Storage Classes in C interview questions

Storage classes in C define the scope, visibility, lifetime, and default initial value of variables. There are four storage classes in C:

1. auto
2. extern
3. static
4. register

1. auto Storage Class

• Definition: auto is the default storage class for local variables.
• Scope: Local to the block where it is defined.
• Lifetime: Exists until the block/function ends.
• Visibility: Not accessible outside the block.
• Default Value: Garbage (undefined).

Example of Storage classes :

#include <stdio.h>

void testFunction() {
    auto int x = 10;  // 'auto' is optional, same as int x = 10;
    printf("Value of x: %d\n", x);
}

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

Key Points of Storage classes :

• Automatically allocated when the function/block is called.
• Destroyed when the function/block exits.
• Cannot be accessed outside the function/block.

Interview Questions based on Storage classes:

1. What is the default storage class of a local variable in C?
2. Can an auto variable be accessed outside its function?
3. How does an auto variable differ from a static variable?

2. extern Storage Class

• Definition: extern is used to declare a global variable in one file and define it in another.
• Scope: Global (visible throughout the program).
• Lifetime: Exists as long as the program runs.
• Visibility: Accessible in all files (if declared with extern).
• Default Value: Zero (0).

Example (Multiple Files Usage) for Storage classes :

File: main.c

#include <stdio.h>

// Declaration of external variable (defined in another file)
extern int count;

void display() {
    printf("Count: %d\n", count);
}

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

File: data.c

// Definition of external variable
int count = 10;

Compile & Run

gcc main.c data.c -o output
./output

Output: Count: 10

Key Points of Storage classes :

• Used for global variables and function definitions across multiple files.
• Declaration using extern does not allocate memory, only references it.
• Definition (without extern) allocates memory.

Interview Questions of Storage classes:

1. What is the difference between declaration and definition of a variable?
2. Can extern be used for functions? (Yes, by default functions are extern)
3. What happens if an extern variable is not defined anywhere?

3. static Storage Class

• Definition: static variables retain their values across function calls.
• Scope:
  • Local Static Variable: Limited to the function/block where it is defined.
  • Global Static Variable: Limited to the file where it is defined.
• Lifetime: Exists throughout the program execution.
• Visibility:
  • Local Static: Visible only in the function.
  • Global Static: Not accessible outside the file.
• Default Value: Zero (0).

Example 1: Local Static Variable

#include <stdio.h>

void counter() {
    static int count = 0;  // Retains value between function calls
    count++;
    printf("Count: %d\n", count);
}

int main() {
    counter(); // Output: Count: 1
    counter(); // Output: Count: 2
    counter(); // Output: Count: 3
    return 0;
}

Example 2: Global Static Variable

File: static_var.c

#include <stdio.h>

static int globalCount = 10;  // This variable is NOT accessible in other files

void display() {
    printf("Global Count: %d\n", globalCount);
}

File: main.c

#include <stdio.h>

// extern int globalCount;  // This will cause a linker error!

int main() {
    // printf("Global Count: %d\n", globalCount);  // Not accessible
    return 0;
}

Key Observation: globalCount is limited to static_var.c and cannot be accessed in main.c.

Key Points of Storage classes:

• Local Static Variables retain values between function calls.
• Global Static Variables restrict visibility to the file.
• Useful in scenarios like counters, cache implementations, and module-specific global variables.

Interview Questions of Storage classes:

1. What is the difference between static and auto variables?
2. Can we use a static variable inside a function?
3. What happens if we declare a global variable as static?

4. register Storage Class

• Definition: Suggests that the variable be stored in CPU registers instead of RAM.
• Scope: Local to the block where it is defined.
• Lifetime: Exists until the function/block ends.
• Visibility: Not accessible outside the function/block.
• Default Value: Garbage (undefined).
• Key Restriction: Cannot use & (address-of) operator on register variables.

Example:

#include <stdio.h>

void testFunction() {
    register int x = 5;  // Hints compiler to store in CPU register
    printf("Value of x: %d\n", x);
}

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

Example: Error Case (& Operator)

#include <stdio.h>

int main() {
    register int x = 10;
    printf("%p\n", &x);  // ❌ Error: Cannot get address of register variable
    return 0;
}

Error: cannot take the address of register variable x.

Key Points of Storage classes:

• register is just a hint; compiler may ignore it.
• Useful for performance-critical variables like loop counters.
• No guarantee that the variable will be stored in registers.

Interview Questions of Storage classes:

1. What is the purpose of the register storage class?
2. Can we declare a pointer to a register variable? (No)
3. What happens if the compiler cannot store a register variable in a CPU register?

