Math constant PI value in C

Calculating PI value is one of the complex problem and wikipedia talks about the approximations done for it and says it's difficult to calculate PI accurately.

How does C calculate PI? Does it calculate it every time or is it using a less accurate fixed value?

What does M_PI mean in C?

M_PI. Pi, the ratio of a circle's circumference to its diameter. M_PI_2. Pi divided by two. M_PI_4.

Is pi a constant variable?

Pi is an irrational number. Furthermore, it is a transcendental number. Pi is defined as the ratio of the circumference of any circle to its diameter. As all circles are similar and therefore proportional in dimensions, pi is therefore always the same for all circles and is a constant.

In C Pi is defined in math.h: #define M_PI 3.14159265358979323846