delphi - 为什么 TCusomWinSocket.ReceiveBuf 不返回 0？

Question

说到套接字，我最喜欢 TClientSocket 和 TServerSockets，因为它们的用法很简单。

我的任务很简单。我需要通过这 2 个组件发送一个文件(RAW)，所以我有 2 个例程，如下所示:

procedure csRead(Sender: TObject; Socket: TCustomWinSocket);
var
   MSCli : TMemoryStream;
   cnt   : Integer;
   buf   : array [0..1023] of byte;
begin
    MSCli := TMemoryStream.Create;
    try
    repeat
      cnt := Socket.ReceiveBuf(buf[0], 1024); //This loop repeats endlesly
      MSCli.Write(buf[0], cnt)
    until cnt = 0;
    finally
    MSCli.SaveToFile('somefile.dmp');
    MSCli.Free;
    end;
end;

当然还有发件人:

  //...some code
    MSSErv.LoadFromFile('some file');
    MSServ.Position := 0;
    Socket.SendStream(MSServ);
  end;

阅读器中的循环不断重复，我不知道为什么。问题的根源可能是什么？

Answer 1

<小时/>

SendStream() 从来都不是一个特别好的选择。它的目的是发送整个TStream，然后在完成后释放它。但是，如果套接字设置为非阻塞模式并且套接字在发送期间阻塞，则 SendStream() 会立即退出并且不会释放 TStream。您必须再次调用 SendStream() 才能从 SendStream() 中断处继续发送 TStream。但还有其他条件可能导致 SendStream() 退出并释放 TStream，并且您并不真正知道它何时释放或没有释放 TStream，因此尝试使用相同的 TStream 对象再次调用 SendStream() 变得非常危险。更安全的方法是不惜一切代价避免 SendStream() 并直接在您自己的循环中调用 SendBuf()。

话虽如此，SendStream() 不会通知接收方将发送多少字节，因此接收方不知道何时停止读取(除非您在发送 后关闭连接>TStream)。更好的选择是先发送预期的字节数，然后再发送 TStream 数据。这样，接收方可以先读取字节数，然后在接收到指定字节数时停止读取。例如:

procedure ReadRawFromSocket(Socket: TCustomWinSocket; Buffer: Pointer; BufSize: Integer);
var 
  buf: PByte; 
  cnt: Integer; 
begin 
  buf := PByte(Buffer); 
  while BufSize > 0 do
  begin 
    cnt := Socket.ReceiveBuf(buf^, BufSize);
    if cnt < 1 then begin
      if (cnt = -1) and (WSAGetLastError() = WSAEWOULDBLOCK) then
      begin
        Application.ProcessMessages;
        Continue;
      end;
      Abort;
    end;
    Inc(buf, cnt);
    Dec(BufSize, cnt);
  end; 
end;

function ReadInt64FromSocket(Socket: TCustomWinSocket): Int64;
begin
  ReadRawFromSocket(Socket, @Result, SizeOf(Int64));
end;

procedure ReadMemStreamFromSocket(Socket: TCustomWinSocket: Stream: TMemoryStream);
var
  cnt: Int64; 
begin
  cnt := ReadInt64FromSocket(Socket);
  if cnt > 0 then
  begin
    Stream.Size := cnt; 
    ReadRawFromSocket(Socket, Stream.Memory, cnt);
  end; 
end;

procedure csRead(Sender: TObject; Socket: TCustomWinSocket); 
var 
  MSCli : TMemoryStream; 
begin 
  MSCli := TMemoryStream.Create; 
  try 
    ReadMemStreamFromSocket(Socket, MSCli);
    MSCli.SaveToFile('somefile.dmp'); 
  finally
    MSCli.Free; 
  end; 
end; 

procedure SendRawToSocket(Socket: TCustomWinSocket; Buffer: Pointer; BufSize: Integer);
var 
  buf: PByte; 
  cnt: Integer; 
begin 
  buf := PByte(Buffer); 
  while BufSize > 0 do
  begin 
    cnt := Socket.SendBuf(buf^, BufSize);
    if cnt < 1 then begin
      if (cnt = -1) and (WSAGetLastError() = WSAEWOULDBLOCK) then
      begin
        Application.ProcessMessages;
        Continue;
      end;
      Abort;
    end;
    Inc(buf, cnt);
    Dec(BufSize, cnt);
  end; 
end;

procedure SendInt64ToSocket(Socket: TCustomWinSocket; Value: Int64);
begin
  SendRawToSocket(Socket, @Value, SizeOf(Int64));
end;

procedure SendMemStreamToSocket(Socket: TCustomWinSocket: Stream: TMemoryStream);
begin
  SendInt64FromSocket(Socket, Stream.Size);
  SendRawToSocket(Socket, Stream.Memory, Stream.Size);
end;

begin
  ...
  MSSErv.LoadFromFile('some file'); 
  MSServ.Position := 0; 
  SendMemStreamToSocket(Socket, MSServ);
  ...
end;