奇怪了，sizeof与结构的问题

[jixiaoqiang]

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;

[trainbox]

delphi编译器有字段对齐的功能，默认是八位对齐

[trainbox]

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