我先申了全局的：
struct Stdinfo
{
  char num[MAX];  /* for students number */
  char names[MAX];  /* for students name */
  float score;
  struct Stdinfo *nextAddr;
};

/* here is the definition of the first structure pointer */
struct Stdinfo *tosp;


下面两个是我输入链表内容的函数：
void add_record(struct Stdinfo *p)
{
  char names[MAX];
  char num[MAX];
  float score;
  void push(struct Stdinfo *,char *,char *,float);

  getchar();
  printf("\n--------------------------------------------------------------------------------\n");
  printf("\nEnter as many information as you wish.\n");
  while (1)
  {
    printf("\nPlease enter the student's number:");
    printf("\n(To stop entering,enter a single x)");
    scanf("%s",num);
    getchar();
    if (strcmp(num,"x")==0)
      break;
    printf("Please enter the student's name:");
    gets(names);
    printf("Please enter the student's score:");
    scanf("%f",&score);
    getchar();
   
    printf("\nNow you have input the imformation of this student successful.\n");
    printf("--------------------------------------------------------------------------------\n");
    push(tosp,num,names,score);
  }
}

void push(struct Stdinfo *p,char *num,char *names,float score)
{
  struct Stdinfo *newaddr;
  newaddr=(struct Stdinfo *) malloc(sizeof(struct Stdinfo));
  if (newaddr==(struct Stdinfo *) NULL)
  {
    printf("\nFailed to allocate memory for this structure.\n");
    exit(1);
  }
  strcpy(newaddr->num,num);
  strcpy(newaddr->names,names);
  newaddr->score=score;
  newaddr->nextAddr=p;
  tosp=newaddr;
}
上面两个是我输入链表内容的函数

下面是我保存链表的函数：
void save_record(struct Stdinfo *tosp)
{
  FILE *infile;

  infile=fopen("c:\\record.txt","w");   /* to record the information in the "record.txt" file */
  if(infile==NULL)
    {
      printf("\nFailed to open the file.\n");
      exit(1);
    }
    while(tosp!=NULL)
    {
      fwrite(tosp,sizeof(struct Stdinfo),1,infile);
      tosp=tosp->nextAddr;
    }
    fclose(infile);
}

现在是以保存就出现Null pointer assignment，且少存了最后输入的一组数据。

push 函数有问题，你没有考虑表头表尾问题

我在main函数里写了tosp=NULL；

nextAddr关键是这个