How to convert a record from host to network byte

问题:

I have a record:

  Tconnecting = record
  var
    a: int64;
    b: integer;
    c: integer;
  end;

which I need send to server using UDP protocol

I fill it

  packet.a := StrToInt64('0x1234567890');
  packet.b := 0;
  packet.c := RandomRange(1, 9999999);

and sending it

  SetLength(send_data, sizeof(packet));
  send_data := RawToBytes(packet, SizeOf(packet));
  udp.SendBuffer(make_it_big_endian(send_data)); <-- the question... "network byte order"

or maybe I'm doing something wrong? I need to send "bign endian" packet

pack("N*", int64, int, int); (this is in PHP)

Thanks.

回答1:

Assuming your code is running in a little-endian OS/Machine (like Windows/Intel), you can just reverse the individual bytes of your packet, for example, include the FMX.Types unit in your uses clause:

type
  Tconnecting = record
    a: int64;
    b: integer;
    c: integer;
  end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Packet, BEPacket: Tconnecting;
begin
  packet.a := $0102030405060708;
  packet.b := $01020304;
  packet.c := $05060708;
  //big endian packet  
  BEPacket := Packet;

  ReverseBytes(@BEPacket.a, SizeOf(BePacket.a));
  ReverseBytes(@BEPacket.b, SizeOf(BePacket.b));
  ReverseBytes(@BEPacket.c, SizeOf(BePacket.c));

  Memo1.Lines.Add(Format('a: %16.16x => %16.16x', [Packet.a, BEPacket.a]));
  Memo1.Lines.Add(Format('b: %8.8x => %8.8x', [Packet.b, BEPacket.b]));
  Memo1.Lines.Add(Format('c: %8.8x => %8.8x', [Packet.c, BEPacket.c]));
end;

回答2:

What you are actually doing is converting from host byte order to network byte order. All standard sockets libraries provide helper functions to do that.

For example, Winsock offers htons, htonl etc. And in the opposite direction you have ntohs, ntohl etc. If you are using Indy, then the equivalent functions are GStack.HostToNetwork and GStack.NetworkToHost.

You should serialize each field into a byte stream, with each field being transformed by the appropriate hton* function. Then you can put the byte stream on the wire.