How to transfer data using Indy TCP Server/Client? - delphi

Question

I want to transfer data from the TIdTCPServer to the TIdTCPClient.
On the server side I have:
procedure TForm1.IdTCPServer1Execute(AContext: TIdContext);
var x:Integer;
Received:String;
SendBuff:TBytes;
hFile:THandle;
fSize:Int64;
begin
fSize:=0;
if MOpenFileForRead(hFile,MGetExePath+'\test.jpg') then begin
fSize:=MFileSize(hFile);
SetLength(SendBuff,fSize);
MReadFile(hFile,SendBuff[0],fSize);
MCloseFile(hFile);
end;
// ... here the SendBuff contains valid data, I checked.
repeat
Received:=AContext.Connection.Socket.ReadLn;
if not AContext.Connection.Connected then Exit;
if Received=CMD_TEST_FILE then begin
AContext.Connection.Socket.Write(fSize);
AContext.Connection.Socket.WriteBufferOpen;
AContext.Connection.Socket.Write(SendBuff);
AContext.Connection.Socket.WriteBufferClose;
end;
until False;
end;
And the client side:
procedure TForm1.Button2Click(Sender: TObject);
var fSize:Int64;
RecvBuff:TBytes;
hFile:THandle;
begin
IdTCPClient1.Socket.WriteLn(CMD_TEST_FILE);
fSize:=IdTCPClient1.Socket.ReadInt64;
SetLength(RecvBuff,fSize);
IdTCPClient1.Socket.ReadBytes(RecvBuff,fSize);
if MCreateFile(hFile, MGetExePath+'\new.jpg') then begin
MWriteFile(hFile,RecvBuff[0],fSize);
MCloseFile(hFile);
end;
Memo1.Lines.Add('ok');
end;
... but it's not working. I checked the read and write data functions used and they are ok. At the server the buffer is set ok, the file size arrives at client ok, but the content of the buffer at client is only zeros.
P.S: I want to send the file in this way not with stream or anything else.

Answer 1

If you look at the signature of ReadBytes(), it has an optional AAppend parameter that is True by default:
procedure ReadBytes(var VBuffer: TIdBytes; AByteCount: Integer; AAppend: Boolean = True); virtual;
When true, it reads bytes from the socket and appends them to the end of the existing byte array. Since you are pre-allocating the array, the initial bytes are undefined and the file bytes follow after the undefined bytes.
To fix this, you need to either:
Stop pre-allocating the byte array, let ReadBytes() allocate it for you.
procedure TForm1.Button2Click(Sender: TObject);
var
fSize: Int64;
RecvBuff: TBytes;
hFile: THandle;
begin
IdTCPClient1.Socket.WriteLn(CMD_TEST_FILE);
fSize := IdTCPClient1.Socket.ReadInt64;
// SetLength(RecvBuff,fSize); // <-- remove this line
IdTCPClient1.Socket.ReadBytes(RecvBuffer, fSize);
if MCreateFile(hFile, MGetExePath+'\new.jpg') then
begin
MWriteFile(haile, RecvBuff[0], fSize);
MCloseFile(hFile);
end;
Memo1.Lines.Add('ok');
end;
pre-allocate the array, but set AAppend to False so the bytes fill the existing array instead of append to it.
procedure TForm1.Button2Click(Sender: TObject);
var
fSize: Int64;
RecvBuff: TBytes;
hFile: THandle;
begin
IdTCPClient1.Socket.WriteLn(CMD_TEST_FILE);
fSize := IdTCPClient1.Socket.ReadInt64;
SetLength(RecvBuff, fSize);
IdTCPClient1.Socket.ReadBytes(RecvBuff, fSize, False);
if MCreateFile(hFile, MGetExePath+'\new.jpg') then
begin
MWriteFile(haile, RecvBuff[0], fSize);
MCloseFile(hFile);
end;
Memo1.Lines.Add('ok');
end;
Update: That being said, I strongly suggest you use a TStream instead, despite you saying you do not want to. It will greatly simplify the code and memory management, without breaking the communication protocol you have chosen to use:
procedure TForm1.IdTCPServer1Connect(AContext: TIdContext);
begin
AContext.Data := TFileStream.Create(MGetExePath+'\test.jpg', fmOpenRead or fmShareDenyWrite);
AContext.Connection.IOHandler.LargeStream := True;
end;
TForm1.IdTCPServer1Execute(AContext: TIdContext);
var
Received: String;
begin
Received := AContext.Connection.IOHandler.ReadLn;
if Received = CMD_TEST_FILE then
begin
AContext.Connection.IOHandler.Write(TStream(AContext.Data), 0, True);
end;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
FileName: string;
Strm: TStream;
begin
FileName := MGetExePath+'\new.jpg';
Strm := TFileStream.Create(FileName, fmCreate);
try
try
IdTCPClient1.IOHandler.WriteLn
(CMD_TEST_FILE);
IdTCPClient1.IOHandler.ReadStream(Strm, -1, False);
finally
Strm.Free;
end;
except
DeleteFile(FileName);
raise;
end;
Memo1.Lines.Add('ok');
end;