ioctlsocket with FIONREAD report 0 byte in buffer

Question

ioctlsocket with FIONREAD report 0 byte in buffer

sikai han 1

I am writing a Delphi windows console app with three client sockets. One socket is created in its own thread, the other two are created in another thread. Both threads are running its own windows message pump. All three sockets use WSAAsyncSelect registered with events FD_READ|FD_WRITE|FD_CONNECT. Each socket will also create its own windows handle for the purpose of WSAAsyncSelect
My testing program would run like the following:
create three sockets -> send/receive data for a few minutes -> tear them down then do it all over again.
The test can only last a few hours. then it will hit the following problem:
One of the socket (either the one created in the first thread or the first one of the two created in the second thread) after socket connection and one success write, it can only receive maximum one read, then followed by two or three consecutive read event notifications within a millisecond. The ioctlsocket always report 0 bytes for read in these read notification callback. Though the Wireshark has verified the data has been successfully sent to my client with tcp ack.
Many thanks for your help.

Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2021-11-30T09:02:31.987+00:00

Could you please show a minimal, reproducible sample?

sikai han 1

thanks for your reply, here is my delphi code, it's too big for one post i have to do it in multiple batchs

 procedure TCustomWinSocket.Open;
    begin
      //to setup a client socket connection
      FAsyncStyles := [asRead, asWrite, asConnect, asClose];
      FSocket := Socket(PF_INET, SOCK_STREAM, IPPROTO_IP);
      FAddr.sin_family := PF_INET;
      FAddr.sin_addr.s_addr := INADDR_ANY;
      FAddr.sin_port := htons(2000);
      FAddr.sin_addr.s_addr := inet_addr('127.0.0.1');
      Msg := CM_SOCKETMESSAGE;
      //the native AllocateHwnd is replaced with MakeAllocateHwndThreadsafe
      //see link https://stackoverflow.com/questions/8820294/how-can-i-make-allocatehwnd-threadsafe
      Wnd := AllocateHwnd(WndProc);
      WSAAsyncSelect(FSocket, Wnd, Msg, Longint(Byte(FAsyncStyles)));
      Winapi.WinSock.Connect(FSocket, FAddr, sizeof(FAddr));
    end;

    procedure TCustomWinSocket.WndProc(var Message: TMessage);
    begin
      try
        Dispatch(Message);
      except
        if Assigned(ApplicationHandleException) then
          ApplicationHandleException(Self);
      end;
    end;

sikai han 1

//CM_SOCKETMESSAGE HANDLER
//delphi's way to register windows message for dispatch CM_SOCKETMESSAGE
procedure CMSocketMessage(var Message: TCMSocketMessage);  message CM_SOCKETMESSAGE;

procedure TCustomWinSocket.CMSocketMessage(var Message: TCMSocketMessage);
begin
  with Message do
    case SelectEvent of
      FD_CONNECT:
        begin
          ....
        end;
      FD_CLOSE:
        begin
          ....
        end;
      FD_READ:
        begin
          //here readCnt could be zero when actually there is data waiting to be read
          ErrorCode := ioctlsocket(FSocket, FIONREAD, Longint(readCnt));
          if ErrorCode <> SOCKET_ERROR then
          begin
            if readCnt > 0 then
              console.writeln(ReceiveText) //see following func def
          end
          else
          begin
            PrintLog('on ioctlsocket with error ' +inttostr(WSAGetLastError));
          end;
        end;
      FD_WRITE:
        begin
          ...
        end;
    end;
end;

function TCustomWinSocket.ReceiveText: AnsiString;
var
begin
  //first read to get the length, then use it to set up return  buf size
  SetLength(Result, ReceiveBuf(Pointer(nil)^, -1));
  //second read to populate the return buf
  SetLength(Result, ReceiveBuf(Pointer(Result)^, length(Result)));
end;

sikai han 1

function TCustomWinSocket.ReceiveBuf(var Buf; Count: integer): integer;
var
  ErrorCode, iCount: integer;
begin
  Lock;
  try
    Result := 0;
    if (Count = -1) and FConnected then
      ioctlsocket(FSocket, FIONREAD, Longint(Result))
    else
    begin
      if not FConnected then
        exit;
      if ioctlsocket(FSocket, FIONREAD, iCount) = 0 then
      begin
        if (iCount > 0) and (iCount < Count) then
          Count := iCount;
      end;

      Result := recv(FSocket, Buf, Count, 0);
      if Result = SOCKET_ERROR then
      begin
        ErrorCode := WSAGetLastError;
        if ErrorCode <> WSAEWOULDBLOCK then
        begin
          Error(Self, eeReceive, ErrorCode);
          PrintLog('ReceiveBuf Disconnect' + LocalAddress + ':' +
            inttostr(LocalPort) + ' ' + inttostr(ErrorCode));
          Disconnect(FSocket);
          if ErrorCode <> 0 then
            raise ESocketError.CreateResFmt(@sWindowsSocketError,
              [SysErrorMessage(ErrorCode), ErrorCode, 'recv']);
        end;
      end;
    end;
  finally
    Unlock;
  end;
end;

