Structures vs. Unions in C: A Practical Guide

What is a Structure?

A structure is a user‑defined data type in C that groups logically related data items of different types into a single unit. Each member occupies its own contiguous memory location, so a structure can hold multiple values simultaneously.

Example: a student record that stores a name, roll number, and marks under one variable name.

What is a Union?

A union is also a user‑defined type, but all members share the same memory space. The union’s size equals the size of its largest member. This feature makes unions ideal for memory‑constrained scenarios where only one member is needed at a time.

Declaring a Structure

struct Student {
    char name[60];
    int  roll_no;
    float marks;
};

Notice the use of the struct keyword and the semicolon after the closing brace.

Example: Using a Structure

#include <stdio.h>

struct Student {
    char name[60];
    int  roll_no;
    float marks;
} sdt;

int main(void) {
    printf("Enter the following information:\n");
    printf("Student name: ");
    fgets(sdt.name, sizeof(sdt.name), stdin);

    printf("Roll number: ");
    scanf("%d", &sdt.roll_no);

    printf("Marks: ");
    scanf("%f", &sdt.marks);

    printf("\nEntered data:\n");
    printf("Name: %s", sdt.name);
    printf("Roll number: %d\n", sdt.roll_no);
    printf("Marks: %.1f\n", sdt.marks);
    return 0;
}

Output (example):

Enter the following information:
Student name: James
Roll number: 21
Marks: 67

Entered data:
Name: James
Roll number: 21
Marks: 67.0

Declaring a Union

union Item {
    int    x;
    float  y;
    char   ch;
};

Example: Using a Union

#include <stdio.h>

union Item {
    int    x;
    float  y;
    char   ch;
};

int main(void) {
    union Item it;
    it.x = 12;
    it.y = 20.2f;
    it.ch = 'a';

    printf("x = %d\n", it.x);
    printf("y = %f\n", it.y);
    printf("ch = %c\n", it.ch);
    return 0;
}

Because all members share memory, setting ch overwrites the previous values of x and y. Only the last stored value is valid.

Structure vs. Union

Feature	Structure	Union
Declaration keyword	struct	union
Memory layout	Each member gets its own space	All members share one space
Changing one member	Does not affect others	Overwrites other members
Initialization	All members can be initialized simultaneously	Only the first member can be initialized at a time
Total size	Sum of all member sizes	Size of the largest member
Typical use	Store related data of various types	Store one of several possible types
Access pattern	Any member can be accessed at any time	Only one member is valid at a time
Flexible array support	Supported	Not supported

When to Use Each

• Structure: When you need to maintain multiple related fields simultaneously, such as a record of a student or an employee.
• Union: When you have a value that could be of several types but only one at a time, such as a variant type or a memory‑constrained sensor value.

Pros and Cons

Structure Advantages

• Groups related data under a single name, improving readability.
• Easy to pass entire records to functions.
• Arrays of structures enable storage of many records.
• Supports flexible arrays for variable‑length data.

Structure Disadvantages

• Requires more memory, as every member is stored.
• Changes to the structure definition propagate throughout the code.
• Can become unwieldy in large projects with deeply nested structures.

Union Advantages

• Minimal memory footprint: only the largest member’s size.
• Useful for implementing type tags or variant data.
• Can simplify code that uses a single value of varying type.

Union Disadvantages

• Only one member can hold a valid value at any time.
• Harder to debug due to shared memory.
• Cannot use flexible arrays.