Re: [ns] weird interactions

[Ratul Mahajan <ratul@aciri.org>]

I finally found out how stupid I can be.
(the culprit line pointed out below).

-ratul

On Fri, 13 Oct 2000, Ratul Mahajan wrote:

> 
> I am seeing some behavior for which i have no explaination for. I would
> appreciate if someone can help.
> 
> I write a new queue and a new agent (also changed the makefile). And after
> instantiating the agent, i pass that as parameter to the queue which
> stores a pointer to it. All is fine till here. When a packet comes to this
> queue, I do an upcall to the agent. In this upcall i try to access a
> member variable of the agent, at which point the code seg faults.
> 
> I have appended both the .cc and .tcl file i am wrote. They are rather
> small and simple (i tore them down to the minumum before posting). Again,
> any help or directions appreciated.
> 
> thanks,
> -ratul
> 
> -----------------------
> 
> trial.cc
> ========
> 
> #include "agent.h"
> #include "queue.h"
> 
> class PushbackAgent : public Agent {
> 
>  public:
>   PushbackAgent();
>   int last_index_;
>   
>   void upcall() {
>     printf("In Agent upcall\n");
>     printf("Last Index = %d\n", last_index_);
>   }
>   
> };
> 
> static class PushbackClass : public TclClass {
> public:
>   PushbackClass() : TclClass("Agent/Pushback") {}
>   TclObject* create(int, const char*const*) {
>     return (new PushbackAgent());
>   }
> } class_Pushback;
> 
> PushbackAgent::PushbackAgent() : Agent(PT_PING), last_index_(0) {
>   bind("last_index_", &last_index_);
> }
> 
> 
> //##################### PushbackQueue #####################
> 
> class PushbackQueue: public Queue {
>  public:
>   PushbackQueue( const char* const);
> 
>   void enque(Packet *p);
>   Packet * deque() {return NULL;}
> 
>   PushbackAgent * pushback_; 
> };
> 
> static class PushbackQueueClass : public TclClass {
> public:
>   PushbackQueueClass() : TclClass("Queue/Pushback") {}
>   TclObject * create(int argc, const char*const* argv) {
>     if (argc==4) {
>       printf("Missing Argument for Pushback Queue Constructor\n");
>       exit(-1);
>     }
>     return (new PushbackQueue(argv[4]));
>   }
> 
> } class_pushback_queue;
> 
> PushbackQueue::PushbackQueue(const char* const pba) {
>   
>   PushbackAgent * pushback_ = (PushbackAgent *)TclObject::lookup(pba);

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	decalring another pushback_ instead of assigning value to old one.

>   if (pushback_ == NULL) {
>     printf("Wrong Argument for Pushback Queue Constructor\n");
>     exit(-1);
>   }
>   printf("pushback queue instantiated %d\n",pushback_->last_index_);
> }
> 
> void 
> PushbackQueue::enque(Packet *p) {
> 
>   printf("In queue enque\n");
>   pushback_->upcall();
> }
> 
> 
> --------------------------------------------------------
> 
> first.tcl
> ========
> 
> set ns [new Simulator]
> 
> set n0 [$ns node]
> set n1 [$ns node]
> 
> set pushback0 [new Agent/Pushback]
> $ns attach-agent $n0 $pushback0
> 
> $ns simplex-link $n0 $n1 0.5Mb 10ms Pushback $pushback0
> $ns simplex-link $n1 $n0 0.5Mb 10ms DropTail
> 
> set src1 [$ns create-connection TCP $n0 TCPSink $n1 0]
> set ftp1 [$src1 attach-app FTP]
> 
> $ns at 1.0 "$ftp1 start"
> $ns at 20.0 "$ns halt; exit 0"
> $ns run
> 
> 
> -----------------------------------------------
> output
> -----
> 
> warning: no class variable Agent/Pushback::last_index_
> 
>         see tcl-object.tcl in tclcl for info about this warning.
> 
> pushback queue instantiated 0
> In queue enque
> In Agent upcall
> Segmentation fault (core dumped)
> 
> 
> -> so it fails at the point i try to access "last_index_".
> any clues?
> 
>