Dharani Tech Edu Hub

Unions in C

---

Introduction

In C programming, a union is a user-defined data type similar to a structure.
But unlike structures, all members share the same memory location.

👉 This helps in saving memory.

---

What is a Union?

A union is:
👉 A user-defined data type where all members share the same memory location

---

Why Use Unions?

• ✔ Save memory
• ✔ Store different types at different times
• ✔ Efficient memory usage
• ✔ Useful in embedded systems

---

Syntax of Union

union union_name {
    data_type member1;
    data_type member2;
};

---

Example: Union Declaration & Usage

#include <stdio.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data d;

    d.i = 10;
    printf("Integer = %d\n", d.i);

    d.f = 5.5;
    printf("Float = %.1f\n", d.f);

    return 0;
}

---

Output

Integer = 10
Float = 5.5

---

⚠️ Important Note

👉 Only one member holds value at a time
👉 Changing one value overwrites others

---

Example Showing Overwrite

#include <stdio.h>

union Data {
    int i;
    float f;
};

int main() {
    union Data d;

    d.i = 10;
    d.f = 3.5;

    printf("i = %d\n", d.i);
    printf("f = %.1f\n", d.f);

    return 0;
}

---

Output

i = (garbage value)
f = 3.5

👉 i value is lost

---

Structure vs Union Table

Feature	Structure	Union
Memory	Separate memory	Shared memory
Usage	All members usable	One at a time
Size	Sum of members	Largest member

---

Important Notes

• Only one member is valid at a time
• Union size = largest data member
• Saves memory but limits usage

---

Common Mistakes

• ❌ Using multiple members together
• ❌ Expecting all values to be stored
• ❌ Confusing with structure
• ❌ Wrong memory understanding

---

Pro Tips

• ✔ Use unions when memory is limited
• ✔ Use structures for multiple values
• ✔ Understand memory sharing
• ✔ Practice union examples

---

Conclusion

Unions are useful for memory optimization in C programming. They allow efficient use of memory when only one value is needed at a time.

Use unions wisely for better performance.

👉 This article is part of Dharani Tech Edu Hub — where learning programming is made simple and practical.