C language Two-level pointers and sample code _c language

Pointers can point to an ordinary type of data, such as int, double, char, or to a pointer type of data, such as int *, double *, char *, and so on.

If a pointer is pointing to another pointer, we call it a level two pointer, or a pointer to a pointer.

Suppose a variable a,p1 of type int is a pointer variable to a, P2 is a pointer variable that points to a P1, and their relationship is shown in the following illustration:

Convert this relationship to C language code:

int a =100;
int *P1 = &a;
int **P2 = &p1;

A pointer variable is also a variable that consumes storage space and can use & to get its address. C language does not limit the series of pointers, each additional level of pointers, in the definition of pointer variables will have to add an asterisk *. P1 is a pointer to the normal type of data, defined with a *;P2 is a level two pointer, pointing to the first pointer P1, defined with two *.

If we want to define a three-level pointer P3 and point it to P2, you can write this:

int ***P3 = &p2;

The four-level pointer is similar:

int ****P4 = &p3;

In actual development, first-level pointers and level two pointers are frequently used, with almost no advanced pointers.

To get the data that the pointer points to, a level pointer plus a *, a level two pointer plus two *, a three-level pointer plus three *, and so on, look at the code:

#include <stdio.h>
int main () {
  int a =100;
  int *P1 = &a;
  int **p2 = &p1;
  int ***p3 = &p2;
  printf ("%d,%d,%d,%d\n", A, *p1, **P2, ***p3);
  printf ("&P2 =% #X, p3 =% #X \ n", &P2, p3);
  printf ("&P1 =% #X, p2 =% #X, *p3 =% #X \ n", &p1, p2, *p3);
  printf ("&a =% #X, p1 =% #X, *p2 =% #X, **p3 =% #X \ n", &a, p1, *p2, **p3);
  return 0;
}

Run Result:

100, 100, 100, 100.
&P2 = 0x28ff3c, p3 = 0x28ff3c
&P1 = 0x28ff40, p2 = 0x28ff40, *p3 = 0x28ff40
&a = 0x28ff44, p1 = 0x28ff44, *p2 = 0x28ff44, **p3 = 0x28ff44

Take the three-level pointer P3 as an example to analyze the above code. P3 is equivalent to * (* (*P3)). *p3 Gets the P2 value, which is the P1 address; * (*P3) Gets the value of the P1, which is the address of a, after three "value" operations, * (* (p3)) is the value of a.

Suppose the addresses of a, p1, p2, p3 are 0x00a0, 0X1000, 0X2000, 0x3000, and the relationship between them can be described in the following illustration:

Inside the box is the value of the variable itself, and the box below is the address of the variable.

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