volatile keyword in c

1Using volatile  Keyword is yet another way (like synchronized,
2atomic wrapper) of making class thread safe. Thread safe
3means that a method or class instance can be used by 
4multiple threads at the same time without any problem.
5
6

1C's volatile keyword is a qualifier that is applied to a variable when it is declared. It tells the compiler that the value of the variable may change at any time--without any action being taken by the code the compiler finds nearby.

1//volatile keyword usage in C
2#include<stdio.h>
3
4int main(){
5    //different methods of declaring and initializing volatile variables
6
7    //method 1 - volatile int
8    int volatile number1 = 10;
9
10    //method 2 - volatile int
11    volatile int number2;
12    number2 = 20;
13
14    //method 3 - volatile pointer
15    int volatile *p1;
16    p1 = &number1;
17
18    //method 4 - volatile double pointer
19    volatile int **p2;
20    p2 = &p1;
21    
22    printf("%d %d %d %d",number1,number2,*p1,**p2);
23    return 0;
24}