一个简单的C程序问题，在IAR Embedded Workbench 编译器中出错的问题

#define Uchar unsigned char
typedef struct
{
Uchar year_k;
Uchar year;
Uchar month;
Uchar daytime;
Uchar hour;
Uchar minute;
}_boot_time;
Uchar a=1;
_boot_time boot_time={a,2,3,4,5,6};
void main()
{
Uchar *p_time_=(Uchar*)&boot_time;
}
这样编译会出错，说是结构体中必须是常量；但是 如果改为下面

#define Uchar unsigned char
typedef struct
{
Uchar year_k;
Uchar year;
Uchar month;
Uchar daytime;
Uchar hour;
Uchar minute;
}_boot_time;
Uchar a=1;

void main()
{
_boot_time boot_time={a,2,3,4,5,6};

Uchar *p_time_=(Uchar*)&boot_time;
}
这样又对 ，是为什么啊？为什么放在main函数里面又对了 求解！！！！