Storage Classes in C: Auto, Extern, Static, Register

What are Storage Classes?

A storage class defines the scope, lifetime, visibility, and default value of a variable.

auto

Default storage class for local variables; created when the block starts and destroyed when it ends.

extern

Declares a global variable that is defined in another file or later in the same file.

static

Retains its value between function calls and is initialized only once.

void counter() {
    static int count = 0;
    count++;
    printf("%d", count);
}

register

Suggests to the compiler that the variable be stored in a CPU register for faster access (modern compilers largely optimize this automatically).