//TCustomWinSocket.Open will be posted to the following thread with PostThreadMessage using the following message pump
while GetMessage(Msg, 0, 0, 0) do
begin
TranslateMessage(Msg);
DispatchMessageA(Msg);
end;

Your answer

Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2021-11-30T09:02:31.987+00:00

Could you please show a minimal, reproducible sample?
sikai han 1 Reputation point

2021-11-30T23:43:56.607+00:00

thanks for your reply, here is my delphi code, it's too big for one post i have to do it in multiple batchs

procedure TCustomWinSocket.Open; begin //to setup a client socket connection FAsyncStyles := [asRead, asWrite, asConnect, asClose]; FSocket := Socket(PF_INET, SOCK_STREAM, IPPROTO_IP); FAddr.sin_family := PF_INET; FAddr.sin_addr.s_addr := INADDR_ANY; FAddr.sin_port := htons(2000); FAddr.sin_addr.s_addr := inet_addr('127.0.0.1'); Msg := CM_SOCKETMESSAGE; //the native AllocateHwnd is replaced with MakeAllocateHwndThreadsafe //see link https://stackoverflow.com/questions/8820294/how-can-i-make-allocatehwnd-threadsafe Wnd := AllocateHwnd(WndProc); WSAAsyncSelect(FSocket, Wnd, Msg, Longint(Byte(FAsyncStyles))); Winapi.WinSock.Connect(FSocket, FAddr, sizeof(FAddr)); end; procedure TCustomWinSocket.WndProc(var Message: TMessage); begin try Dispatch(Message); except if Assigned(ApplicationHandleException) then ApplicationHandleException(Self); end; end;
sikai han 1 Reputation point

2021-11-30T23:46:05.07+00:00

//CM_SOCKETMESSAGE HANDLER //delphi's way to register windows message for dispatch CM_SOCKETMESSAGE procedure CMSocketMessage(var Message: TCMSocketMessage); message CM_SOCKETMESSAGE; procedure TCustomWinSocket.CMSocketMessage(var Message: TCMSocketMessage); begin with Message do case SelectEvent of FD_CONNECT: begin .... end; FD_CLOSE: begin .... end; FD_READ: begin //here readCnt could be zero when actually there is data waiting to be read ErrorCode := ioctlsocket(FSocket, FIONREAD, Longint(readCnt)); if ErrorCode <> SOCKET_ERROR then begin if readCnt > 0 then console.writeln(ReceiveText) //see following func def end else begin PrintLog('on ioctlsocket with error ' +inttostr(WSAGetLastError)); end; end; FD_WRITE: begin ... end; end; end; function TCustomWinSocket.ReceiveText: AnsiString; var begin //first read to get the length, then use it to set up return buf size SetLength(Result, ReceiveBuf(Pointer(nil)^, -1)); //second read to populate the return buf SetLength(Result, ReceiveBuf(Pointer(Result)^, length(Result))); end;
sikai han 1 Reputation point

2021-11-30T23:47:34.763+00:00

function TCustomWinSocket.ReceiveBuf(var Buf; Count: integer): integer; var ErrorCode, iCount: integer; begin Lock; try Result := 0; if (Count = -1) and FConnected then ioctlsocket(FSocket, FIONREAD, Longint(Result)) else begin if not FConnected then exit; if ioctlsocket(FSocket, FIONREAD, iCount) = 0 then begin if (iCount > 0) and (iCount < Count) then Count := iCount; end; Result := recv(FSocket, Buf, Count, 0); if Result = SOCKET_ERROR then begin ErrorCode := WSAGetLastError; if ErrorCode <> WSAEWOULDBLOCK then begin Error(Self, eeReceive, ErrorCode); PrintLog('ReceiveBuf Disconnect' + LocalAddress + ':' + inttostr(LocalPort) + ' ' + inttostr(ErrorCode)); Disconnect(FSocket); if ErrorCode <> 0 then raise ESocketError.CreateResFmt(@sWindowsSocketError, [SysErrorMessage(ErrorCode), ErrorCode, 'recv']); end; end; end; finally Unlock; end; end;

//TCustomWinSocket.Open will be posted to the following thread with PostThreadMessage using the following message pump
while GetMessage(Msg, 0, 0, 0) do
begin
TranslateMessage(Msg);
DispatchMessageA(Msg);
end;

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ioctlsocket with FIONREAD report 0 byte in buffer

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