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📘 Unions in C Programming – Memory Explanation, Fully Commented Programs, Viva Questions & MCQs (Beginner to Advanced Guide)

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🧑‍💻 Introduction

In C programming, we use structures to store different data types together.

But sometimes:

• We need to store different data types

• Only one value is required at a time

• Memory optimization is important

👉 That’s where Union comes into the picture.

A Union is a user-defined data type in C where:

🧠 All members share the same memory location.

This makes union memory-efficient compared to structures.

In this complete beginner to advanced guide, you will learn union memory concepts, fully commented programs, viva questions, MCQs, and real-world applications.

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📌 What is a Union in C?

A Union is a special data type that allows storing different types of data in the same memory location, but only one member can hold a value at a time.

Unlike structure:

Structure → separate memory
Union → shared memory

Internally, the compiler allocates memory equal to the largest member and all other members reuse that same memory space starting from the same base address.

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🔹 Syntax of Union

union UnionName        // Declare union
{
    data_type member1; // First member
    data_type member2; // Second member
    data_type member3; // Third member
};

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🧠 Memory Concept of Union (Very Important)

🧠 How Union Saves Memory (Visual Explanation)

Before understanding unions deeply, let’s visually compare how memory is allocated in structure and union. This diagram clearly shows why unions are memory-efficient and how members share the same memory block.

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🔍 What This Diagram Shows

1️⃣ In a structure, each member gets separate memory.
👉 Total size = sum of all members + padding.

2️⃣ In a union, all members share the same memory location.
👉 Total size = size of the largest member.

3️⃣ When a new value is assigned in a union,
⚠ The previous value is overwritten.

🚨 In a union, only one member holds a valid value at a time.

💡 Note: The structure size may include padding for memory alignment, while union size depends only on its largest member.

In union, all members start at the same base address.

🔸 Structure

• Each member has separate memory.

• Total size = Sum of all members (plus padding).

🔸 Union

• All members share same memory block.

• Total size = Size of largest member.

📌 When one member is updated, the previous value is overwritten.

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💻 Program 1: Basic Union Example (Fully Commented)

#include <stdio.h>                 // Header file for input-output functions

union Data                         // Define a union named Data
{
    int i;                         // Integer member
    float f;                       // Float member
    char str[20];                  // Character array member
};

int main()                         // Main function starts
{
    union Data data;               // Declare union variable

    data.i = 10;                   // Assign integer value
    printf("Integer: %d\n", data.i); // Print integer value

    data.f = 3.14;                 // Assign float value (overwrites integer)
    printf("Float: %.2f\n", data.f); // Print float value

    return 0;                      // End program
}

📌 Observation:
Assigning data.f overwrites data.i because memory is shared.

✅ Expected Output

Integer: 10
Float: 3.14

📌 Explanation:

After assigning data.f = 3.14, the previous integer value is overwritten because both members share the same memory.

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📏 Program 2: Checking Size of Union (Fully Commented)

#include <stdio.h>                          // Standard I/O header

union Example                               // Define union
{
    int a;                                  // 4 bytes
    float b;                                // 4 bytes
    char c;                                 // 1 byte
};

int main()                                  // Main function
{
    printf("Size of union: %lu", sizeof(union Example)); // Print union size
    return 0;                               // End program
}

✅ Expected Output (Usually)

Size of union: 4

📌 Explanation:

Largest member = int (4 bytes) or float (4 bytes)
So union size = 4 bytes.

(Note: It may vary slightly depending on compiler/system.)

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🔍 Structure vs Union – Complete Comparison

Feature	Structure	Union
Memory Allocation	Separate memory for each member	Shared memory for all members
Total Size	Sum of sizes of all members	Size of largest member only
Data Storage	All values stored at once	Only one value stored at a time
Memory Efficiency	Less efficient	More efficient
Risk of Overwrite	No	Yes
Keyword Used	struct	union
Access to Members	All members accessible anytime	Only last assigned member reliable
Initialization	Multiple members can be initialized	Only one member can be initialized
Use Case	When all data needed together	When only one data needed at a time
Programming Safety	Safer	Less safe if misused

💪 Practice Programs on Union (New Section)

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💻 Practice Program 1

Demonstrating How Union Members Overwrite Each Other

This program shows how assigning a new value to one union member overwrites the previous value because all members share the same memory location.

#include <stdio.h>                  // Input-output header

union Info                          // Define union
{
    int age;                        // Integer member
    float salary;                   // Float member
    char grade;                     // Character member
};

int main()                          // Main function
{
    union Info person;              // Declare union variable

    person.age = 25;                // Assign age
    printf("Age: %d\n", person.age); // Print age

    person.salary = 50000.50;       // Assign salary (overwrites age)
    printf("Salary: %.2f\n", person.salary); // Print salary

    person.grade = 'A';             // Assign grade (overwrites salary)
    printf("Grade: %c\n", person.grade); // Print grade

    return 0;                       // End program
}

✅ Expected Output
Age: 25
Salary: 50000.50
Grade: A
📌 Explanation:
Each new assignment overwrites the previous value because union members share memory.

