#include <stdio.h>
#include <string.h>   
#include <stdlib.h>
struct student
    {
    char name[20];
    int num;
    char flag;
    }stu[3];
void main()
{
FILE *fin;
FILE *fout;
    char name[20];
    int num;
    char flag;

if((fin=fopen("fscanf.txt","r"))==NULL)
        {printf("cannot open the file!\n");
        exit (0);}

if((fout=fopen("fprintf.txt","w"))==NULL)
        {printf("cannot open the file!\n");
        exit (0);}

        int i=0;
        while(3==fscanf(fin,"%s%d%c",name,&num,&flag))
        {
        strcmp(stu[i].name,name);
        stu[i].num=num;
        stu[i].flag=flag;
        fprintf(fout,"%s%d%c",stu[i].name,stu[i].num,stu[i].flag);
        printf("%s  %d  %c\n",stu[i].name,stu[i].num,stu[i].flag);
        i++;
        }

}

while(3==fscanf(fin,"%s%d%c",name,&num,&flag))
这句有问题，最好不要用fscanf，scanf混输字符数据和数值数据，参考下c primer plus.