C++ linux Typedef 定义数组大小判断

头文件内定义了:

typedef struct {
unsigned char headtag;
unsigned int ipacket;
unsigned short packetLen;
unsigned short checkcode;
} tagUDPHead;
cpp 文件里面:
159 cout<<"tagUDPHead:"<<sizeof(tagUDPHead)<<endl; 
(gdb) n
tagUDPHead:12
(gdb) p sizeof(unsigned char)
$1 = 1
(gdb) p sizeof(unsigned int) 
$2 = 4
(gdb) p sizeof(unsigned short)
$3 = 2
怎么加起来应该是9,为什么打印结果是12呢?
我继续修改cpp,下面是GDB的显示:
168 cout<<"tagUDPHead1:"<<sizeof(tagUDPHead1)<<endl;
(gdb) l 158 
153 m_SocketID = -1;
154 }
155 }
156
157 struct tagUDPHead1 {
158 unsigned char headtag;
159 unsigned int ipacket;
160 unsigned short packetLen;
161 unsigned short checkcode;
162 } ;
(gdb) n 
tagUDPHead1:12
169 cout<<"headtag:"<<sizeof(a.headtag)<<endl;
(gdb)

headtag:1
170 cout<<"ipacket:"<<sizeof(a.ipacket)<<endl;
(gdb)

ipacket:4
171 cout<<"packetLen:"<<sizeof(a.packetLen)<<endl;
(gdb)

packetLen:2
172 cout<<"checkcode:"<<sizeof(a.checkcode)<<endl;
(gdb)

checkcode:2