1. High Level View – Scope, Lifetime & Storage Class

Before going into each keyword, let’s clarify a few important terms:

C gives us four main storage class specifiers:

• auto – default for local (block) variables.
• static – keep value between function calls, or limit scope of globals.
• extern – reference a global variable defined in another file.
• register – request storage in CPU register (historical/optimization hint).

2. auto – The Default Local Variable Storage Class

The auto storage class is automatically applied to local variables declared inside a function (or a block) when you don’t specify any other storage class.

Example: normal local variable (implicitly auto)

#include <stdio.h>

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

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

You almost never write auto explicitly because it’s the default:

auto int x = 10;  // legal, but rarely used

Scope, lifetime and default value of auto variables

3. static (local) – Remember Value Between Function Calls

The static storage class does two different things depending on where it is used. First, we’ll see the local static variable case inside a function.

Example: static local variable vs normal local variable

#include <stdio.h>

void normalCounter() {
    int count = 0;       // auto / local
    count++;
    printf("normalCounter: %d\\n", count);
}

void staticCounter() {
    static int count = 0;  // static local
    count++;
    printf("staticCounter: %d\\n", count);
}

int main() {
    int i;

    for (i = 0; i < 3; i++) {
        normalCounter();
    }

    for (i = 0; i < 3; i++) {
        staticCounter();
    }

    return 0;
}

Typical output:

normalCounter: 1
normalCounter: 1
normalCounter: 1

staticCounter: 1
staticCounter: 2
staticCounter: 3

Explanation:

• normalCounter: count is auto; it is created fresh each time the function is called, so it always starts at 0 and becomes 1.
• staticCounter: count is static; it is created only once, retains its value between calls, and keeps incrementing.

Scope, lifetime and default value of static local variables

4. static (global) – Limit Visibility to This File Only

When static is used with a global variable or a function (outside all functions), it changes the linkage: the variable or function becomes file-local.

Global variable without static

int gCounter = 0;   // global, external linkage (visible from other files)

Global variable with static

static int gCounter = 0;  // global, internal linkage (visible only in this file)

In bigger projects with multiple C files, this is important for controlling what is visible outside the file, similar to putting some functions as “private” to that file.

5. extern – Using Global Variables Across Multiple Files

The extern keyword is used to declare a global variable that is defined in some other file.

Example with two files (conceptual)

File: globals.c

int totalDevices = 0;       // definition (storage allocated here)
float siteVoltage = 415.0f; // definition

File: main.c

#include <stdio.h>

// declare that these variables exist somewhere else
extern int totalDevices;
extern float siteVoltage;

int main() {
    printf("Devices: %d\\n", totalDevices);
    printf("Site voltage: %.1f V\\n", siteVoltage);
    return 0;
}

When the program is linked, main.c and globals.c are combined, and the extern declarations refer to the actual definitions in globals.c.

Common mistakes with extern

• Writing extern int x = 5; in multiple files – this becomes a definition; you must have only one definition.
• Forgetting to provide a real definition somewhere – leading to linker errors like “undefined reference to x”.

6. register – Requesting CPU Register Storage

The register storage class is a hint to the compiler that a variable will be used very often, so it may be beneficial to keep it in a CPU register for faster access.

void sum() {
    register int i;
    int total = 0;

    for (i = 0; i < 1000; i++) {
        total += i;
    }
}

Important points:

• Modern compilers are smart enough to decide register usage on their own, so register is often ignored.
• You cannot take the address of a variable declared as register (e.g., &i is not allowed) because it might not be in memory.
• register variables are like auto variables in scope and lifetime: local to the block, destroyed when the block exits.

7. Summary Table – auto vs static vs extern vs register

8. Real Use Cases for Storage Classes (Technician-Style Examples)

8.1 static local – counting function calls

Suppose you have a function that logs sensor faults. You want to know how many times it has been called without using a global variable.

void logFault(const char *msg) {
    static int faultCount = 0;   // remembers count between calls
    faultCount++;

    printf("FAULT %d: %s\\n", faultCount, msg);
}

Every time logFault is called, faultCount increases and keeps its value.

8.2 static global – file-local configuration

In a file handling device configuration, you may want to keep some global data private to that file:

/* device_config.c */

static int currentProfileId = 0;   // visible only inside this file

void setProfile(int id) {
    currentProfileId = id;
}

int getProfile() {
    return currentProfileId;
}

Here, other files can call setProfile and getProfile but cannot directly access currentProfileId. This is good encapsulation.

8.3 extern – sharing device count across files

device_data.c:

int totalDevices = 0;  // definition

main.c:

extern int totalDevices;  // declaration

int main() {
    totalDevices = 34;   // set total devices in the system
    // ...
    return 0;
}

Now both device_data.c and main.c can share the same global count.

9. Common Mistakes with Storage Classes

• Assuming auto variables start at 0: They don’t – they usually hold garbage values. Always initialize local variables.
• Overusing global variables: Too many global variables (with or without extern) makes code hard to maintain and debug.
• Using static when you really need dynamic: Static local variables keep values between calls. This can cause hidden state that’s hard to test.
• Relying on register for performance: Modern compilers are better than human guesses. Use register rarely, if at all.
• Multiple definitions with extern: Only one source file should define a global; other files should just declare it with extern.

10. Learning Checklist – Storage Classes in C

• I can explain the difference between scope and lifetime of a variable.
• I know that local variables without any storage class keyword are auto by default.
• I understand that static local variables retain their values between function calls.
• I can use static with global variables to restrict their visibility to a single file.
• I know how extern helps share global variables across multiple files.
• I understand that register is an optimization hint and may be ignored by the compiler.
• I can choose the correct storage class based on where and how long a variable should live.

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“Flat modern 16:9 illustration on a light background. In the center, a computer screen shows a small C code snippet with four highlighted keywords: `auto`, `static`, `extern`, and `register`. Around the screen, four labeled sections appear: one bubble for ‘auto’ showing a local variable inside a function block, one for ‘static’ showing a bar that persists across multiple function calls, one for ‘extern’ showing a global variable being shared between two C files, and one for ‘register’ showing a CPU chip icon with a small variable label inside it. Little timeline and scope icons (like a clock for lifetime and a bracket for scope) appear next to each bubble. A South Indian / Tamil technician character points at the screen with a pointer stick. At the top, bold title text: ‘Storage Classes in C’ and smaller subtitle: ‘auto, static, extern, register | Lifetime & Scope | Tamil Technicians’. Clean vector style, minimal colors, educational, high quality.”