What are Storage Classes?

Storage classes in C define a variable's scope, lifetime, default value and storage location.

auto Storage Class

auto is the default storage class for local variables.

#include <stdio.h>

int main() {
    auto int x = 10;   // same as int x = 10;
    printf("x = %d", x);
    return 0;
}

register Storage Class

register requests the compiler to store a variable in CPU register for faster access. Modern compilers decide optimization themselves, so this keyword is rarely needed.

#include <stdio.h>

int main() {
    register int i;
    for(i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    return 0;
}

static Storage Class

A static local variable keeps its value between function calls.

#include <stdio.h>

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

int main() {
    counter();
    counter();
    counter();
    return 0;
}

extern Storage Class

extern tells the compiler that a global variable is declared somewhere else, possibly in another file.

// file1.c
int total = 100;

// file2.c
#include <stdio.h>
extern int total;

int main() {
    printf("Total = %d", total);
    return 0;
}

Summary

• auto is default for local variables
• register is for fast access request
• static preserves value for whole program lifetime
• extern refers to a global variable defined elsewhere
• Storage class controls scope and lifetime