kamal7725

 Profile Answers by kamal7725 Questions by kamal7725

• Nov 29th, 2007

 

A volatile variable is nothing but a local variable which gets destroyed as soon as its scope reachs to end.
we use the constant qualifier to make the value unchangable. This value could be volatile.

amritshukla

 Profile Answers by amritshukla 

• Sep 24th, 2010

 

There is no easy way to make a direct comparison between volatile and const, although I suppose in some sense they are exact opposites.

Modifying a variable definition with const means that the compiler will flag as an error any attempt to directly modify it later in the code. So, something like this:

void somefunc(void) { const int x = 5; x = 7; }

will generate an error on compilation since you're trying to modify a value you've told the compiler will remain constant.

The volatile keyword is especially important in interrupt-driven and multithreaded applications. It tells the compiler to always fetch the latest value of a variable from memory, and never assume that it won't change. It really has alot to do with compiler optimizations. For instance, if I have something like this:

int somefunc(void) { int y; y = 7; return (y * 2); }

An optimizing compiler could rightfully make some very easy assumptions and just return the value 14 all the time without any calculation, etc. On the other hand, if I declare y as "volatile" then the compiler will generate code to load the latest value from memory and do the calculation. This is because I'm telling the compiler "something else might change this value in between when I set it to 7 and when the function returns."