C pointer constant and constant pointer

Two ways to declare constants in C language

const   INT   valueint   const   Value

If you want to declare a constant pointer, which is a pointer to a constant, you can refer to the constant declaration above to modify

const   INT   *ptrint   const   *PTR

Consider *ptr as a whole, then ptr in *ptr is a pointer to a constant. As the name implies, a pointer to a constant, then it is not possible to modify this value by this pointer.

#include <stdio.h>int main () {int val = 123;int Const *PTR = &val;*ptr = 111;return 0;} Error: Error:read-only variable is not assignable        *ptr = 111;        ~ ~ ~ ^1 Error generated.

However, you can still modify the value of this amount in other ways. For example

#include <stdio.h>int main () {int val = 123;int Const *PTR = &val;val = 222;printf ("%d\n", *ptr); return 0;} Output: 222

The meaning of a constant pointer is that you cannot modify the value it points to by using this pointer.

Pointer constants

It can be understood that a constant is defined first, and that the constant is a pointer type.

int val = 123;int * Const PTR = & val;

PTR is just a constant, and its value is a fixed memory address.

#include <stdio.h>int main () {char *message = "Hello, world\n";//Here PTR is the memory address of "hello,world\n" char * const PTR = Messag e;printf ("%s\n", ptr);//Even if you modify the value of the message and do not modify the PTR value, PTR still points to "Hello, world\n" message = "Hello, maomao\n";p rintf ("%s\n", PTR); return 0;} Output Hello, Worldhello, world

Constant pointer to constant

Char Const  *  const PTR;

The pointer is a constant, and you cannot modify the value pointed to by the pointer.

C pointer constant and constant pointer