Knowledge summary of "level two pointers" for the initialization of chained storage for linked lists

First explain the "level two pointer":

A first-level pointer is associated with its value (an address) in the name of the data, the data can be any type and for any purpose, but the two-level pointer to the data associated with only one type of use, is the address.

Pointers are two purposes: to provide read or overwrite of the target, then the level two pointer is to provide a read or overwrite of the memory address.

The representation of a pointer is the address, the core is the pointer to the relationship, and the function of the operator "*" is to access the object pointed to by the relationship. If there is a pointing relationship to B, A is the address of B, and "*a" means indirectly accessing B through this pointing relationship. If the value of B is also a pointer to C, then B is the address of C, and "*b" represents an indirect access to C, if C is a variable of type integer, real, or struct, or the type of data that holds these types.array Elements, then B (that is, the address of C) is a normal pointer, called First -level pointers,The variable used to hold the first-level pointer is called first-level pointer variable 。 A (that is, the address of B) is a pointer to a pointer, called Level Two pointers, a variable for holding a level two pointer is called Two-level pointer variable． If a, B, C are variables, that is, C is a normal variable, B is a levelpointer Variable, where the address of c is stored, A is a two-level pointer variable, which holds the address of B, then these 3 variables occupy their own storage unit in memory respectively. The above content from Baidu Encyclopedia。The first time you encounter a "level two pointer" in the Learning Data Structure List section, the code is as follows:

typedef struct NODE
{
DataType data;
struct Node *next;
}listnode,*linklist;

void Initlist (linklist *head)//Head is a level two pointer

{
if ((*head) = (linklist) malloc (sizeof (ListNode))) ==null)
exit ( -1);
(*head)->next=null; 
}
The book explains that head is a pointer to the head node of a single-linked list that receives the address of the head-pointer variable in the main program to initialize a single-linked list, and *head the head-pointer variable that corresponds to the single-linked list in the main program. 
read a few times do not understand, Baidu a bit:

There is no reference in the C language, so you must use a level two pointer when trying to change the value of a pointer.

Reference types can be used in C + + to implement.

Demonstrate an example of the error if the above initialization code is written like this:

void Initlist (linklist head)

{

  Exit (-1);
(head)->next=null;    
}
When this code is executed, the header pointer in the main program is not processed because the parameter is only a copy of the argument and the operation on the parameter does not affect the argument.
Let's look at a simple example when formally explaining the Initlist initialization code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void Memorylocate (char **ptr)
{
*ptr= (char *) malloc (10*sizeof (char));
}
int main (int argc,char **argv)
{
Char *buffer;
memorylocate (&buffer);
strcpy (buffer, "12345");
printf ("Buffer%s\n", buffer);
return 0;
}

Operation Result:buffer 12345

If you change to

void Memorylocate (char *ptr)
{
ptr= (char*) malloc (10*sizeof (char));
}
int main (int argc,char **argv)
{
Char *buffer;
memorylocate (buffer);
strcpy (buffer, "12345");
printf ("Buffer%s\n", buffer);
return 0;
}

strcpy can't execute ... Description buffer not allocated to storage space ...

In this way, the initial explanation of how to explain why using the two-level pointer will be enlightened.

Knowledge summary of "level two pointers" for the initialization of chained storage for linked lists