这个指针类型的变量如何指向指针

Question

struct node
{
   int data;
   struct node *link;
};

struct node *addnode(struct node **head);

int main()
{
    struct node *head = NULL;

    addnode(&head);

    return 0;
}


struct node *addnode(struct node **ptrTohead)
{
       if (*ptrTohead == NULL)
       {
          struct node *newNode = (struct node*) malloc(sizeof(struct node));
          *ptrTohead = newNode;
       }

}

我正在用 C 语言实现链表并遇到了这段代码：我不明白 &head 的类型是 struct node ** 毕竟 *head 是一个存储地址的指针，而 @987654325 @ 获取头变量的地址。那么它是一个指向指针的指针呢？

这就是我的想象：

//                head ----> |___2______| 
/memory address/  100           200
// &head is 100 and is of type struct node *

Answer 1

研究这个简单的演示程序。

#include <stdio.h>

int main(void) 
{
    int x = 10;
    
    int *p = &x;

    printf( "The variable x stores the value %d\n", x );
    printf( "Its address is %p\n", ( void * )&x );
    
    printf( "\nThe pointer p stores the address of the variable x %p\n", 
            ( void * )p );
    printf( "Dereferencing the pointer p we will get x equal to %d\n" , *p );
    printf( "The address of the pointer p itself is %p\n", ( void * )&p );
    
    int **pp = &p;
    
    printf( "\nThe pointer pp stores the address of the pointer p %p\n", 
            ( void * )pp );
    printf( "Dereferencing the pointer pp we will get the address stored in p %p\n",
            ( void * )*pp );
    printf( "Dereferencing the pointer pp two times "
            "we will get  x equal to %d\n", **pp );

    int y = 20;
    
    printf( "\nBecause dereferencing the pointer pp\n"
            "we have a direct access to the value stored in p\n"
            "we can change the value of p\n" );

    *pp = &y;           

    printf( "\nNow the pointer p stores the address of the variable y %p\n", 
            ( void * )p );
    printf( "Dereferencing the pointer p we will get y equal to %d\n" , *p );
    printf( "The address of the pointer p itself was not changed %p\n", ( void * )&p );
    
    return 0;
}

程序输出可能看起来像

The variable x stores the value 10
Its address is 0x7fffd5f6de90

The pointer p stores the address of the variable x 0x7fffd5f6de90
Dereferencing the pointer p we will get x equal to 10
The address of the pointer p itself is 0x7fffd5f6de98

The pointer pp stores the address of the pointer p 0x7fffd5f6de98
Dereferencing the pointer pp we will get the address stored in p 0x7fffd5f6de90
Dereferencing the pointer pp two times we will get  x equal to 10

Because dereferencing the pointer pp
we have a direct access to the value stored in p
we can change the value of p

Now the pointer p stores the address of the variable y 0x7fffd5f6de94
Dereferencing the pointer p we will get y equal to 20
The address of the pointer p itself was not changed 0x7fffd5f6de98

也就是说变量p和pp都是指针。区别在于指针p 指向int 类型的对象（指向变量x 或y），而指针pp 指向类型为p 的变量@987654331 @。取消引用这些指针中的任何一个，您都可以直接访问指向的对象并可以更改它（如果它不是常量对象）。

Answer 2

在main 中，head 是一个指向结构节点的指针。表达式&head 是指向结构节点的指针的地址。 ptrTohead 是函数addnode 的形参，是指向结构节点的指针。 &head 的值可以分配给ptrTohead，因为它是指向结构节点的指针的地址，而指向结构节点的指针的地址是用于初始化指向结构节点的指针的适当值。

注意&head 不是指向结构节点指针的指针。它是指向结构节点的指针的地址。另请注意，*head 不是指针。 *head 是一个结构节点，而head 是一个指向结构节点的指针。