How make a TServerSocket multithread?

Question

I'm testing how get periodical screenshots of my Android smartphone and already that this server is receiving a big flux of screenshots form my device, happens that these images not appear in TImage, i think that is because TServerSocket (part where receives images) not is in a thread. And yes, i'm sending these screenshots correctly:

Java (Android):

    bitmap = Bitmap.createBitmap(mWidth + rowPadding / pixelStride, mHeight, Bitmap.Config.ARGB_8888);
    bitmap.copyPixelsFromBuffer(buffer);

    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    bitmap.compress(Bitmap.CompressFormat.PNG, 100, bos);
    byte[] array = bos.toByteArray();

    DataOutputStream dos = new DataOutputStream(clientSocket.getOutputStream());
    dos.writeInt(array.length);
    dos.write(array, 0, array.length);

    dos.flush();

And here is my Delphi code where must receive the periodical screenshots:

var
  Form1: TForm1;

   stSize: integer;   
   Stream: TMemoryStream; 
   Receiving: boolean; 
   png: TPngImage; 

   FSize: Integer;
   writing: Boolean;

implementation

{$R *.dfm}

procedure TForm1.ServerSocket1ClientConnect(Sender: TObject; Socket: TCustomWinSocket);
var
Item: TListItem;  
begin
    Item := ListView1.Items.Add;   
    Item.Caption := IntTostr(socket.Handle); 
    Item.SubItems.Add(Socket.RemoteAddress); 
    Item.SubItems.Add(socket.RemoteHost);  
    Item.Data := Socket.Data; 
end;

procedure TForm1.ServerSocket1ClientDisconnect(Sender: TObject; Socket: TCustomWinSocket);
var
Item: TListItem;
begin  
    Item:= ListView1.FindCaption(0, inttostr(socket.Handle), false, true, false);
    if item <> nil then
    Item.Delete;
end;

procedure TForm1.ServerSocket1ClientError(Sender: TObject; Socket: TCustomWinSocket; ErrorEvent: TErrorEvent; var ErrorCode: Integer);
begin
    showmessage('socket erro'); 
    ErrorCode := 0;  
end;

procedure TForm1.Activate1Click(Sender: TObject);
begin
    ServerSocket1.Active := true;  
end;

procedure TForm1.Deactive1Click(Sender: TObject);
begin
    ServerSocket1.Active := false;  
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
    Stream:= TMemoryStream.Create;
    writing:= False;
end;

procedure TForm1.SendMyReqst1Click(Sender: TObject);
begin
    if ListView1.Selected = nil then exit;    
    ServerSocket1.Socket.Connections[ListView1.ItemIndex].SendText('screencapture' + #13#10); 
end;

procedure TForm1.ServerSocket1ClientRead(Sender: TObject; Socket: TCustomWinSocket);
var
  BytesReceived: Longint;
  CopyBuffer: Pointer;
  ChunkSize: Integer;
  TempSize: Integer;
const
  MaxChunkSize: Longint = 8192;
begin

  If FSize=0 then
  begin
    begin
      Socket.ReceiveBuf(TempSize,SizeOf(TempSize));
      TempSize := ntohl(TempSize);
      Stream.SetSize(TempSize);
      FSize:= TempSize
    End;
  End;

  If (FSize>0) and not(writing) then
  begin
    GetMem(CopyBuffer, MaxChunkSize);
    writing:= True;
    While Socket.ReceiveLength>0 do
    Begin
      ChunkSize:= Socket.ReceiveLength;
      If ChunkSize > MaxChunkSize then ChunkSize:= MaxChunkSize;
      BytesReceived:= Socket.ReceiveBuf(CopyBuffer^,ChunkSize);
      Stream.Write(CopyBuffer^, BytesReceived);
      Dec(FSize,BytesReceived);
    End;
  end;

  If FSize=0 then begin

  Stream.Position := 0;
  png:=TPngImage.Create;
  png.LoadFromStream(Stream);
  img1.Picture.Assign(png);
  img1.Refresh;
  Stream.SetSize(0);
  png.Free;
  FSize:= 0;
end;

FreeMem(CopyBuffer, MaxChunkSize);
Writing:= False;
end;

end.

This Delphi code above works fine, but to receive only 1 screenshot, not a big flux.

UPDATE:

this is my code base for obtain periodical screenshots on Android.

PS: See that he use a infinite loop.

Answer 1

The Delphi code you have shown DOES NOT correctly account for the streaming nature of TCP, or for multiple clients:

• it is NOT reading FSize correctly. More than 1 read may be needed to get all 4 bytes.

• it DOES NOT use FSize to limit the number of bytes read for the PNG stream. You need to read exactly how many bytes FSize specifies, no more, no less. It is reading as long as the client is still sending bytes, even if they belong to subsequent messages. It needs to stop reading when it reaches the end of the stream, and then reset for the next message.

