Unions in C Programming

A Union in C programming is a user-defined derived data type that allows different data types to be stored in the exact same memory location. While syntactically similar to structures (using the union keyword instead of struct), a union differs fundamentally in how memory is allocated: in a structure, each member possesses its own dedicated memory space, whereas all members of a union share a single, common memory block.

Syntax of a Union in C


union union_name
{
    data_type member1;
    data_type member2;
    data_type member3;
};

Memory Allocation: How Unions Save RAM

Size of a Union: The compiler allocates memory equivalent to the size of its largest member (with padding for memory alignment). At any given moment in time, only one member can hold a meaningful value.

Overwriting: Assigning a value to one member immediately overwrites the data stored in all other members.

Structure vs. Union: The Definitive Comparison (BCA Exam Special)

Feature	Structure (`struct`)	Union (`union`)
Keyword	`struct`	`union`
Memory Allocation	Each member gets separate memory.	All members share the same memory.
Total Size	≥ Sum of sizes of all members.	Equal to size of largest member.
Value Access	All members accessible simultaneously.	Only one member accessible at a time.
Initialization	All members can be initialized together.	Only the first member can be initialized.

Comprehensive Working Example: Understanding Shared Memory

This complete program demonstrates how assigning a value to one member corrupts/overwrites earlier members:


#include <stdio.h>
#include <string.h>

/* Define union with 3 different data types */
union DataRecord
{
    int i;
    float f;
    char str[20];
};

int main()
{
    union DataRecord record;

    printf("Size of union DataRecord = %zu bytes\n\n", sizeof(record));

    /* 1. Storing integer */
    record.i = 101;
    printf("record.i = %d\n", record.i);

    /* 2. Storing float - overwrites record.i */
    record.f = 220.5f;
    printf("record.f = %.2f\n", record.f);

    /* 3. Storing string - overwrites record.f and record.i */
    strcpy(record.str, "C Programming");
    printf("record.str = %s\n\n", record.str);

    /* Demonstrating corruption when reading all at once */
    printf("--- Reading all members after writing string ---\n");
    printf("record.i   : %d (Corrupted garbage)\n", record.i);
    printf("record.f   : %f (Corrupted garbage)\n", record.f);
    printf("record.str : %s (Valid current value)\n", record.str);

    return 0;
}

Practice Session for Beginner Students

Test your understanding of unions and memory sharing.

Part 1: Spot the Bug & Predict the Output

Problem 1: What is the output of sizeof(union Test)?


union Test {
    char a;
    int b;
    double c;
};

Answer & Explanation: Output is 8 bytes (on typical systems). The members are char (1 byte), int (4 bytes), and double (8 bytes). A union allocates memory based on its largest member (double = 8 bytes).

Part 2: Multiple Choice Questions (BCA Exam Preparation)

Q1. Which member of a union can be initialized at the time of declaration?
A) Any member
B) Only the largest member
C) Only the first declared member
D) All members simultaneously
Correct Answer: C) Only the first declared member.

Q2. What operator is used to access union members via a union pointer?
A) Dot operator (.)
B) Arrow operator (->)
C) Asterisk (*)
D) Ampersand (&)
Correct Answer: B) Arrow operator (->).

Part 3: Frequently Asked University Viva Questions

1. When should a programmer choose a union over a structure?
Answer: When you have an entity that can hold only one of several possible data types at any given time (e.g., a variant record, sensor readings that report either an integer code or a float measurement), using a union saves substantial memory.

2. Can a union contain a structure as a member, or vice versa?
Answer: Yes. C supports nested structures and unions (structures inside unions, or unions inside structures), which is common in low-level operating system headers and network packet parsers.