Callback function and Threading

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Hello everyone,

I am having a callback function which is supposed to receive the data and to assign it to a local variable. Until here, everything is fine.

However, after the assignment, when I want to "process" the data (that is, taking out some entries or such primitive operations), the callback skips some of the data and leaves some parts of it. I suspect the problem is related to threading issue. I know, I am not supposed to "process" the data in the callback, but even when I create a separate function for that, when I call the function after the call back, it doesn't get invoked, so nothing happens. I either place it in the wrong place, or something else is going on.

Here is my callback:

void multibeamCallback(const sensor_msgs::LaserScan::ConstPtr& data)
{
    // get the beam ranges from the callback msg into the local vector
    ranges = data->ranges;
    minimum_beam_range = data->range_min;
}

and the function to process the received data:

void correlate()
{
    for (int i=0; i < ranges.size(); i++)
    {
        // the beams 3599 and 3600 are errornous, they always have the range of 0, therefore they need to be taken out from the array
        if(ranges[i] == minimum_beam_range)
            ranges.erase(ranges.begin() + (i-1));

        // from the ranges, filter in the beams which do not travel the max. range, which means they get blocked by some obstacle
        // if the range of a beam is not the maximum value, it means that the beam was blocked by an obstacle, hence the detection
        if(ranges[i] < maximum_beam_range)
        {
            blockedBeams.push_back(make_pair(i, ranges[i])); // store the beam info in another vector for further use, i.e. beam id: 190, range:34
        }   
    }
}

And finally, my main:

int main(int argc, char **argv)
{
  ros::init(argc, argv, "RadarListener");
  ros::NodeHandle n("~");
  ros::Subscriber sub = n.subscribe("/multibeam", 1000, multibeamCallback);
  correlate();
  ros::spin();

  return 0;
}

Maybe the error is with me using spin() in a wrong way or some threading issue which I skip.

Any thoughts?