LNet file sending (Again)
lazarus / 38-Networking and Web Programming

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dimonar · 2011-04-10 17:56

Hello.
I know, that it's a frequently asked question. But I have a problem.

So. I have a "client", that want to send a file (in general it can be
any file, but for the beginning I want to send a short txt file). I
have a form, button and TLTCPComponent. On FormCreate client tries to
connect to "server". Button click causes an CanSend event.
Code: [Select]procedure TForm1.TCPCanSend(aSocket: TLSocket);
var
  i,n : integer;
  sendfile : file;
  Buff : array of char;
begin
If sending then //check if the button clicked
begin
  i := 0;
  AssignFile(sendfile, 'D:/jj.html'); //example file
  Reset(sendfile,1);
    Repeat
      begin
        Repeat // I try to read portion of data. May be 65k bytes, may
be less.
          SetLength(Buff,Length(Buff)+1); //increase buffer
          BlockRead(sendfile, Buff[i], 1); //read part of the file
into buffer
          inc(i,1);
        until (i=65000) or EoF(sendfile);
        n := TCP.Send(Buff[0], Length(Buff)); //sending
      end;
    until Eof(sendfile) or (n = 0) or (FSendIndex > Length(Buff));
    CloseFile(sendfile);
end;
end;

On the other side "server" is trying to construct a file from received
parts.
Code: [Select]procedure TForm1.TCPReceive(aSocket: TLSocket);
Var
  filename : string;
  savefile : file;
  letter : Array of Char;
begin
 SetLength(letter,151); // For example length is 151. Like a base file
length
 If aSocket.Get(letter,151) > 0 then
 begin
  filename := 'jjj.html';
   AssignFile(savefile,filename);
    if not FileExists(filename) then
    begin
      Rewrite(savefile);
    end;
    Reset(savefile, SizeOf(letter));
    BlockWrite(savefile,letter,1);
    CloseFile(savefile);
 end;
end;

As a result I've got an "SIGSEGV" error on the "server" side while
receive. So I do not understand how to send a file correctly.

Could you, please, help me to solve this problem.
And sorry for my english and bad coding habit.

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Almindor · 2011-04-10 20:17

There are multiple things wrong I see here.

1. In client/oncansend you need to do .Send(buffer) either until the
whole file is sent, or Send = 0 (which means "wait" and another
OnCanSend will get automagically called out when you can continue) for
more info read: http://lnet.wordpress.com/usage/sockets-protocols-and-
sending
2. In server you open the file on each receive, you should only open
it once, and then write to it (make it a field of the form for
example) until the whole thing is received, or something bad happens
(OnDisconnect or OnError should close the file, or even delete it as
failed).

It's simplest to do with a TFileStream IMO.

Hope this helps.