VS2019 C6011错误删除空指针“NewNode”

Question

你好?我在研究双链接列表

无论如何，当我过去在节点上放置数据时，出现了C6011错误:删除空指针'NewNode‘

所以我引用了https://learn.microsoft.com/ko-kr/visualstudio/code-quality/c6011?view=vs-2019和

尝试检查空值。但没成功..。

我检查了错误窗口的一些细节，比如我添加的图片。

他们说'NewNode将为空‘，'NewNode将仍然为空，即使它被取消引用’。

​

代码如下所示

1.头文件(DoubleLinkedList.h)

#pragma once
#ifndef DOUBLY_LINKEDLIST_H
#define DOUBLY_LINKEDLIST_H

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
typedef int ElementType;

typedef struct MyNode
{
    ElementType Data;
    struct MyNode* Prev;
    struct MyNode* Next;
}Node;

Node* DLL_CreateNode(ElementType NewData); //1.create node
void DLL_DestroyNode(Node* Node);//2.destroy node
void DLL_AppendNode(Node** Head, Node* NewNode);//3.append node
/*
4.insert node
connect
prev-insert-next  
*/
void DLL_InsertAfter(Node* Current, Node* NewNode);


void DLL_RemoveNode(Node** Head, Node* Remove);//5.remove node
Node* DLL_GetNodeAt(Node* Head, int location);//6.search node 
int DLL_GetNodeCount(Node* Head);//7.count the number of nodes
#endif

2.DLL.cpp(用于头文件)

#include "DoubleLinkedList.h"

Node* DLL_CreateNode(ElementType NewData)    
{
    //create node with memory allocation
    Node* NewNode = (Node*)malloc(sizeof(Node));

  //nullptr check for c6011(https://learn.microsoft.com/ko-kr/visualstudio/code-quality/c6011?)
    if (NewNode)
    {
        NewNode->Data = NewData;
        NewNode->Prev = NULL;
        NewNode->Next = NULL;
    }


    return NewNode;
}

void DLL_DestroyNode(Node* Node)
{
    free(Node);
}

void DLL_AppendNode(Node** Head, Node* NewNode)
{
    if ((*Head) == NULL)
    {
        *Head = NewNode;
    }
    else
    {
        Node* Tail = (*Head);

        while (Tail->Next != NULL)
        {
            Tail = Tail->Next;
        } 
        Tail->Next = NewNode;
        NewNode->Prev = Tail;
    }
}

void DLL_InsertAfter(Node* Current, Node* NewNode)
{

    NewNode->Prev = Current->Next;
    NewNode->Next = Current;

    if (Current->Next != NULL)
    {
        Current->Next->Prev = NewNode;
        Current->Next = NewNode;
    }
}

void DLL_RemoveNode(Node** Head, Node* Remove)
{

    if (*Head == Remove)
    {

        *Head = Remove->Next;
        if (*Head != NULL)
        {
            (*Head)->Prev = NULL;
        }
        Remove->Next = NULL;
        Remove->Prev = NULL;
    }
    else
    {
        Node* Temp = Remove;

        if (Remove->Prev != NULL)
        {

            Remove->Prev->Next = Temp->Next;
        }

        if (Remove->Next != NULL)
        {

            Remove->Next->Prev = Temp->Prev;
        }

        Remove->Prev = NULL;
        Remove->Next = NULL;
    }
}

Node* DLL_GetNodeAt(Node* Head, int location)
{
    Node* Current = Head;

    while (Current != NULL && (--location) >= 0)
    {
        //--location >=0 is for check index
        Current = Current->Next;
    }
    return Current;
}

int DLL_GetNodeCount(Node* Head)
{
    int count = 0;

    Node* Current = Head;

    while (Current != NULL)
    {
        Current = Current->Next;
        count++;
    }

    return count;
}

3.实习档案(.c)

#include "DoubleLinkedList.h"

int main()
{
    int i = 0;
    int Count = 0;
    Node* List = NULL;
    Node* Current = NULL;
    Node* NewNode = NULL;

   //create&append nodes
    for (i = 0; i < 5; i++) {
        NewNode = DLL_CreateNode(i);
        DLL_AppendNode(&List, NewNode);
    }


    Count = DLL_GetNodeCount(List);
    for (i = 0; i < Count; i++) {
        Current = DLL_GetNodeAt(List, i);
        printf("List[%d] : %d\n", i, Current->Data);
    }



    Current = DLL_GetNodeAt(List, 2);
    NewNode = DLL_CreateNode(3000);

    DLL_InsertAfter(Current, NewNode);

    Count = DLL_GetNodeCount(List);
    for (i = 0; i < Count; i++) {
        Current = DLL_GetNodeAt(List, i);
        printf("List[%d] : %d\n", i, Current->Data);
    }

    for (i = 0; i < Count; i++) {
        Current = DLL_GetNodeAt(List, 0);

        if (Current != NULL) {
            DLL_RemoveNode(&List, Current);
            DLL_DestroyNode(Current);
        }
    }

    return 0;
}

谢谢!

Answer 1

这是静态分析器发出的警告。编译器说可能有一些潜在的错误。

实际上，在DLL_CreateNode中有一个对malloc的调用，它可能会失败并返回NULL。在函数中检查它，这很好。

但是，如果malloc失败，您仍然将向main.c中的调用方返回一个NULL。

NewNode = DLL_CreateNode(i);   //<-- this could return NULL
DLL_AppendNode(&List, NewNode);

在这种情况下，这个NULL将作为第二个参数提供给DLL_AppendNode。

如果此时存在一个现有的head节点，那么在DLL_AppendNode中这一行：

NewNode->Prev = Tail;

它将提供一个EXC_BAD_ACCESS异常。

因此，静态分析器正确地发现了一个潜在的问题。

有几种方法可以解决这个问题。一种解决方案是检查NULL，在stderr上打印错误消息，并使用错误代码退出程序。