字节对齐掌握4个原则：
1 与首地址的偏移量必须是变量长度的整数倍
2 变量按定义的顺序存放
3 整个结构体的长度必须是最长成员变量长度的整数倍
4 如果有struct嵌套，则子struct中的变量必须在整个struct符合以上三个原则，但是子struct自身各个变量之间的存储位置不会变。
e.g.
struct NSSB{
long a;
short b;
char c;
short d;
char ch[3];
}obj,*p;
cout<<"sizeof long "<<sizeof(long)<<endl;
cout<<"sizeof double "<<sizeof(double)<<endl;
cout<<"sizeof float "<<sizeof(float)<<endl;
cout<<"sizeof int "<<sizeof(int)<<endl;
cout<<"sizeof short "<<sizeof(short)<<endl;
cout<<"sizeof char "<<sizeof(char)<<endl;
cout<<"sizeof p "<<sizeof(p)<<endl;
cout<<"sizeof *p "<<sizeof(*p)<<endl;
p=(struct NSSB *)0x1000000;
// cout<<"next struct is "<<(p + 0x1)<<endl;
//obj.c = 'C';
cout<<"a "<<&obj.a<<endl;
cout<<"b "<<&obj.b<<endl;
cout<<"c "<<&obj.c<<endl;
cout<<"d "<<&obj.d<<endl;
cout<<"ch "<<&obj.ch<<endl;
运行结果：
[root@127 test]# ./test.x
sizeof long 4
sizeof double 8
sizeof float 4
sizeof int 4
sizeof short 2
sizeof char 1
sizeof p 4
sizeof *p 16
a 0xbfcee3c0
b 0xbfcee3c4
c d��ο
d 0xbfcee3c8
ch 0xbfcee3ca