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 volatile can be changed from outside the compiled code (for example, a program may map a volatile variable to a memory mapped register.) The compiler won't apply certain optimizations to code that handles a volatile variable - for example, it won't load it into a register without writing it to memory. This is important when dealing with hardware registers.