send messages problem

[Alexandre DESTRUEL]

Hello
I write this code to sned a message directly to a gate:

void RIO::handleMessage(cMessage *msg)
{
    cGate *OBCIn = gateHalf("OBC", cGate::INPUT);
    cGate *OBCOut = gateHalf("OBC", cGate::OUTPUT);

    send(cMessage *msg, *OBCIn, int index =7);

On the User manual, the definition to send message knowledge the gatename is : send(cMessage *msg, const char *gatename, int index=0);
How can i get the index value of the corresponding gate ? I write 7 but i don't know witch one is.
Thks
Alex