You will see static and extern used commonly. There are two other storage class specifiers that occur less often. The auto specifier is almost never used because it tells the compiler that this is a local variable. auto is short for “automatic” and it refers to the way the compiler automatically allocates storage for the variable. The compiler can always determine this fact from the context in which the variable is defined, so auto is redundant.

A register variable is a local (auto) variable, along with a hint to the compiler that this particular variable will be heavily used so the compiler ought to keep it in a register if it can. Thus, it is an optimization aid. Various compilers respond differently to this hint; they have the option to ignore it. If you take the address of the variable, the register specifier will almost certainly be ignored. You should avoid using register because the compiler can usually do a better job of optimization than you.