#include <stdio.h>
void main()
{
    void copy_string(char from[],char to[]);
    char from[]="I am a teacher.";
        char to[]="You are a student.";
        char *a=from,*b=to;
        printf("string a=%s\nstring b=%s\n",a,b);
        printf("copy string a to string b:\n");
        copy_string(a,b);
        printf("string a=%s\nstring b=%s\n",a,b);
}
void copy_string(char *from,char *to)
{
    for(;*from!='\0';from++,to++)
        *to=*from;
    *to='\0';

}

-------------------Configuration: cb - Win32 Debug--------------------
Linking...
test-27.obj : error LNK2001: unresolved external symbol "void __cdecl copy_string(char * const,char * const)" (?copy_string@@YAXQAD0@Z)
Debug/cb.exe : fatal error LNK1120: 1 unresolved externals
初学，不懂这些