• it DOES NOT handle the possibility of multiple clients sending screenshots at the same time. It is sharing variables with multiple clients, thus allowing them to corrupt each other's messages.

In short, the code is COMPLETELY broken, regardless of multi-threading. Which, BTW, is not a factor if you use the server in non-blocking mode (which the code likely is, as that is the server's default mode, and the code is not using any of the server's thread-related events).

The code does not need to be multi-threaded to work correctly. It needs to be rewritten to operate correctly.

Try something more like this:

type
  TInt32Bytes = record
    case Integer of
      0: (Bytes: array[0..SizeOf(Int32)-1] of Byte);
      1: (Value: Int32);
  end;

  TSocketState = (ReadingSize, ReadingStream);

  TSocketData = class
  public
    Stream: TMemoryStream;
    Png: TPngImage;
    State: TSocketState;
    Size: TInt32Bytes;
    Offset: Integer;
    constructor Create;
    destructor Destroy; override;
  end;

constructor TSocketData.Create;
begin
  Stream := TMemoryStream.Create;
  Png := TPngImage.Create;
end;

destructor TSocketData.Destroy;
begin
  Stream.Free;
  Png.Free;
end;

procedure TForm1.ServerSocket1ClientConnect(Sender: TObject; Socket: TCustomWinSocket);
var
  Item: TListItem;
begin
  Socket.Data := TSocketData.Create;
  Item := ListView1.Items.Add;
  Item.Data := Socket;
  Item.Caption := IntToStr(Socket.Handle);
  Item.SubItems.Add(Socket.RemoteAddress);
  Item.SubItems.Add(Socket.RemoteHost);
end;

procedure TForm1.ServerSocket1ClientDisconnect(Sender: TObject; Socket: TCustomWinSocket);
var
  Item: TListItem;
begin
  Item := ListView1.FindData(0, Socket, true, false);
  if Item <> nil then Item.Delete;
  TSocketData(Socket.Data).Free;
end;

procedure TForm1.ServerSocket1ClientError(Sender: TObject; Socket: TCustomWinSocket; ErrorEvent: TErrorEvent; var ErrorCode: Integer);
begin
  ErrorCode := 0;
  Socket.Close;
end;

procedure TForm1.Activate1Click(Sender: TObject);
begin
  ServerSocket1.Active := true;
end;

procedure TForm1.Deactive1Click(Sender: TObject);
begin
  ServerSocket1.Active := false;
end;

procedure TForm1.SendMyReqst1Click(Sender: TObject);
var
  Index: Integer;
begin
  Index := ListView1.ItemIndex;
  if Index = -1 then Exit;
  ServerSocket1.Socket.Connections[Index].SendText('screencapture' + #13#10);
end;

procedure TForm1.ServerSocket1ClientRead(Sender: TObject; Socket: TCustomWinSocket);
var
  BytesReceived: Integer;
  BufferPtr: PByte;
  SD: TSocketData;
  Item: TListItem;
begin
  SD := TSocketData(Socket.Data);

  if SD.State = ReadingSize then
  begin
    while SD.Offset < SizeOf(Int32) do
    begin
      BytesReceived := Socket.ReceiveBuf(SD.Size.Bytes[SD.Offset], SizeOf(Int32) - SD.Offset);
      if BytesReceived <= 0 then Exit;
      Inc(SD.Offset, BytesReceived);
    end;
    SD.Size.Value := ntohl(SD.Size.Value);
    SD.State := ReadingStream;
    SD.Offset := 0;
    SD.Stream.Size := SD.Size.Value;
  end;

  if SD.State = ReadingStream then
  begin
    if SD.Offset < SD.Size.Value then
    begin
      BufferPtr := PByte(SD.Stream.Memory);
      Inc(BufferPtr, SD.Offset);
      repeat
        BytesReceive := Socket.ReceiveBuf(BufferPtr^, SD.Size.Value - SD.Offset);
        if BytesReceived <= 0 then Exit;
        Inc(BufferPtr, BytesReceived);
        Inc(SD.Offset, BytesReceived);
      until SD.Offset = SD.Size.Value;
    end;
    try
      SD.Stream.Position := 0;
      SD.Png.LoadFromStream(SD.Stream);
    except
      SD.Png.Assign(nil);
    end;
    Item := ListView1.Selected;
    if (Item <> nil) and (Item.Data = Socket) then
      img1.Picture.Assign(SD.Png);
    SD.State := ReadingSize;
    SD.Offset := 0;
  end;
end;

procedure TForm1.ListView1SelectItem(Sender: TObject; Item: TListItem; Selected: Boolean);
begin
  if (Item <> nil) and Selected then
    img1.Picture.Assign(TSocketData(TCustomWinSocket(Item.Data).Data).Png);
end;