Delphi XE2 Indy10 udp client-server interchange using SendBuffer-ReceiveBuffer - udp

Question

I use Delphi XE2 and Indy10 UDP protocol.
I can't receive server echo on client side if I use ReceiveBuffer method.
I got "Socket Error # 10040" although I send back very small echo
message from server to client.
Console application to illustrate my problem is below.
Thanks in advance.
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, IdGlobal, IdBaseComponent, IdComponent, IdSocketHandle,
IdUDPClient, IdUDPServer, IdUDPBase, IdStack;
type
TUDP_Serv = class(TIdUDPServer)
procedure udpSvUDPRead(AThread: TIdUDPListenerThread;
AData: TIdBytes; ABinding: TIdSocketHandle);
end;
var
udpServer: TUDP_Serv;
udpCl: TIdUDPClient;
bSnd, bRcv: TBytes;
s: string;
k: integer;
//==============================================================================
procedure TUDP_Serv.udpSvUDPRead(AThread: TIdUDPListenerThread; AData: TIdBytes;
ABinding: TIdSocketHandle);
begin
writeln(' Server read: ' + ToHex(AData, length(AData)));
with ABinding do SendTo(PeerIP, PeerPort, AData);
end;
//==============================================================================
begin
try
udpServer := TUDP_Serv.Create;
with udpServer do begin
OnUDPRead := udpSvUDPRead; DefaultPort := 20001; BufferSize := 2048;
ThreadedEvent := true; Active := True;
if Active then writeln('Server started on port: ' + IntToStr(DefaultPort));
end;
udpCl := TIdUDPClient.Create;
with udpCl do begin
BufferSize := 2048; Host := '127.0.0.1'; Port := 20001;
end;
SetLength(bSnd, 5); bSnd[0] := $31; bSnd[1] := $0;
bSnd[2] := $33; bSnd[3] := $0; bSnd[4] := $0;
repeat
writeln(' Client send: ' + ToHex(bSnd, length(bSnd)));
with udpCl do SendBuffer(Host, Port, bSnd); sleep(100);
try
k := udpCl.ReceiveBuffer(bRcv, 10);
if k > 0 then writeln(' Client read: ' + ToHex(bRcv, length(bRcv)));
except
on E: exception do begin
writeln(Format(' Client read err: %s',[E.Message]));
end;
end;
readln(s);
until s <> '';
except
on E: Exception do begin
Writeln(E.ClassName, ': ', E.Message); readln(s);
end;
end;
end.
Screen output:
Server started on prot: 20001
Client send: 3100330000
Server read: 3100330000
Client read err: Socket Error # 10040
Message too long.

Answer 1

10040 is WSAEMSGSIZE, which means the buffer you tried to receive into was smaller than the actual size of the message that was received.
You are not allocating any memory for bRcv before calling ReceiveBuffer(), so you are trying to receive into a 0-byte buffer, hense the error. You need to pre-allocate bRcv to at least the same size as your messages, if not larger.
ReceiveBuffer() does not allocate a new TBytes for each received message. You have to allocate the buffer yourself beforehand and then ReceiveBuffer() will merely fill it in, returning how many bytes were actually received into it.