C Storage Classes auto, register, static, extern

What are Storage Classes?

A storage class defines the scope (visibility), lifetime (how long the variable exists in memory), and default initial value of a variable. C has four storage classes:

Storage Class	Keyword	Scope	Lifetime	Default Value
Automatic	auto	Local (block)	Until block ends	Garbage
Register	register	Local (block)	Until block ends	Garbage
Static	static	Local or file	Entire program	0
External	extern	Global (all files)	Entire program	0

1. auto - Automatic Storage

auto is the default storage class for all local variables. You almost never write it explicitly - every local variable is auto by default. The variable is created when the block is entered and destroyed when the block exits.

auto Storage Class

#include <stdio.h>

void demo() {
    auto int x = 10;  // same as: int x = 10;
    printf("x = %d\n", x);
    // x is destroyed when demo() returns
}

int main() {
    demo();  // x = 10
    demo();  // x = 10 (fresh copy each call)
    return 0;
}

2. register - Register Storage

register is a hint to the compiler to store the variable in a CPU register instead of RAM for faster access. Modern compilers largely ignore this hint and optimize on their own. Key restriction: you cannot take the address of a register variable (& is not allowed).

register Storage Class

#include <stdio.h>

int main() {
    register int i;  // hint: store i in CPU register
    int sum = 0;

    for (i = 1; i <= 100; i++) {
        sum += i;
    }
    printf("Sum 1..100 = %d\n", sum);  // 5050

    // ERROR: cannot take address of register variable
    // printf("%p", &i);  // compile error!

    return 0;
}

3. static - Static Storage

static has two distinct uses:

Static local variable - retains its value between function calls. Initialized only once.
Static global variable / function - restricts visibility to the current file only (file-scope linkage).

static Local Variable - Retains Value

#include <stdio.h>

void counter() {
    static int count = 0;  // initialized ONCE, persists across calls
    count++;
    printf("Call count: %d\n", count);
}

int main() {
    counter();  // Call count: 1
    counter();  // Call count: 2
    counter();  // Call count: 3
    return 0;
}

/*
Without static: count resets to 0 every call -> always prints 1
With static:    count persists -> prints 1, 2, 3
*/

3. static - Static Storage - C Example

// file: utils.c
#include <stdio.h>

// static function - only visible within utils.c
static void helper() {
    printf("Internal helper\n");
}

// static global - only visible within utils.c
static int filePrivate = 42;

void publicFunction() {
    helper();
    printf("filePrivate = %d\n", filePrivate);
}

// file: main.c
// extern void helper();  // ERROR - helper is static, not accessible
extern void publicFunction();  // OK - publicFunction is not static

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

4. extern - External Storage

extern declares a variable or function that is defined in another file. It tells the compiler "this exists somewhere - the linker will find it." Use it to share global variables across multiple source files.

extern - Sharing Variables Across Files

// globals.c - defines the global variable
int appVersion = 3;  // definition (allocates memory)

void printVersion() {
    printf("App version: %d\n", appVersion);
}

4. extern - External Storage - C Example

// main.c - uses the global variable from globals.c
#include <stdio.h>

extern int appVersion;       // declaration (no memory allocated)
extern void printVersion();  // declaration

int main() {
    printVersion();          // App version: 3
    appVersion = 4;          // modify the shared variable
    printVersion();          // App version: 4
    return 0;
}

// Compile: gcc globals.c main.c -o app

Quick Comparison

Feature	auto	register	static	extern
Memory location	Stack	CPU register (hint)	Data segment	Data segment
Scope	Block	Block	Block or file	Global (all files)
Lifetime	Block	Block	Program	Program
Default value	Garbage	Garbage	0	0
Can take address?	Yes	No	Yes	Yes

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