Counting connections and commands in a simulator (Delphi 7/Windows XP)

Question

I was working in a simulator for testing connections and commands sending to a server. The simulator has some counters like Total of sent commands, successfully sent commands, fail sent commands, connection attempts, successful connections, etc...

The code that I used is the following:

procedure TALClient.SendCommand;
begin
  Try
    dlgMain.IncrementIntConx; //Increments conn attemps
    FTCP.Connect(1000);
    If FTCP.Connected Then
      Begin
        dlgMain.IncrementConections;  //increments successfully connections

        try
          dlgMain.IncrementIntSendCommand;  //Increments command sent attemps (A)
          FTCP.SendCmd(FCmd.FNemo + ' ' + FCmd.FParams);  // (Z)
          dlgMain.IncrementSendComm;  //Increments sent Commands (B)

          try
            FParent.CS.Acquire;
            FParent.FStatistic[Tag, FCmd.FTag].LastCodeResult := FTCP.LastCmdResult.NumericCode;
            FParent.FStatistic[Tag, FCmd.FTag].LastMsgResult := FTCP.LastCmdResult.Text.Text;
            FParent.CS.Release;
            if ((FTCP.LastCmdResult.NumericCode) = (497)) then
              Synchronize(UpdateCorrectCounters)  //increments successfully responds from server
            else
              Synchronize(UpdateErrorCounters);  //increments failed responds from server
          except
            Synchronize(UpdateErrorCounters);
          end;

        except
          dlgMain.IncrementFailCommand; //increments failed commands (C)
        end;
      End
    Else
      Synchronize(UpdateErrorCounters); //Increment failed responses from sever
  Finally
    If FTCP.Connected Then
      FTCP.Disconnect;
  End
end;

I have changed the code to many many other ways, but it never works fine. The big problem is that the total count of sent commands is not equal to successfully sent commands plus failed sent commands. (in the code: A is not equal to B plus C). There are responses that I have never "seen" in the line marked as (Z), maybe "lost" responses...

So, what I am doing wrong?

Answer 1

I guess you are using multiple threads for your simulator. This looks like the classic Lost Updates problem to me. You have to synchronize the counter-incrementing code.

Incrementing a variable is NOT thread-safe:

Temp := CounterValue;
// If another thread intercepts here, we've got a lost update
Temp := Temp + 1;
CounterValue := Temp;

See this MSDN article to read more about concurrency issues.

Answer 2

If you only use counting you can use the Windows functions InterlockedIncrement and InterlockedDecrement and you won't need any locking.