how to retrieve contact data from engine (NOT MESSAGE)!

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how to retrieve contact data directly from physic engine (not in a message).

I tried to use the map structure but this function also use messages.

sensors::SensorPtr sensor = sensors::SensorManager::Instance()->GetSensor(this->model->GetLink("0")->GetSensorName(0));

sensors::ContactSensorPtr contactSensor = boost::dynamic_pointer_cast<sensors::ContactSensor>(sensor);

std::map<string, physics::Contact> contacts = contactSensor->GetContacts(this->contactSensor->GetCollisionName(0));

for (std::map<string, physics::Contact>::iterator iter = contacts.begin();iter != contacts.end(); ++iter)
{
       <<"count       " << iter->second.count <<endl
       <<"time        " << iter->second.time <<endl;
}

The probleme with messages, it may miss some data. I want that each time I run the simulation I get the same data sequence, but this is not the case. For instance look at this curves; data in the first run are different from the second even if its the same simulation