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💻 Practice Program 2

Finding the Size of a Union

This program demonstrates that the size of a union is equal to the size of its largest data member.

#include <stdio.h>                      // Header file

union Test                              // Define union
{
    int a;                              // Integer member
    double b;                           // Double member (largest)
    char c;                             // Character member
};

int main()                              // Main function
{
    printf("Size of union: %lu bytes\n", sizeof(union Test)); // Print size
    return 0;                           // End
}

✅ Expected Output (Usually)
Size of union: 8 bytes
📌 Explanation:
Largest member = double (usually 8 bytes)

So union size = 8 bytes.

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💻 Practice Program 3 (Union Inside Structure)

Using a Union Inside a Structure

This program shows how a union can be declared inside a structure to save memory when only one value is needed at a time.

#include <stdio.h>                      // Header file

struct Student                          // Define structure
{
    int roll;                           // Roll number
    union                               // Anonymous union
    {
        int marks;                      // Marks
        char grade;                     // Grade
    } result;                           // Union variable inside struct
};

int main()                              // Main function
{
    struct Student s;                   // Declare struct variable

    s.roll = 101;                       // Assign roll number
    s.result.marks = 85;                // Assign marks

    printf("Roll: %d\n", s.roll);       // Print roll
    printf("Marks: %d\n", s.result.marks); // Print marks

    return 0;                           // End
}

✅ Expected Output
Roll: 101
Marks: 85
📌 Explanation:
Here, only marks is assigned, so it prints correctly.

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💻 Practice Program 4 (Structure Inside Union)

Declaring a Structure as a Member of a Union

This program demonstrates how a structure can be included inside a union and how assigning one member overwrites another.

#include <stdio.h>                      // Header

struct Address                          // Define structure
{
    int houseNo;                        // House number
    char city[20];                      // City name
};

union Data                              // Define union
{
    int id;                             // ID member
    struct Address addr;                // Structure member
};

int main()                              // Main function
{
    union Data d;                       // Declare union variable

    d.id = 1001;                        // Assign ID
    printf("ID: %d\n", d.id);           // Print ID

    d.addr.houseNo = 221;               // Assign house number (overwrites ID)
    printf("House No: %d\n", d.addr.houseNo); // Print house number

    return 0;                           // End
}

✅ Expected Output
ID: 1001
House No: 221
📌 Explanation:
After assigning addr.houseNo, the previous id value is overwritten.

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💻 Practice Program 5 (Union Passed to Function)

Passing a Union Variable to a Function

This program shows that a union can be passed to a function just like a structure, either by value or by reference.

#include <stdio.h>                  // Header file

union Number                        // Define union
{
    int i;                          // Integer member
    float f;                        // Float member
};

void display(union Number n)        // Function receiving union
{
    printf("Integer value: %d\n", n.i); // Print integer
}

int main()                          // Main function
{
    union Number num;               // Declare union variable
    num.i = 50;                     // Assign integer value

    display(num);                   // Pass union to function

    return 0;                       // End program
}

✅ Expected Output
Integer value: 50
📌 Explanation:
The union is passed by value, and the integer member is printed.

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🌍 Real-Life Example of Union

Suppose we are designing a Payment System.

A person can pay using:

• Cash

• Credit Card

• UPI

But at one time, only one method is used.

union Payment              // Define a union named Payment
{
    int cash;              // Stores cash amount (used when payment is made by cash)
    long cardNumber;       // Stores credit/debit card number (used for card payment)
    char upiID[20];        // Stores UPI ID as a string (used for UPI payment)
};                         // End of union definition

This saves memory because only one payment method is active at a time.

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🧑‍🏫 Detailed Viva Questions on Union (With Answers)

✅ Q1. What is a Union in C?

A union is a user-defined data type where all members share the same memory location. Only one member can contain a valid value at a time.

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✅ Q2. What is the main difference between structure and union?

Structure allocates separate memory to each member, while union shares memory among all members.

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✅ Q3. How is the size of a union calculated?

The size of a union equals the size of its largest member.

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✅ Q4. Can we access all members at the same time?

No. Only one member can be used at a time.

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✅ Q5. Why do we use union?

Union is used for memory optimization, especially in embedded systems and hardware-level programming.

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✅ Q6. What happens if we access inactive member?

It may produce a garbage value or undefined behavior.

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✅ Q7. Can union be inside structure?

Yes. A union can be declared inside a structure.

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✅ Q8. Can structure be inside union?

Yes. A structure can be a member of a union.

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✅ Q9. What are disadvantages of union?

• Only one active member

• Risk of overwrite

• Can confuse beginners

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✅ Q10. Where are unions used in real applications?