Comparison Table of Storage classes :

Storage Class	Scope	Lifetime	Default Value	Special Features
auto	Local	Function/block	Garbage	Default storage class for local variables
extern	Global	Whole program	Zero (0)	Used for global variables across multiple files
static	Local/Global	Whole program	Zero (0)	Retains value between function calls, file scope for global static variables
register	Local	Function/block	Garbage	Suggests CPU register usage, address (&) not allowed

FAQ (Frequently Asked Questions) on Storage Classes in C and C++:

1. What are storage classes in C/C++?

Storage classes define the scope, lifetime, and visibility of variables and functions in a program. They determine where and how a variable is stored in memory.

2. What are the different types of storage classes in C/C++?

There are four main storage classes:

1. auto – Default for local variables (stored in stack).
2. register – Hints to store the variable in a CPU register for faster access.
3. static – Retains the value of a variable between function calls.
4. extern – Refers to a global variable defined in another file.

3. What is the default storage class for variables in C?

By default, local variables have the auto storage class.

4. What is the difference between auto and register storage classes?

Feature	auto	register
Storage	Stack	CPU register (if available)
Scope	Local	Local
Lifetime	Function call	Function call
Access via &	Yes	No (register variables don’t have a memory address)
Speed	Normal	Faster (if stored in a register)

5. Can we take the address of a register variable?

No. The register storage class suggests the variable be stored in a CPU register, which doesn’t have a memory address.

register int x = 10;
printf("%p", &x); // ❌ Error: Cannot take address of register variable

6. What is the purpose of static storage class in C/C++?

The static storage class is used to:

1. Retain a variable’s value across function calls (inside a function).
2. Restrict the scope of a global variable to the same file (internal linkage).

✅ Example (Inside a function):

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

int main() {
    counter(); // Output: 1
    counter(); // Output: 2
    return 0;
}

✅ Example (File Scope – Internal Linkage):

static int var = 10; // Only accessible within this file

7. What is the difference between static and extern variables?

Feature	static	extern
Scope	Limited to the same file	Accessible across multiple files
Lifetime	Throughout the program	Throughout the program
Use Case	Keeps variable private to the file	Shares variable across files

8. Can we use static inside a class in C++?

Yes. In C++, static can be used inside a class to create class variables (shared across all objects).

✅ Example (C++ static member variable):

class Test {
    static int count; // Shared by all objects
public:
    void increment() { count++; }
    void show() { std::cout << count << std::endl; }
};
int Test::count = 0; // Must be defined outside the class

9. What happens if we declare a variable as extern but don’t define it?

If a variable is declared with extern but not defined anywhere, the compiler will throw a linking error.

✅ Example (Correct Usage across files):
File 1 (file1.c):

int num = 10; // Global definition

File 2 (file2.c):

extern int num; // Declaration
printf("%d", num); // ✅ Works fine

❌ Incorrect (Missing Definition):

extern int num;
printf("%d", num); // ❌ Linker Error: Undefined reference to 'num'

10. Can a function be declared as static?

Yes. A static function has file scope, meaning it can only be accessed within the file where it is declared.

✅ Example:

static void helper() {
    printf("This function is only accessible in this file.");
}

11. What is the difference between static and global variables?

Feature	static (Global Scope)	Global Variable
Scope	Limited to the file	Accessible in all files
Lifetime	Program lifetime	Program lifetime
Use Case	Hides from other files	Accessible everywhere

✅ Example:

static int a = 10; // Accessible only in this file
int b = 20;        // Accessible in all files if declared with `extern`

12. Can a global variable be static and extern at the same time?

No. static restricts visibility to the file, while extern is used to access a variable globally across multiple files. They conflict with each other.

13. Where are storage class variables stored in memory?

Storage Class	Stored In
auto	Stack
register	CPU Register (if available)
static	Data Segment (BSS or initialized)
extern	Data Segment

14. What is the difference between static and const in C/C++?

Feature	static	const
Scope	Can be global or local	Local by default
Lifetime	Throughout the program	Depends on where it is defined
Modifiable?	Yes	No (Read-only)

✅ Example (Difference):

static int x = 10;  // Accessible only in this file
const int y = 20;   // Cannot be modified

15. Can static and volatile be used together?

Yes. static controls lifetime, while volatile tells the compiler not to optimize the variable.

✅ Example (Using static volatile):

static volatile int flag = 1;

🔹 Useful in embedded systems where the variable is modified by hardware (e.g., interrupts).

16. Can we use extern with functions?

Yes. By default, functions in C have external linkage, so extern is redundant but still valid.

