Why is volatile needed in C?

Question

Why is volatile needed in C? What is it used for? What will it do?

Answer 1

A marginal use for volatile is the following. Say you want to compute the numerical derivative of a function f :

double der_f(double x)
{
    static const double h = 1e-3;
    return (f(x + h) - f(x)) / h;
}

The problem is that x+h-x is generally not equal to h due to roundoff errors. Think about it : when you substract very close numbers, you lose a lot of significant digits which can ruin the computation of the derivative (think 1.00001 - 1). A possible workaround could be

double der_f2(double x)
{
    static const double h = 1e-3;
    double hh = x + h - x;
    return (f(x + hh) - f(x)) / hh;
}

but depending on your platform and compiler switches, the second line of that function may be wiped out by a aggressively optimizing compiler. So you write instead

    volatile double hh = x + h;
    hh -= x;

to force the compiler to read the memory location containing hh, forfeiting an eventual optimization opportunity.