Problem when sending a file with TCP
lazarus / 38-Networking and Web Programming

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stab · 2013-09-17 05:47

Hi all,

Could someone please explain what I'm doing wrong when trying to send
a small png-image(579 bytes)
with lNet-tcpsockets on same pc(Win 7/64 bits).

The server sends the file in one go but

In both client and server are declared:
Code: [Select] 
private
    { private declarations }
    FServerReceivedStr : string;
    FImgStream : TMemoryStream;
    FBuffer : array of byte;
    FSize : integer;
    FPos : integer;

and FImgStream is created in FormOnCreate;

From the client is sent a request for the file:
Code: [Select]procedure TfrmlNetClient.btnGetImageClick(Sender:
TObject);
begin
  FSize := 0;
  SetLength(FBuffer, 10000);
  Memo1.Lines.Add('Client1 requesting image');
  FRequestingImage := true;
  LTCPClient1.SendMessage('send image');
end;

and in the server the request is received:
Code: [Select]procedure TfrmlNetServer.LTCPServerReceive(aSocket:
TLSocket);
var
  n : integer;
begin
  if aSocket.GetMessage(FServerReceivedStr) > 0 then
  begin
    if FServerReceivedStr <> 'send image' then
      Memo1.Lines.Add('Server received: ' + FServerReceivedStr)
    else
    begin
      FImgStream.LoadFromFile(ExtractFilePath(Application.Exename) +
'test.png');
      n := FImgStream.Size;                      n is 579 as it should
be
      SetLength(FBuffer, FImgStream.Size);
      n := Length(FBuffer);                      n is 579 as it should
be
      FImgStream.Position := 0;
      FImgStream.Read(FBuffer, FImgStream.Size);
      n := Length(FBuffer);                      !!! SIGSEV !!! How
come???
      FPos := 0;
// send image
      LTCPServerCanSend(aSocket);
    end;
  end;
end;

procedure TfrmlNetServer.LTCPServerCanSend(aSocket: TLSocket);
var
  n : integer; // will store results of .Send()
begin
  if FServerReceivedStr = 'send image' then
  begin
  { repeat the sending until we sent the whole buffer, or .Send return
0 -- means we filled the send buffer for now. NOTE: FSendIndex  has to
begin at 1 for a string! }
    repeat
    { try to send the rest of the buffer we didn't send yet }
      n := Length(FBuffer) - FPos;               !!! SIGSEV again!!!
      if n > 0 then
        n := LTCPServer.Send(FBuffer[FPos], n);
    { if we sent something, move us in the buffer so we don't re-send
the same part }
      Inc(FPos, n);
    until (n = 0) or (FPos > Length(FBuffer));
  end
  else
    LTCPServer.SendMessage(Edit1.Text, aSocket);
end;


and eventually the image is received in the client:
Code: [Select]procedure TfrmlNetClient.LTCPClient1Receive(aSocket:
TLSocket);
var
  s : string;
  n, t : integer;
begin
  if not FRequestingImage then
  begin
    if aSocket.GetMessage(s) > 0 then
      Memo1.Lines.Add('Client1 received: ' + s);
  end
  else
  begin
    n := aSocket.Get(FBuffer, 100);
    FSize := FSize + n;
    if n > 0 then
    begin
      FImgStream.Write(FBuffer, n);
      Exit;
    end;

    FRequestingImage := false;
    n := FImgStream.Size;
    FImgStream.Position := 0;
    Image1.Picture.Bitmap.LoadFromStream(FImgStream);
    FBuffer := nil;
  end;
end;

My questions:
Why do I get a SIGSEV in LTCPServerReceive?
Am I supposed to first send number of bytes in the file to client for
it to know how many bytes should be read?
Is one, on the receiving side, also forced to read block of data
because of input buffer overflow?

Very thankful for guidance

Regards
stab

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ludob · 2013-09-17 06:19

QuoteCode: [Select]FImgStream.Read(FBuffer, FImgStream.Size);You are
overwriting the pointer to the buffer, FBuffer, instead of writing in
the buffer. Use
Code: [Select]FImgStream.Read(FBuffer[0], FImgStream.Size);

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stab · 2013-09-17 10:34

Thanks alot, I became blind

Regards
stab