title: Segmentation fault in Ns2 program
body: <p>I have referred to a program of ns2 the program is giving me Segmentation fault(Core Dumped) when i changed this -> $ns at 0.5 “$cbr0 start” to this->$ns at 0.5 "$cbr0 start". Are the "" any problem .If i change "" to  “” following is the error</p>
  
  <p>ERRORS</p>
  
  <p>(_o5 cmd line 1)
      invoked from within
  "_o5 cmd at 0.5 “_o84 start”"
      invoked from within
  "catch "$self cmd $args" ret"
      invoked from within
  "if [catch "$self cmd $args" ret] {
  set cls [$self info class]
  global errorInfo
  set savedInfo $errorInfo
  error "error when calling class $cls: $args" $..."
      (procedure "_o5" line 2)
      (SplitObject unknown line 2)
      invoked from within
  "_o5 at 0.5 “<em>o84 start”"
      ("eval" body line 1)
      invoked from within
  "eval $scheduler</em> at $args"
      (procedure "_o3" line 3)
      (Simulator at line 3)
      invoked from within
  "$ns at 0.5 “$cbr0 start”"
      (file "b.tcl" line 71)</p>
  
  <p>CODE:</p>
  
  <pre><code>set ns [new Simulator]
  #Define different colors for data flow 
  $ns color 1 Blue
  $ns color 2 Red
  
  #open the nam trace file 
  set nf [open out.nam w]
  $ns namtrace-all $nf 
  
  #define a finish procedure 
  proc finish {} {
          global ns nf 
          $ns flush-trace 
          #close the trace file 
          close $nf 
          #execute nam on the trace file 
          exec nam out.nam&amp;
          exit 0
          }
  
  #create four nodes 
  set n0 [$ns node]
  set n1 [$ns node]
  set n2 [$ns node]
  set n3 [$ns node]
  
  #create links between nodes 
  $ns duplex-link $n0 $n2 1Mb 10ms DropTail
  $ns duplex-link $n1 $n2 1Mb 10ms DropTail
  $ns duplex-link $n3 $n2 1Mb 10ms SFQ
  
  #orientation
  
  $ns duplex-link-op $n0 $n2 orient right-down
  $ns duplex-link-op $n1 $n2 orient right-up
  $ns duplex-link-op $n2 $n3 orient right
  
  #Monitors the queue for the link betweeen node 2 &amp; 3
  $ns duplex-link-op $n2 $n3 queuePos 0.5
  
  #Create udp agent and attatch it to node 0
  set udp0 [new Agent/UDP]
  $udp0 set class_ 1
  $ns attach-agent $n0 $udp0
  
  #Create CBR traffic source &amp; attatch it to upd0 
  set cbr0 [new Application/Traffic/CBR]
  $cbr0 set packetSize_ 500
  $cbr0 set interval_ 0.005
  $cbr0 attach-agent $udp0
  
  #Create udp agent and attatch it to node 1 
  set udp1 [new Agent/UDP]
  $udp1 set class_ 2
  $ns attach-agent $n1 $udp1
  
  #Create CBR traffic source &amp; attatch it to upd1
  set cbr1 [new Application/Traffic/CBR]
  $cbr1 set packetSize_ 500
  $cbr1 set interval_ 0.005
  $cbr0 attach-agent $udp1
  
  #Create a CBR traffic source &amp; attatch it to udp 1 
  set null0 [new Agent/Null]
  $ns attach-agent $n3 $null0
  #connect the traffic source with the traffic sink 
  $ns connect $udp0 $null0
  $ns connect $udp1 $null0
  
  #Schedule events for the CBR agent 
  $ns at 0.5 “$cbr0 start”
  $ns at 1.0 “$cbr1 start”
  $ns at 4.0 “$cbr1 stop”
  $ns at 4.5 “$cbr0 stop”
  
  #call finish after 5 seconds of simulation time 
  $ns at 5.0 “finish”
  
  #Run the simulator
  $ns run 
  </code></pre>