First-chance exception in main.exe: 0xC0000005: Access Violation.的有关问题，需回答啊求你们了

First-chance exception in main.exe: 0xC0000005: Access Violation.的问题，急需回答啊求你们了
#include"head.h"
CLASS *head=NULL,*point_class_save,*point_class_go;
FILE *fp=NULL;
extern FILE *fp22;
extern FILE *fp33;
extern FILE *fp44;
void build_class()
{ 
char class_fall[20];
if(fp==NULL)
{
if((fp=fopen("classes.txt","r"))==NULL)
{
            printf("File open error!\n");
        exit(0);
}
}
if(fscanf(fp,"%s",class_fall)==4)
{
    if(head==NULL)
{
        head=(CLASS *)malloc(sizeof(CLASS));
strcpy(point_class_save->class_no,class_fall);
    point_class_save=head;
    point_class_save->next1=NULL;
}
    else
{
      point_class_go=(CLASS*)malloc(sizeof(CLASS));
    point_class_go->next1=NULL;
    point_class_save->next1=point_class_go;
    point_class_save=point_class_go;
strcpy(point_class_save->class_no,class_fall);
}
    build_student();
}
else
{
fclose(fp);
fclose(fp22);
fclose(fp33);
fclose(fp44);
return ;
}
}

这是出错那个文件，运行到if(fscanf(fp,"%s",class_fall)==4)出现First-chance exception in main.exe: 0xC0000005: Access Violation.不知道什么回事，求解啊大神们，急急急!!!! 是你的基础知识不扎实呀。求结贴给分。。。。