Question

我发现了如何解决我的问题，但我不知道它是如何工作的或原因。我非常感谢有人看看这个：

我正在创建一个链接列表，其中包含一个指向列表头部的全局指针。我在主线程中创建了一个虚拟节点。我想要发生的是能够调用printList（）并且如果除了虚拟节点之外没有其他节点则打印“Person：0”（基本上说列表是空的）。

[编辑 - ＆gt;这是我的简洁问题：为什么printList（）在main（）中识别Person * head = NULL而在使用它来设置当前指针等于head时不是全局指针？]

使用此代码，我得到以下输出

int main(){
    setvbuf(stdout, NULL, _IONBF, 0); 

    //Person *head = NULL; 
    printf("\nmain head:%p \n", head);

    head = (Person *)malloc(sizeof(Person));
    printf("\nmain head:%p \n", head);
    head->name[0] = '\0'; 
    head->next = NULL;
    head->previous = NULL;

输出：

main head:0000000000000000 

main head:00000000003F1390 
Enter add, insert or delete for Person functions: print

printList head:00000000003F1390 Person:1 Total People:1
Enter add, insert or delete for Person functions:

在main（）中声明Person * head并将其初始化为NULL，我得到了所需的结果。 为什么会这样？为什么我不能初始化全局指针并获得相同的期望结果？

int main(){
    setvbuf(stdout, NULL, _IONBF, 0); 

    Person *head = NULL; 
    printf("\nmain head:%p \n", head);

    head = (Person *)malloc(sizeof(Person));
    printf("\nmain head:%p \n", head);
    head->name[0] = '\0'; 
    head->next = NULL;
    head->previous = NULL;

关注输出：

main head:0000000000000000 

main head:00000000005E1390 
Enter add, insert or delete for Person functions: print

printList head:0000000000000000 Total People:0
Enter add, insert or delete for Person functions:

以下是整个计划供参考：

#include "stdio.h" 
#include "stdlib.h"
#include "string.h"

typedef struct S_PersonInLine{
    char name[16];

    struct S_PersonInLine *next;
    struct S_PersonInLine *previous;
}Person;

//pointer to head of the list
//This isn't a global head pointer(wrong). It doesn't go into main (wrong). False it does go into main but it doesn't give the intended result from the printList
Person *head = NULL; //this allows the functions to access the head pointer

//prototypes 
Person *makePerson();
void *addPerson();
void *insert();
void *delete();
void printList();
void cleanUp();


int main(){
    setvbuf(stdout, NULL, _IONBF, 0); //Figure out what this thing does again and why its necessary
    Person *head = NULL; 
    printf("\nmain head:%p \n", head);

    head = (Person *)malloc(sizeof(Person));
    printf("\nmain head:%p \n", head);
    head->name[0] = '\0'; 
    head->next = NULL;
    head->previous = NULL;

    char input[16];
    char command[16];

    printf("Enter add, insert or delete for Person functions: ");
    while( fgets(input , 15 , stdin) ){ 
    sscanf(input, "%s", command);
        if ( strcmp(command, "quit") == 0 ){
            printf("\n\nBreaking....");
            break;
        } else if ( strcmp(command, "print") == 0 ){
            printList();
        }

    printf("Enter add, insert or delete for Person functions: ");
    }


    return 0;
}

void printList(){
    Person *current = head;
    printf("\nprintList head:%p ", head);
    int count = 0;


    while(current != NULL){
        count++;
        printf("Person:%d %s", count, current->name);
        current = current->next;
    }
    printf("Total People:%d\n", count);
}

Answer 1

没有状态传递到printList函数，因此变量head引用全局实例。它不适合您的原因是因为您的main函数未使用全局实例。当您在Person *head = NULL;函数中键入main时，它会声明一个局部变量（即，您没有修改全局实例）。相反，您只需键入head = NULL;。

即可初始化全局实例

Answer 2

建议从main（）中删除以下行（完全）像他们 1）掩盖全局变量'head' 2）将第一个条目放入链表这就是计数等的原因什么都没有特别插入链表

Person *head = NULL; 
printf("\nmain head:%p \n", head);

head = (Person *)malloc(sizeof(Person));
printf("\nmain head:%p \n", head);
head->name[0] = '\0'; 
head->next = NULL;
head->previous = NULL;

注意：在打印功能中，检查'head'的内容在进入任何引用某个偏移量的循环之前不是NULL 从'head'开始检查，代码将尝试取消引用内存地址0，导致未定义的行为，可以/将导致seg故障事件

C中全局指针的逻辑错误

2 个答案: