at to get those samples' color ?

Thanks