multithreading - Delphi (XE2) Indy (10) Multithread Ping

Question

I have a room with 60 computers/devices (40 computers and 20 oscilloscopes Windows CE based) and I would like to know which and every one is alive using ping. First I wrote a standard ping (see here Delphi Indy Ping Error 10040), which is working fine now but takes ages when most computers are offline.

So what I am trying to do is to write a MultiThread Ping but I am quite struggling with it. I have seen only very few examples over the internet and no one was matching my needs, that's why I try to write it myself.

I use XE2 and Indy 10 and the form is only constitued of a memo and a button.

unit Main;

interface

uses
  Winapi.Windows, System.SysUtils, System.Classes, Vcl.Forms,
  IdIcmpClient, IdGlobal, Vcl.StdCtrls, Vcl.Controls;

type
  TMainForm = class(TForm)
    Memo1: TMemo;
    ButtonStartPing: TButton;
    procedure ButtonStartPingClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

type
  TMyPingThread = class(TThread)
  private
    fIndex : integer;
    fIdIcmpClient: TIdIcmpClient;
    procedure doOnPingReply;
  protected
    procedure Execute; override;
  public
    constructor Create(index: integer);
  end;

var
  MainForm: TMainForm;
  ThreadCOunt : integer;

implementation

{$R *.dfm}

constructor TMyPingThread.Create(index: integer);
begin
  inherited Create(false);

  fIndex := index;
  fIdIcmpClient := TIdIcmpClient.Create(nil);
  fIdIcmpClient.ReceiveTimeout := 200;
  fIdIcmpClient.PacketSize := 24;
  fIdIcmpClient.Protocol := 1;
  fIdIcmpClient.IPVersion := Id_IPv4;

  //first computer is at adresse 211
  fIdIcmpClient.Host := '128.178.26.'+inttostr(211+index-1);

  self.FreeOnTerminate := true;
end;

procedure TMyPingThread.doOnPingReply;
begin
  MainForm.Memo1.lines.add(inttostr(findex)+' '+fIdIcmpClient.ReplyStatus.Msg);
  dec(ThreadCount);

  if ThreadCount = 0 then
    MainForm.Memo1.lines.add('--- End ---');
end;

procedure TMyPingThread.Execute;
begin
  inherited;

  try
    fIdIcmpClient.Ping('',findex);
  except
  end;

  while not Terminated do
  begin
    if fIdIcmpClient.ReplyStatus.SequenceId = findex then Terminate;
  end;

  Synchronize(doOnPingReply);
  fIdIcmpClient.Free;
end;

procedure TMainForm.ButtonStartPingClick(Sender: TObject);
var
  i: integer;
  myPing : TMyPingThread;
begin
  Memo1.Lines.Clear;

  ThreadCount := 0;
  for i := 1 to 40 do
  begin
    inc(ThreadCount);
    myPing := TMyPingThread.Create(i);
    //sleep(10);
  end;
end;

end.

My problem is that it "seems" to work when I uncomment the "sleep(10)", and "seems" not to be working without it. This for sure means I am missing a point in the threading I have written.

In other words. When Sleep(10) is in the code. Every time I clicked the button to get to check the connections the result was correct.

Without the sleep(10), it is working "most" of the time but some times the result is wrong giving me a ping echo on offline computers and no ping echo on online computer, as is the ping reply was not assigned to the correct thread.

Any comment or help is welcome.

----- EDIT / IMPORTANT -----

As a general follow up of this question, @Darian Miller started a Google Code project here https://code.google.com/p/delphi-stackoverflow/ which is a working basis. I mark his answer as the "accepted answer" but users should refer to this open source project (all the credit belongs to him) as it will surely be extended and updated in the future.

Answer 1

The root problem is that pings are connectionless traffic. If you have multiple TIdIcmpClient objects pinging the network at the same time, one TIdIcmpClient instance can receive a reply that actually belongs to another TIdIcmpClient instance. You are trying to account for that in your thread loop, by checking SequenceId values, but you are not taking into account that TIdIcmpClient already does that same check internally. It reads network replies in a loop until it receives the reply it is expecting, or until the ReceiveTimeout occurs. If it receives a reply it is not expecting, it simply discards that reply. So, if one TIdIcmpClient instance discards a reply that another TIdIcmpClient instance was expecting, that reply will not get processed by your code, and that other TIdIcmpClient will likely receive another TIdIcmpClient's reply instead, and so on. By adding the Sleep(), you are decreasing (but not eliminating) the chances that pings will overlap each other.

For what you are attempting to do, you won't be able to use TIdIcmpClient as-is to have multiple pings running in parallel, sorry. It is simply not designed for that. There is no way for it to differentiate reply data the way you need it. You will have to serialize your threads so only one thread can call TIdIcmpClient.Ping() at a time.

If serializing the pings is not an option for you, you can try copying portions of TIdIcmpClient's source code into your own code. Have 41 threads running - 40 device threads and 1 response thread. Create a single socket that all of the threads share. Have each device thread prepare and send its individual ping request to the network using that socket. Then have the response thread continuously reading replies from that same socket and routing them back to the appropriate device thread for processing. This is a bit more work, but it will give you the multiple-ping parallelism you are looking for.

If you don't want to go to all that trouble, an alternative is to just use a third-party app that already supports pinging multiple machines at the same time, like FREEPing.