• Embedded systems

• Communication protocols

• Device drivers

• Memory-sensitive programs

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🧠 10 Advanced Trick Viva Questions

1️⃣ Can union contain pointers? → ✅ Yes

A union member can be a pointer variable like int *ptr because pointers are valid data types in C.

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2️⃣ Can union contain array? → ✅ Yes

Arrays can be declared as union members just like in structures.

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3️⃣ Can union contain another union? → ✅ Yes

A union can include another union as one of its members (nested union).

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4️⃣ Can union be global? → ✅ Yes

A union can be declared outside all functions and used globally in the program.

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5️⃣ Can union be passed to function? → ✅ Yes

Union variables can be passed to functions either by value or by reference (pointer).

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6️⃣ Can multiple members be initialized? → ❌ Only first member directly

In C, only the first member of a union can be initialized at declaration time.

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7️⃣ Is union faster than structure? → ❌ Not necessarily

Union saves memory but does not automatically improve execution speed.

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8️⃣ Does union support nesting? → ✅ Yes

A union can contain another union or structure inside it.

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9️⃣ Can union contain structure? → ✅ Yes

A structure can be a member of a union.

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🔟 Is union object-oriented? → ❌ No

Union is a feature of procedural C programming, not object-oriented programming.

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📝 15 MCQs on Union in C (With Answers & Explanation)

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1. Union size depends on:

A) Sum
B) Largest member
C) Smallest member
D) None

✅ Answer: B) Largest member
📘 Explanation: The size of a union equals the size of its largest data member.

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2. How many members can store value at once?

A) All
B) Two
C) One
D) None

✅ Answer: C) One
📘 Explanation: Only one member can hold a valid value at a time because all members share the same memory.

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3. Union is mainly used for:

A) Speed
B) Memory saving
C) Looping
D) Security

✅ Answer: B) Memory saving
📘 Explanation: Union allows multiple variables to share the same memory space, reducing memory usage.

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4. Union keyword is:

A) struct
B) union
C) enum
D) typedef

✅ Answer: B) union
📘 Explanation: The keyword union is used to declare a union in C.

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5. Members of union share:

A) Separate memory
B) Same memory
C) Heap memory
D) Stack memory

✅ Answer: B) Same memory
📘 Explanation: All members of a union use the same memory location.

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6. Can union contain structure?

A) Yes
B) No
C) Only in C++
D) None

✅ Answer: A) Yes
📘 Explanation: A union can contain structures, arrays, and other data types as members.

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7. Union size of (int, char)?

A) 1
B) 4
C) 5
D) 8

✅ Answer: B) 4
📘 Explanation: Since int is larger than char, the union size becomes the size of int (usually 4 bytes).

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8. Is union memory efficient?

A) Yes
B) No

✅ Answer: A) Yes
📘 Explanation: Union is memory efficient because it stores only one member at a time in shared memory.

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9. Can union be global?

A) Yes
B) No

✅ Answer: A) Yes
📘 Explanation: A union can be declared globally just like structures and variables.

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10. Can union be nested?

A) Yes

B) No

✅ Answer: A) Yes
📘 Explanation: A union can be nested inside another union or structure.

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11. Can we use sizeof on union?

A) Yes
B) No

✅ Answer: A) Yes
📘 Explanation: The sizeof operator can be used to determine the size of a union.

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12. Does union allow multiple active members?

A) Yes
B) No

✅ Answer: B) No
📘 Explanation: Only one member can be active at a time because they share the same memory block.

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13. Largest member determines:

A) Speed
B) Size
C) Security
D) Loop

✅ Answer: B) Size
📘 Explanation: The largest member in a union determines the total memory size allocated.

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14. Union overwrites data because:

A) Separate memory
B) Shared memory
C) Dynamic memory
D) Stack

✅ Answer: B) Shared memory
📘 Explanation: When a new value is assigned, it overwrites the previous data stored in the shared memory.

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15. Union is best used in:

A) Web design
B) Embedded systems
C) Animation
D) Gaming

✅ Answer: B) Embedded systems
📘 Explanation: Unions are commonly used in embedded systems where memory optimization is critical.

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🏆 Advantages of Union

✔ Memory efficient
✔ Useful in embedded systems
✔ Reduces memory consumption
✔ Good for low-level programming

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❌ Disadvantages of Union

✖ Only one active member
✖ Risk of data loss
✖ Can create confusion

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🎓 Final Interview-Ready Summary

🔹 Union is a user-defined data type
🔹 All members share same memory
🔹 Only one value stored at a time
🔹 Size equals largest member
🔹 Used for memory optimization

If structure organizes data,
👉 Union optimizes memory.

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📌 Keep Learning. Keep Coding. Keep Growing.

✨ Written by Krishna Popat
🌱 Founder, Learning Growth Hub