xdm看看为啥我这程序没结果

```c
#include<stdio.h>
void copy(char *p,char *q);
int main()
{
    char *a="I am a student";
    char *b="I am a teacher";
    printf("a is %s\nb is %s\n",a,b);
    copy(a,b);
    printf("\nnew a is %s\nb is %s\n",a,b);
    return 0;
}
void copy(char *p,char *q)
{
    int i=0;
    while(*p!='\0')
    {
        *p=*q;
        p++;
        q++;
    }
    *p='\0';
}

```

a,b都是常量数组，不可以修改