Volatile qualifier in C

I am going to talk about volatile keyword in C which is asked in most of the technical interviews by companies.Although we encountered with this keyword often but most of us don't know why it is being used in declaring variables.So it can be a good question for the interview.

The volatile keyword is used to prevent compiler from applying optimizations on objects which can get changed that cannot be determined by the compiler.Objects declared as volatile are excluded from optimizations because they can changed by the code outside of the scope of the current code.The system always read the value of volatile object from memory location rather then the temporary register where as if the object is not volatile then compiler will do optimization in such a way that it will uses the value from temporary register to speed up the program.

Let's understand it by sample programs.In the below code we are compiling the code without optimization and we are declaring a variable as constant.We know that we cannot change the value of const variable in program.But that occurs when the compiler is compiling code with optimization.In the code below pointer is changing the value of constant variable as the code is without optimization.

/*compiling code without optimization */

int main()

{

const int x=100;

int *p;

p=&x;

*p=101;

printf("%d",x);

}

In the above program the output will be 101.

If the above program is compiled using optimization then the output will be 100 as pointer cannot modify the const variable.Now here the role of volatile keyword comes into play.When we declare a variable with keyword volatile then the value of constant variable can be changed by pointer although we are compiling our code using optimization.That is the variable which is declared volatile is excluded from compiler optimization.The code below illustrates the use of volatile keyword.

/* compiler with optimizations */

int main()

{

const volatile int x=100;

int *p;

p=&x;

*p=101;

}

  

The output of the above program is 101