#include<iostream.h>
#define M 2;
    struct student
    {
        char name[80];
        int age;
        char add[80];
    };
void main()
{int i;student stu[M];char s[10];
    cout<<"a system of students!";
      for(i=0;i<M;i++)
      {
     cout<<"input the name: ";
     cin>>stu[i].name;
     cout<<"input the age: ";
     cin>>stu[i].age;
     cout<<"input the address: ";
     cin>>stu[i].add;
      }   
      for(i=0;i<M;i++)
      {
          cout<<stu[i].name<<","<<stu[i].age<<","<<stu[i].add<<endl;
      }
}

宏定义后面多了一个分号，把那个分号删了就OK了

同楼上，而且你的s字符数组没用啊

谢谢各位啦！