title: Network Reachability error: in iphone os 4
body: <p>my project was buied in os 3.0</p>
  
  <p>but now i want to do some changes with os 4.</p>
  
  <p>so when i run my project in xcode 3.2.4.</p>
  
  <p>i get those errors......</p>
  
  <pre>
  ld: warning: in /Users/abc/SystemConfiguration.framework/SystemConfiguration, missing required architecture i386 in file
  
  ld: warning: in /Users/abc/libz.1.2.3.dylib, missing required architecture i386 in file
  
  
  Undefined symbols:
    "_SCNetworkReachabilityScheduleWithRunLoop", referenced from:
        -[ReachabilityQuery startListeningForReachabilityChanges:onRunLoop:] in Reachability.o
  
    "_SCNetworkReachabilityGetFlags", referenced from:
        -[Reachability remoteHostStatus] in Reachability.o
        -[Reachability isHostReachable:] in Reachability.o
        -[Reachability isNetworkAvailableFlags:] in Reachability.o
        -[Reachability isAdHocWiFiNetworkAvailableFlags:] in Reachability.o
  
    "_SCNetworkReachabilityCreateWithAddress", referenced from:
        -[Reachability reachabilityRefForAddress:] in Reachability.o
        -[Reachability isNetworkAvailableFlags:] in Reachability.o
        -[Reachability isAdHocWiFiNetworkAvailableFlags:] in Reachability.o
  
    "_SCNetworkReachabilityUnscheduleFromRunLoop", referenced from:
        -[Reachability stopListeningForReachabilityChanges] in Reachability.o
  
    "_SCNetworkReachabilityCreateWithName", referenced from:
        -[Reachability reachabilityRefForHostName:] in Reachability.o
        -[Reachability isHostReachable:] in Reachability.o
  
    "_SCNetworkReachabilitySetCallback", referenced from:
        -[ReachabilityQuery startListeningForReachabilityChanges:onRunLoop:] in Reachability.o
  
  ld: symbol(s) not found
  collect2: ld returned 1 exit status
  </pre>