c - C中全局指针的逻辑错误

Question

我发现了如何解决我的问题，但我不知道它是如何或为什么起作用的。我将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然后打印“Person:0”(基本上说列表是空的)。

[编辑 -> 这是我的简洁问题:为什么 printList() 识别 main() 中的 Person *head = NULL 而不是全局指针，当它使用它来设置当前指针等于 head 时？]

使用这段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，我得到了想要的结果。 为什么会这样？为什么我无法初始化全局指针并获得相同的预期结果？

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;

以下输出:

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:

这里是整个程序供引用:

#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

没有状态被传递到 printList 函数，因此那里的变量 head 指的是全局实例。它不适合你的原因是因为你的 main 函数没有使用全局实例。当您在 main 函数中键入 Person *head = NULL; 时，它会声明一个局部变量(即，您没有修改全局实例)。相反，您应该通过简单地键入 head = NULL; 来初始化全局实例。