✅ Example:

extern void func(); // Declaration
void func() {
    printf("Hello World");
}

17. How does extern "C" work in C++?

C++ uses name mangling, so extern "C" ensures the function is linked using C-style linkage (useful when calling C functions from C++).

✅ Example (C++ Code Calling C Function):

extern "C" void hello(); // Declaring C function

18. Which storage class should be used for embedded programming?

• register → Fast access variables.
• static → Retain values (e.g., sensor data).
• volatile → Prevent compiler optimizations (e.g., ISR flags).
• extern → Share global variables across files.

19. Why is static used in embedded systems?

• Reduces RAM usage by keeping variables in non-volatile memory (data segment).
• Avoids unnecessary stack allocation.
• Improves performance by retaining values across function calls.

20. What are the best practices for using storage classes?

✔️ Use static for file-private variables.
✔️ Use register for frequently accessed variables (if needed).
✔️ Use extern for global variables shared across files.
✔️ Avoid too many global variables (use static instead).

Here are 20 Multiple Choice Questions (MCQs) on Storage Classes in C and C++:

1. What is the default storage class for local variables in C?

a) auto
b) static
c) register
d) extern

✅ Answer: a) auto

2. Which storage class is used to retain a variable’s value across function calls?

a) auto
b) static
c) register
d) extern

✅ Answer: b) static

3. Where are register variables stored?

a) Heap
b) Stack
c) CPU Register
d) Data Segment

✅ Answer: c) CPU Register

4. Which storage class makes a global variable accessible across multiple files?

a) static
b) extern
c) register
d) auto

✅ Answer: b) extern

5. What happens if you take the address of a register variable?

a) It returns the memory address
b) It results in a compilation error
c) It returns NULL
d) It gives unpredictable behavior

✅ Answer: b) It results in a compilation error

6. What is the lifetime of a static variable declared inside a function?

a) Until the function exits
b) Throughout the program execution
c) Until the next function call
d) Depends on compiler optimization

✅ Answer: b) Throughout the program execution

7. Which storage class ensures a function is not accessible from other files?

a) extern
b) register
c) static
d) auto

✅ Answer: c) static

8. Where are static variables stored in memory?

a) Stack
b) Heap
c) Data Segment
d) CPU Register

✅ Answer: c) Data Segment

9. What is the scope of an auto variable?

a) Function-level (local scope)
b) Global scope
c) File-level
d) System-wide

✅ Answer: a) Function-level (local scope)

10. What is the primary purpose of the extern keyword?

a) To make variables private to a file
b) To declare a variable defined in another file
c) To store a variable in a CPU register
d) To keep a variable’s value persistent

✅ Answer: b) To declare a variable defined in another file

11. Which keyword is used to restrict a global variable’s access to the same file?

a) extern
b) auto
c) static
d) volatile

✅ Answer: c) static

12. Which of the following is true about register variables?

a) They are guaranteed to be stored in CPU registers
b) They can be accessed from other files
c) They have a local scope
d) They can be modified by external programs

✅ Answer: c) They have a local scope

13. How many times is memory allocated for a static variable in a function?

a) Every time the function is called
b) Only once
c) Never
d) Once per function call

✅ Answer: b) Only once

14. Can a static variable be initialized inside a function?

a) No, it must be global
b) Yes, but it retains its value across calls
c) No, only extern variables can
d) Yes, but it is destroyed after function exits

✅ Answer: b) Yes, but it retains its value across calls

15. What will happen if a global variable is declared as static?

a) It can be accessed from other files
b) It will be stored in CPU registers
c) It will be limited to the current file
d) It will cause an error

✅ Answer: c) It will be limited to the current file

16. Which storage class should be used for sharing global variables across multiple files?

a) auto
b) static
c) register
d) extern

✅ Answer: d) extern

17. What is the difference between static and extern storage classes?

a) static restricts variable scope, extern extends it
b) static is for global variables only
c) extern variables cannot be modified
d) static variables cannot be initialized

✅ Answer: a) static restricts variable scope, extern extends it

18. In C++, what does static do when used inside a class?

a) Creates a variable that belongs to a single instance
b) Makes the variable global
c) Creates a class variable shared by all objects
d) Prevents inheritance

✅ Answer: c) Creates a class variable shared by all objects

19. Where are extern variables stored?

a) Heap
b) Stack
c) Data Segment
d) Register

✅ Answer: c) Data Segment

20. Which storage class should be used for variables that must be frequently accessed?

a) static
b) register
c) extern
d) auto

✅ Answer: b) register

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