为什么编译是报错，这是什么错误？

Question

inking
Cpp1.obj : error LNK2001: unresolved external symbol "void __cdecl ModifyInformation(struct student * const,int)" (?ModifyInformation@@YAXQAUstudent@@H@Z)
Debug/Cpp1.exe : fatal error LNK1120: 1 unresolved externals
执行 link.exe 时出错.

void modifyInformation(STUDENT stu[],int n)
{
int i,j,k,item,m=0,find=0;
long s1;
FILE *fp;
printf("请输入要修改的学生学号：");
scanf("%ld",&s1);
fp=fopen("E:\demo.txt","r");
if(fp==NULL)
{
printf("打开文件失败，请重试！");
getch();
fclose(fp);
exit(0);
}
while(!feof(fp))//扫描文件
{
fscanf(fp,"%ld",&stu[m].studentID);
m++;
}
for(i=0;i<n;i++)
{

    if(stu[i].studentID==s1)
    {
        j=i;
        find=1;
        printf("1---修改姓名\n");
        printf("2---修改性别\n");
        printf("3---修改地址\n");
        printf("4---修改电话\n");
        printf("5---退出程序\n");
        printf("请输入你的选择：\n");
        scanf("%d",&item);
        switch(item)
        {
        case 1:
            printf("请输入新的姓名：\n");
            scanf("%s",&stu[j].studentName);
            break;
        case 2:
            printf("请输入新的性别：\n");
            scanf("%c",&stu[j].studentSex);
            break;
        case 3:
            printf("请输入新的地址：\n");
            scanf("%s",&stu[j].address[j][20]);
            break;
        case 4:
            printf("请输入新的电话：\n");
            scanf("%ld",&stu[j].phonenumber);
            break;
        case 5:
            printf("End of the program!");
            exit(0);
        }    
    }
}
fclose(fp);
if(find==0)
{
    printf("没有找到要修改的学生记录，请按任意键退出！");
    getch();
}
else
{
    fp=fopen("E:\\demo.txt","w");
    for(k=0;k<n;k++)
    {
        fprintf(fp,"%ld %s %c %s %ld",stu[i].studentID,stu[i].studentName,stu[i].studentSex,
            stu[i].address[i][20],stu[i].phonenumber);//更新数据
    }
    fclose(fp);
    printf("录入学生信息成功，请按任意键返回\n");
    getch();
}

}

typedef struct student
{
long studentID;
char studentName[10];
char studentSex;
char address[N][20];
long phonenumber;
}STUDENT;

Answer 1

ModifyInformation没有定义