Unions in C Language

What is a Union in C?

A union in C is a user-defined data type similar to a struct, but unlike structures where each member has its own storage location, all members of a union share the same memory location.

This means only one member can contain a value at any given time.

Syntax of Union

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

Why Use a Union?

Unions in C are used primarily for efficient memory management when you need to work with multiple data types but only one at a time.

Here are the main reasons to use a union:

1. Memory Efficiency

Unions save memory because all members share the same memory space. Only enough memory is allocated to hold the largest member.

Example:

union Example {
    int i;         // 4 bytes
    float f;       // 4 bytes
    char str[20];  // 20 bytes
};

Memory used: Just 20 bytes (not 28), since all members overlap.

2. Handling Variant Data

If a variable can represent different data types at different times, unions are ideal.

Example: A network packet might be a string, a float, or a structure, depending on the packet type. You can use a union to represent all formats.

3. Creating Tagged Unions (Discriminated Unions)

Use a struct with a tag and a union to represent multiple data types in a type-safe way.

#include <stdio.h>
 
typedef struct {
    int type; // 1=int, 2=char
    union {
        int i;
        char c;
    } data;
} Value;
 
int main() {
    Value v;
    v.type = 1;
    v.data.i = 100;
    printf("Integer: %d\n", v.data.i);
 
    v.type = 2;
    v.data.c = 'A';
    printf("Char: %c\n", v.data.c);
 
    return 0;
}

4. Low-Level Hardware Access

• Unions allow accessing the same memory in different ways, such as bytes vs. words.
• Used in device drivers and embedded programming.

union ByteAccess {
    int word;
    char bytes[4];
};

Union Example in C

#include <stdio.h>
 
union QuizData {
    int score;
    float duration;
    char grade;
};
 
int main() {
    union QuizData quiz;
 
    quiz.score = 95;
    printf("Score: %d\n", quiz.score);
 
    quiz.duration = 2.5;
    printf("Duration: %.1f\n", quiz.duration);
 
    quiz.grade = 'A';
    printf("Grade: %c\n", quiz.grade);
 
    // Previous values might be lost
    printf("After Grade assignment:\n");
    printf("Score (undefined): %d\n", quiz.score);
    printf("Duration (undefined): %.1f\n", quiz.duration);
 
    return 0;
}

Expected Output (platform-dependent for undefined values):

Score: 95
Duration: 2.5
Grade: A
After Grade assignment:
Score (undefined): 1094795585
Duration (undefined): 361.500000

Explanation:

• The union QuizData has three parts: an integer score, a float duration, and a character grade.
• All three parts share the same memory, so only one can safely hold a value at a time.
• The program puts values into score, then duration, and finally grade.
• Each new value replaces the one before it because they all use the same space.
• In the end, only grade shows the right value—score and duration give random or wrong results.

Best Practices When Using Unions

• Track Active Member: Always keep track of which member is currently active to avoid undefined behavior.
• Use with Enumerations: Combine unions with enums to indicate the type of data currently stored.
• Avoid Simultaneous Access: Do not attempt to access multiple members simultaneously.
• Be Cautious with Pointers: When unions contain pointers, ensure proper memory management to prevent leaks or dangling pointers.

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