为什么我的程序突然终止？

Question

我有一个创建单链表的程序，在列表的开头和结尾添加一个节点。该程序正在产生预期的输出，但在产生最后一个输出后崩溃。我在 main() 中创建了链表，我有 2 个函数分别在开头和结尾添加一个节点。

#include<stdio.h>
struct book
{

  char bname[20];
  char aname[20];
  int pages;
  struct book *next;
};


struct book *first;
struct book *current;
struct book *previous;
int main()
{
  int count=0,temp=0;


  for(int i=1;i<4;i++)
  {

    current=(struct book*)malloc(sizeof(struct book));//memory assigned to only the current structure
    if(current==NULL)
    {
      break;
    }
    if(first==NULL)
    {
      first=current; //if the first node value is null then it hadn't yet been processed,so current node is now the first;. 
    }
    if(previous!=NULL)
    {
      previous->next=current;//stores the current structure address to the next member(pointer) of the previous structure address
    }

    printf("\nBook Name:: ");
    scanf("%s",current->bname);

    printf("\nAuthor Name:: ");
    scanf("%s",current->aname);

    printf("\nPages::");
    scanf("%d",&current->pages);

    current->next=NULL;//if this is the last node
    //will again be filled up if there is a next structure

    previous=current;//the current node is the previous node for the next iteration 
  }


  current=first;
  while(current!=NULL)
  {
    count++;
    printf("\nBook Name:: %s Author Name:: %s Pages:: %d",current->bname,current->aname,current->pages);
    previous=current;//store to previous only because of freeing the current structure
    current=current->next;//next structure address of the current node is the current node of the next iteration.Now if this next address doesn't exist,then the current pointer does not exist and the loop terminates


  }
  addnodebeginning(first);

}


/*****************************inserting at the BEGINNING*********************************/

void addnodebeginning(struct book *first)
{
  int count=0,temp=0;
  current=(struct book*)malloc(sizeof(struct book));
  current->next=first;
  first=current;
  printf("\nBook Name:: ");
  scanf("%s",current->bname);

  printf("\nAuthor Name:: ");
  scanf("%s",current->aname);

  printf("\nPages::");
  scanf("%d",&current->pages);


  current=first;
  while(current!=NULL)
  {
    count++;
    printf("\nBook Name:: %s Author Name:: %s Pages:: %d",current->bname,current->aname,current->pages);
    previous=current;//store to previous only because of freeing the current structure
    current=current->next;//next structure address of the current node is the current node of the next iteration.Now if this next address doesn't exist,then the current pointer does not exist and the loop terminates


  }

  addnode_end(first);

}
/*****************************inserting at the end*********************************/

void addnode_end(struct node *first)
{

  current=first;

  while(1)
  {

    if(current->next==NULL)
    {
      struct book *newnode=(struct book*)malloc(sizeof(struct book));

      printf("\nBook Name:: ");
      scanf("%s",newnode->bname);

      printf("\nAuthor Name:: ");
      scanf("%s",newnode->aname);

      printf("\nPages::");
      scanf("%d",&newnode->pages);

      current->next=newnode;
      //newnode->next=NULL;
      break;
    }
    current=current->next;
  }

  current=first;
  while(current!=NULL)
  {

    printf("\nBook Name:: %s Author Name:: %s Pages:: %d",current->bname,current->aname,current->pages);
    previous=current;//store to previous only because of freeing the current structure
    current=current->next;
    free(previous);

  }

}

为什么会这样？

Answer 1

在addnodebeginning 函数中，first 参数按值 传递，这意味着该值是复制 并且您在函数中修改副本。如果你想修改传递的值，你必须通过引用传递它。

这是通过传递一个指向值的指针来完成的，在本例中是指向指针的指针。

Answer 2

首先，请注意警告。他们在那里是有原因的。其次，您只需将其包含在 if 测试的末尾 -

newnode->next=NULL;
 break;

您的程序将不会出现运行时崩溃。

Answer 3

代码错过了#include<stdlib.h>。至少在 64 位系统上调用 malloc() 很可能是致命的。

如果在 64 位系统上并且缺少 malloc() 的原型，编译器会假定 int 作为 malloc() 的返回值。 int 肯定是 32 位的，而 malloc() 尝试返回的地址是 64 位宽，所以返回的地址值很可能在返回时被截断一半。

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还有这一行

void addnode_end(struct node *first)

应该是

void addnode_end(struct book *first)

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更新：

此外，代码也错过了初始化一个新的malloc()ed 节点的成员。通常，其next 成员应显式设置为NULL。