void order ( )
{
    struct carinfo *temp,*p1,*p2;
    p1=head;
    p2=p1->next;
p1=p2=(struct carinfo*) malloc(sizeof(struct carinfo));

    while(p2!=NULL)
    { if(strcmp(p1->num,p2->num)>0)
         memcpy(p1,temp,sizeof(struct carinfo));
         memcpy(p1,p2,sizeof(struct carinfo));
         memcpy(p2,p1,sizeof(struct carinfo));
      p1=p1->next;
      p2=p1->next;
    }
   printf("the ordered content as follow\n");
     struct carinfo *p;
     p=head;
     while(p!=NULL)
     {
         printf("%s %s %s %f %d %s %s\n",p->num,p->name,p->type,p->price,p->date,p->chexing,p->corporation);
             p=p->next;

去看看你发的这个帖子https://bbs.bccn.net/thread-319608-1-1.html

看看下面的回答，就知道为什么了