奇怪了，sizeof与结构的问题

Question

定义了一个结构TMsg_Bind，里面仅仅包含两个结构TMsg_Head,TMsg_Body，结果发现使用
sizeof计算Tmsg_Bind类型的变量BindData的大小时候，不是他两个结构大小的和,请问为什么？？var
  BindData:TMsg_Bind;
begin
  ShowMessage(IntToStr(sizeof(BindData.MsgHead)));  //20
  ShowMessage(IntToStr(sizeof(BindData.MsgBody)));  //41
  ShowMessage(IntToStr(sizeof(BindData)));  //64 ???各结构定义如下:    TMsgHead = record           //所有消息体的消息头  20字节
      iMsgLen:Longint;
      iMsgID:Longint;
      sMsgSerial:Array[0..11] of char;
    end;    TMsgBody_Bind = record        //bind消息体      41字节
      iLoginType:Byte;
      sLoginUser:Array[0..15] of char;
      sLoginPwd:Array[0..15] of char;
      sReserve:Array[0..7] of char;
    end;    TMsg_Bind=record
      MsgHead:TMsgHead;
      MsgBody:TMsgBody_Bind;
    end;

Answer 1

delphi编译器有字段对齐的功能，默认是八位对齐

Answer 2

  TMsgHead = packed record           //所有消息体的消息头  20字节
      iMsgLen:Longint;
      iMsgID:Longint;
      sMsgSerial:Array[0..11] of char;
    end;    TMsgBody_Bind = packed record        //bind消息体      41字节
      iLoginType:Byte;
      sLoginUser:Array[0..15] of char;
      sLoginPwd:Array[0..15] of char;
      sReserve:Array[0..7] of char;
    end;    TMsg_Bind = packed record
      MsgHead:TMsgHead;
      MsgBody:TMsgBody_Bind;
    end;这样定义的话就是两个之和了