[ns] Issue with AppData and Packet::copy() in packet.h

[Andre Reis]

Greetings,

I'm having an issue at a lower level of ns2, version 2.31. In my
application, I create a PacketData object, and send a new packet with
an UDP agent, like this:

PacketData vodinfo(10);
agent_->sendmsg(size_, &vodinfo);

I don't even fill the data in the object yet. At some point in the
simulation, Packet::copy() is called, and the simulator breaks with a
'Could not access memory (EXC_BAD_ACCESS)', which occurs in line 643
in packet.h:

inline Packet* Packet::copy() const
{
        Packet* p = alloc();
        memcpy(p->bits(), bits_, hdrlen_);
        if (data_)
                p->data_ = data_->copy();    <- fails here
        p->txinfo_.init(&txinfo_);

        return (p);
}

It would seem it cannot access 'p->data', which is a private AppData*
variable. Since this is so low-level in ns, I cannot imagine what the
cause of the problem might be.
Can someone provide some clues as to why this is happening?

With regards,
Andre Reis