Variables and Storage Classes 

auto

• default variable type
• local to a function 

static

• retains the values between function calls
• initialized only once
• unavailable outside the scope of the function 

extern

• extern keyword tells the compiler that a variable (or function) exists, even if the compiler has not seen it in the file currently being compiled
• forces external linkage 

volatile

• volatile keyword prevents the compiler from performing any optimization based on the stability of that variable
• value of a volatile variable is always read whenever its value is required 

register

• variables are accessed faster
• often placed in register
• address of a register variable cannot be taken or computed i.e. the unary address-of operator (&) cannot be used with register arrays
• register keyword cannot be used with arrays