c - VS2019 C6011 错误取消引用空指针 'NewNode'

Question

你好？我正在学习双向链表

反正我以前在node上放数据的时候出现C6011错误
[:取消引用空指针'NewNode']

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

试图检查空值。但它没有用..

我检查了错误窗口的一些细节，如我添加的图片

他们说“NewNode 将为 NULL”，“即使取消引用，NewNode 仍将是 NULL”



代码如下

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(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://docs.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.练习文件(.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

这是来自静态分析器的警告。编译器说可能有一些潜在的错误。

事实上，在 DLL_CreateNode 中有一个对 malloc 的调用可能会失败并返回一个 NULL。你在函数内部检查它，这很好。

但是如果 malloc 失败，你仍然会在 main.c 中向调用者返回一个 NULL。

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

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

如果此时有一个现有的头节点，则在 DLL_AppendNode 中的这一行:

NewNode->Prev = Tail;

它会给出一个 EXC_BAD_ACCESS 异常。

静态分析器因此正确地识别了潜在问题。

有几种方法可以处理它。一种解决方案是检查 NULL，在 stderr 上打印错误消息，然后以错误代码退出程序。