Structures and unions are user-defined data types in C that allow programmers to group multiple variables of different data types under a single name. Both are used to organize related data and improve code readability. Although their syntax appears very similar, the way they store and manage data is significantly different.
The primary difference between a structure and a union lies in memory allocation. Structures allocate separate memory for each member, while unions allow all members to share the same memory location. Understanding these differences helps programmers choose the most suitable data type based on memory requirements and application needs.
 

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What is a Structure in C?

A structure is a user-defined data type that groups multiple variables of different data types under a single name. Each member of a structure gets its own separate memory location, allowing all members to store values simultaneously.
Example:

struct Student
{
    int rollNo;
    float marks;
    char grade;
};

In this example, rollNo, marks, and grade each occupy separate memory locations.
Below is the C program to implement structures:

// C program to implement structures
#include stdio.h

struct Student
{
    int rollNo;
    float marks;
};

int main()
{
    struct Student s;

    s.rollNo = 101;
    s.marks = 95.5;

    printf("Roll No: %d\n", s.rollNo);
    printf("Marks: %.2f\n", s.marks);

    return 0;
}

Output:

Roll No: 101
Marks: 95.50

Explanation:
In a structure, both rollNo and marks have separate memory locations. Therefore, storing a value in one member does not affect the value stored in another member.

What is a Union in C?

A union is a user-defined data type in which all members share the same memory location. As a result, only one member can contain a valid value at a time because assigning a value to one member overwrites the values of the others.
Example:

union Data
{
    int num;
    float marks;
    char grade;
};

In this example, all members share the same memory space.
Below is the C program to implement unions:

// C programs to implement unions
#include stdio.h

union Data
{
    int num;
    float marks;
};

int main()
{
    union Data d;

    d.num = 100;
    printf("Number: %d\n", d.num);

    d.marks = 95.5;
    printf("Marks: %.2f\n", d.marks);

    printf("Number After Overwrite: %d\n", d.num);

    return 0;
}

Output:

Number: 100
Marks: 95.50
Number After Overwrite: 1119813632

Explanation:
The union members share the same memory location. When a value is assigned to marks, it overwrites the value previously stored in num. This demonstrates the shared memory behavior of unions.

Difference Between Structure and Union

Basis of Comparison	Structure	Union
Definition	A structure is a user-defined data type in which each member has its own memory location.	A union is a user-defined data type in which all members share the same memory location.
Memory Allocation	Memory is allocated separately for every member of the structure.	A single memory block is shared among all union members.
Memory Usage	The total memory required is approximately equal to the sum of all member sizes.	The total memory required is equal to the size of the largest member.
Value Storage	All members can store values simultaneously without affecting each other.	Only one member can hold a valid value at a time.
Data Modification	Modifying one member does not affect the values of other members.	Modifying one member may overwrite the values of other members.
Size Calculation	The size depends on the combined size of all members along with padding.	The size depends only on the largest member along with padding.
Data Accessibility	All members can be accessed and used at the same time.	Only the most recently assigned member should be accessed.
Memory Efficiency	Structures use more memory because every member has separate storage.	Unions use less memory because all members share storage.
Suitability	Structures are suitable when all members need to be used together.	Unions are suitable when only one member is needed at a time.
Applications	Structures are commonly used in student records, employee databases, and inventory systems.	Unions are commonly used in embedded systems, hardware programming, and memory-constrained applications.

Common Mistakes When Choosing Between Structure and Union

1. Using a Union When Multiple Values Are Needed: If multiple members need to store values at the same time, a structure should be used instead of a union.
2. Assuming Union Members Have Separate Memory: All union members share the same memory location. Treating them like structure members can lead to unexpected results.
3. Ignoring Memory Requirements: Choosing structures in memory-constrained environments may waste memory, while choosing unions unnecessarily may complicate data handling.
4. Accessing Incorrect Union Members: Reading a union member that was not most recently assigned can produce unpredictable results.
5. Confusing Structure Size with Union Size: The size calculation rules for structures and unions are different. Understanding these rules is important for memory management.

Conclusion

Structures and unions are powerful user-defined data types in C that help organize related data. The major difference between them is memory allocation. Structures allocate separate memory to each member, while unions allow all members to share the same memory location.
Structures are ideal when multiple values need to be stored simultaneously, whereas unions are preferred when memory optimization is the primary goal. Choosing between a structure and a union depends on the application’s requirements, memory constraints, and data handling needs.

Frequently Asked Questions

1. What is the main difference between a structure and a union?

The main difference is that structures allocate separate memory for each member, whereas unions share the same memory location among all members.

2. Which uses less memory, a structure or a union?

A union uses less memory because all members share a single memory location.

3. Can all members of a union store values simultaneously?

No, only one member can hold a valid value at a time because all members share the same memory.

4. When should structures be used?

Structures should be used when multiple pieces of related data need to be stored and accessed simultaneously.

5. When should unions be used?

Unions should be used when memory optimization is important and only one member needs to store a value at a time.