C程序运行出错，请教如何解决
编译链接都没错，运行时出现
unhandled exception in 文件名.exe dxc0000005:access violation
这是在c++6.0 运行出现的。在TC下运行正常。
程序如下
#include <stdio.h>  
#include <stdlib.h>

void copy_string(char from[],char to[])
{int i=0;
printf("\nstring from=%d\nstring to=%d\n",from,to);
 
printf("\nstring from=%s\nstring to=%s\n",from,to);
 while (from[i]!='\0')
 { to[i]=from[i];i++;}
  to[i]='\0';
 }

 main ()

{ char *a="I am a techer.";
  char *b="you are a student.";
    printf("\nstring aa=%d\nstring bb=%d\n",a,b);
    copy_string(a,b);
   printf("string a=%s\nstring b=%s\n",a,b);
}

    //char *b="you are a student.";
    char b[]="you are a student.";