代码：
struct basic 结构体已经定义为SBA
#define BUSIN_PATH "E:\\个人程序\\新建文件夹\\Business_input.dat"

    FILE *bufp;
    if((bufp=fopen(BUSIN_PATH,"rb+"))==NULL)
    {
        printf("\t>>>cannot open file");
        system("pause");
    }
    int N=File_size(bufp,sizeof(SBU),"成绩信息");
    SBU*b=(SBU*)malloc(N*sizeof(SBU));  
    fread(b,sizeof(SBU),N,bufp);
    rewind(bufp);
    for(int i=0;i<N;i++)
    {
        printf("%11s\t%11s\t%.1f\t%d\n",b[i].num,b[i].c_num,b[i].score,b[i].unpass);
    }
    //rewind(bufp);
    printf("请输入学号和课程编号进行修改：");
        char str[11],strc[11];
        scanf("%s%s",str,strc);
        fflush(stdin);
        for(int i=0;i<N;i++)
         {
            if((strcmp(str,b[i].num)==0)&&(strcmp(strc,b[i].c_num)==0))
            {   
                printf("学号%s编号%s课程进行以下修改\n",b[i].num,b[i].c_num);
                printf("请依次输入：成绩 是否补考\n");
                scanf("%f%d",&b[i].score,&b[i].unpass);
                printf("第%d条信息已修改成功\n",i);break;
            }
         }
    fwrite(b,sizeof(SCO),N,bufp);
        fclose(bufp);
    free(b);
    system("pause");

被自己蠢哭
fwrite(b,sizeof(SCO),N,bufp);改为fwrite(b,sizeof(SBU),N,bufp);