我正在学习King（2008）"C编程：现代方法"，第2版，我对删除操作与插入操作相比的行为感到困惑。
作者写道（第429页）：
注意，add_to_list并不修改列表指针，而是返回一个指向新创建节点的指针......让add_to_list更新first是一件棘手的事情。
现在，删除第一个节点不会修改原始列表，删除内部或末尾的节点会修改原始列表，但是delete_from_list也复制了first指针，那么为什么它可以修改first（而add_to_list不能）？
我错过了什么？

#include <stdio.h>
#include <stdlib.h>

struct node {
  int value;
  struct node *next;
};

struct node *delete_from_list(struct node *, int);
struct node *add_to_list(struct node *, int n);

int main(int argc, char **argv) {

  // setup a linked list
  
  // list's head
  struct node *first= NULL;

  // first node
  struct node *new_node= malloc(sizeof(struct node));
  new_node->value= 10;
  new_node->next= first;
  first= new_node;

  //second node
  new_node = malloc(sizeof(struct node));
  new_node->value= 20;
  new_node->next= first;
  first= new_node;

  //third node
  new_node= malloc(sizeof(struct node));
  new_node->value= 40;
  new_node->next= first;
  first= new_node;

  //fourth node
  new_node= malloc(sizeof(struct node));
  new_node->value= 30;
  new_node->next= first;
  first= new_node;

  int i;
  struct node *head= first;

  
  printf("\n------------------------\n");
  printf("    Original nodes: ");
  for(i=0; head!= NULL; head= head->next, i++)
    printf("\n%d-ith value: %d ", i, head->value);
  printf("\n------------------------\n");

  struct node *first_no20= delete_from_list(first, 20);
  struct node *head_no20= first_no20;
  
  printf("\n------------------------\n");
  printf("    Nodes without 20: ");
  for(i=0; head_no20!= NULL; head_no20= head_no20->next, i++)
    printf("\n%d-ith value: %d ", i,head_no20->value);
  printf("\n------------------------\n");

  
  printf("\n------------------------\n");
  head=first;
  printf("    Original nodes: ");
  for(i=0; head!= NULL; head= head->next, i++)
    printf("\n%d-ith value: %d ", i, head->value);
  printf("\n------------------------\n");

  struct node *first_no30= delete_from_list(first, 30);
  struct node *head_no30= first_no30;
 
  printf("\n------------------------\n");
  printf("    Nodes without 30: ");
  for(i=0; head_no30!= NULL; head_no30= head_no30->next, i++)
    printf("\n%d-ith value: %d ", i,head_no30->value);
  printf("\n------------------------\n");
  

  printf("\n------------------------\n");
  printf("    Original nodes: ");
  head=first;
  for(i=0; head!= NULL; head= head->next, i++)
    printf("\n%d-ith value: %d ", i, head->value);
  printf("\n------------------------\n");

  return 0;

}

struct node *delete_from_list(struct node *list, int n) {
  struct node *cur, *prev;

  for(cur=list, prev=NULL;
      cur != NULL && cur->value !=n;
      prev= cur, cur= cur->next)
    ;

  if(cur == NULL)
    return list;
  if(prev== NULL)
    list= list->next;
  else
    prev->next= cur->next;
  free(cur);

  return list;
}

struct node *add_to_list(struct node *list, int n) {
  struct node *new_node;

  new_node= malloc(sizeof(struct node));
  if(new_node == NULL) {
    printf("Error: malloc failed in add_to_list\n");
    exit(EXIT_FAILURE);
  }

  new_node->value = n;
  new_node->next= list;

  return new_node;
}