[Click] empty queue

[Panos Μatzakos]

Hello, as i had mentioned in a previous mail i try to implement a mac protocol (802.11 csma/ca) for my thesis and i am using the following flowgraph:

Timedsource -> Queue -> mysocket -> Discard

where mysocket is the main element of my router which implements the algorithm of the protocol and sends packets to PHY through sockets. Then, i needed a way to know from "mysocket" when the queue becomes empty because it is of great importance for my protocol. The simplest way that was suggested to me was this:

mysocket::pull() { 
  Packet * p = input(0).pull();
  if(p == NULL){
      return 0;} 

} 
p is NULL exactly if Queue is empty.  Everything seemed to work nice but when i tried to test my protocol in saturation conditions (in which case the queue is always non empty) by securing that the incoming rate of the queue is much higher than the outcoming rate i noticed that the queue got empty sometimes. Specifically i wrote the following code:

Packet * p = input(0).pull(); 
    if(p == NULL){
        cout<<"NULL!!"<<endl;
        return 0;
    }

and noticed that the "NULL!!" message was printed although it shouldn't. One thing that i am not sure about is what happens exactly when the queue holds capacity, which is a case that obviously interests me because as i said my incoming rate is greater than the outcoming rate. In the description of the queue element it says "Drops incoming packets if the queue already holds CAPACITY packets". What i understand from this is that the packets which are dropped are the new ones which are generated from timedsource and not the ones which are already in the queue, so the queue should still remain non empty. So do you have any idea about what happens and my queue still gets empty under these conditions?

One last thing i need to ask as i want to be sure if i have understood everything right is the following. In my flowgraph the element which initiates the packet flow after the queue element is the Discard element by a pull connection from its input. Is that right?