Storage Classes in C

Storage Classes in C – Complete Guide with MCQs for CDAC C-CAT

A storage class in C determines where a variable is stored, its scope, lifetime, and linkage. Questions on storage classes are frequently asked in CDAC C-CAT and technical interviews.

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What is a Storage Class?

A storage class specifies:

• Scope (where the variable is accessible)
• Lifetime (how long the variable exists)
• Linkage (whether it can be accessed from another file)
• Storage location (memory/register)

C provides four storage classes:

• auto
• register
• static
• extern

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Summary Table

Storage Class	Scope	Lifetime	Linkage	Default Value
auto	Local	Function call	None	Garbage
register	Local	Function call	None	Garbage
static (local)	Local	Entire program	None	0
static (global)	File	Entire program	Internal	0
extern	Global	Entire program	External	0

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auto Storage Class

auto is the default storage class for local variables.

void fun()
{
    auto int x = 10;
    printf("%d", x);
}

The auto keyword is rarely written because local variables are automatically auto.

Properties:

• Local scope
• Created when the function starts
• Destroyed when the function returns
• Default value is garbage if not initialized

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register Storage Class

A register variable requests the compiler to store the variable in a CPU register for faster access.

register int i;

for(i = 0; i < 10; i++)
{
    printf("%d ", i);
}

Properties:

• Local scope
• Function lifetime
• Compiler may ignore the request
• Address cannot be taken using &

Example:

register int x = 10;

printf("%p", &x);

Output

Compilation Error

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static Local Variable

A static local variable is initialized only once and retains its value between function calls.

#include <stdio.h>

void fun()
{
    static int x = 0;
    x++;
    printf("%d ", x);
}

int main()
{
    fun();
    fun();
    fun();
}

Output

1 2 3

Unlike ordinary local variables, x is not recreated on every function call.

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Local Variable Without static

void fun()
{
    int x = 0;
    x++;
    printf("%d ", x);
}

Calling fun() three times prints

1 1 1

because a new variable is created every time.

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static Global Variable

static int count = 100;

A static global variable:

• Exists throughout the program
• Can only be accessed within the same source file
• Has internal linkage

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extern Storage Class

extern tells the compiler that a variable is defined in another file.

File 1

int count = 10;

File 2

extern int count;

printf("%d", count);

Output

10

extern does not allocate memory. It only declares the variable.

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Scope, Lifetime and Linkage

Keyword	Scope	Lifetime	Linkage
auto	Local	Function	None
register	Local	Function	None
static (local)	Local	Entire Program	None
static (global)	File	Entire Program	Internal
extern	Global	Entire Program	External

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Common Mistakes

Mistake 1

Thinking static makes a variable global.

Wrong.

A local static variable still has local scope.

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Mistake 2

Thinking extern creates memory.

Wrong.

It only declares an already existing variable.

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Mistake 3

Taking the address of a register variable.

register int x;

printf("%p", &x);

Compilation Error.

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Interview Questions

Why use static variables?

To preserve values between function calls.

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Can a static local variable be accessed outside the function?

No.

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Does extern allocate memory?

No.

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What is the default value of a static variable?

0

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Can register variables be stored in RAM?

Yes.

The compiler may ignore the register keyword.

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CDAC C-CAT MCQs

MCQ 1

void fun()
{
    static int x;
    x++;
    printf("%d ", x);
}

int main()
{
    fun();
    fun();
}

A. 1 1

B. 1 2

C. 2 2

D. Garbage

Answer: B

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MCQ 2

void fun()
{
    int x = 0;
    x++;
    printf("%d ", x);
}

int main()
{
    fun();
    fun();
}

A. 1 2

B. 2 2

C. 1 1

D. Garbage

Answer: C

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MCQ 3

register int x = 10;

printf("%p", &x);

A. Address

B. Garbage

C. Compilation Error

D. Undefined

Answer: C

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MCQ 4

The default storage class of a local variable is

A. static

B. register

C. auto

D. extern

Answer: C

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MCQ 5

Which storage class retains its value between function calls?

A. auto

B. register

C. static

D. extern

Answer: C

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MCQ 6

Which storage class provides external linkage?

A. auto

B. static

C. register

D. extern

Answer: D

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MCQ 7

Which storage class has internal linkage?

A. extern

B. auto

C. static (global)

D. register

Answer: C

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MCQ 8

Which storage class does not allocate memory?

A. auto

B. static

C. extern

D. register

Answer: C

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MCQ 9

The default value of a static variable is

A. Garbage

B. NULL

C. 0

D. 1

Answer: C

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MCQ 10

Which statement is true?

A. register guarantees storage in CPU registers.

B. extern creates a variable.

C. Local static variables retain their values between function calls.

D. auto variables are initialized to zero.

Answer: C

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Key Takeaways

• Storage classes determine scope, lifetime, linkage, and storage location.
• auto is the default storage class for local variables.
• register requests faster access but is only a compiler hint.
• Local static variables preserve their values between function calls.
• Global static variables have internal linkage and are visible only within the same source file.
• extern declares variables defined in another source file and provides external linkage.
• Storage classes are a common source of conceptual and MCQ-based questions in CDAC C-CAT.