#include<stdio.h>
void copy_string(char from[],char to[])
{
    int i=0;
    while(from[i]!='\0')
    {
        to[i]=from[i];
        i++;
    }
    to[i]='\0';
}
int main()
{
    char *a="I am a teacher.";
    char *b="you are a student.";
    printf("string a=%s\nstring b=%s\n",a,b);
    copy_string(a,b);
    printf("string a=%s\nstring b=%s\n",a,b);
}
以上程序在gcc下编译可以通过的，但输出的结果却和预期的相差很远！！！
预期输出为：
string a=I am a teacher.
string b=you are a student.
string a=I am a teacher.
string b=I am a teacher.
实际输出为:
string a=I am a teacher.
string b=you are a student.

    char *a="I am a teacher.";
    char *b="you are a student.";
to
    char a[]="I am a teacher.";
    char b[]="you are a student.";
有些情况下也是不行的，保证to的空间要大于from的

    char *a="I am a teacher.";
    char *b="you are a student.";这样是不能修改值的，这些字符串是存在只读的数据段上的，前面已经讨论过了