Storage Classes in C

Storage classes define the scope (visibility) and lifetime of variables and/or functions within a C program.

12.1 The Four Storage Classes

Storage Class	Keyword	Storage	Initial Value	Scope	Lifetime
Automatic	auto	RAM	Garbage	Local	Within block
Register	register	Register	Garbage	Local	Within block
Static	static	RAM	Zero	Local	Till end of program
External	extern	RAM	Zero	Global	Till end of program

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12.2 auto

The default storage class for all local variables.

auto int age; // Same as 'int age;'

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12.3 register

Directs the compiler to store the variable in a CPU register instead of RAM for faster access. Used for highly-accessed variables like loop counters.

• Note: You cannot get the address of a register variable using &.

register int counter;

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12.4 static

A static local variable retains its value even after the function exits. It is initialized only once.

void count() {
    static int x = 0;
    x++;
    printf("%d ", x);
}
// Calling count() three times prints: 1 2 3

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12.5 extern

Used to give a reference to a global variable that is visible to all program files. When you use extern, the variable is not initialized because the name points to a location previously defined.

// File1.c
int count = 5;

// File2.c
extern int count;
void printCount() { printf("%d", count); }

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Practical Example: Static inside a loop

#include <stdio.h>

void func() {
    static int i = 1; // initialized only once
    int j = 1;        // initialized every time
    printf("static=%d, auto=%d\n", i, j);
    i++;
    j++;
}

int main() {
    func();
    func();
    func();
    return 0;
}
/**
Output:
static=1, auto=1
static=2, auto=1
static=3, auto=1
**/

Check the Lecture Slides for detailed performance benchmarks of register vs RAM storage.