下面的程序出错，是什么原因？ RetLen := FSocket.ReceiveBuf(Sig, SizeOf(Sig));(30分)

function TSocketTransport.Receive(WaitForInput: Boolean; Context: Integer): IDataBlock;
var
RetLen, Sig, StreamLen: Integer;
P: Pointer;
FDSet: TFDSet;
TimeVal: PTimeVal;
RetVal: Integer;
begin
Result := nil;
TimeVal := nil;
FD_ZERO(FDSet);
FD_SET(FSocket.SocketHandle, FDSet);
if not WaitForInput then
begin
New(TimeVal);
TimeVal.tv_sec := 0;
TimeVal.tv_usec := 1;
end;
RetVal := select(0, @FDSet, nil, nil, TimeVal);
if Assigned(TimeVal) then
FreeMem(TimeVal);
if RetVal = SOCKET_ERROR then
raise ESocketConnectionError.Create(SysErrorMessage(WSAGetLastError));
if (RetVal = 0) then Exit;
RetLen := FSocket.ReceiveBuf(Sig, SizeOf(Sig));
if RetLen <> SizeOf(Sig) then
raise ESocketConnectionError.CreateRes(@SSocketReadError);
if (Sig and CallSig <> CallSig) and
(Sig and ResultSig <> ResultSig) then
raise Exception.CreateRes(@SInvalidDataPacket);
RetLen := FSocket.ReceiveBuf(StreamLen, SizeOf(StreamLen));
if RetLen = 0 then
raise ESocketConnectionError.CreateRes(@SSocketReadError);
if RetLen <> SizeOf(StreamLen) then
raise ESocketConnectionError.CreateRes(@SSocketReadError);
Result := TDataBlock.Create as IDataBlock;
Result.Size := StreamLen;
Result.Signature := Sig;
P := Result.Memory;
Inc(Integer(P), Result.BytesReserved);
while StreamLen > 0 do
begin
RetLen := FSocket.ReceiveBuf(P^, StreamLen);
if RetLen = 0 then
raise ESocketConnectionError.CreateRes(@SSocketReadError);
if RetLen > 0 then
begin
Dec(StreamLen, RetLen);
Inc(Integer(P), RetLen);
end;
end;
if StreamLen <> 0 then
raise ESocketConnectionError.CreateRes(@SInvalidDataPacket);
InterceptIncoming(Result);
end;

出错代码：
RetLen := FSocket.ReceiveBuf(Sig, SizeOf(Sig));
if RetLen <> SizeOf(Sig) then
raise ESocketConnectionError.CreateRes(@SSocketReadError);
这个代码是被TSocketConnection调用的，我在MIDAS程序中用到了此对象。

先看看！

Sig应该是一个指针吧，就象RetLen := FSocket.ReceiveBuf(P^, StreamLen);

应该是PChar类型的