Structures and unions allow developers to create their own custom data types in C. They are widely used in real-world applications such as databases, file handling, compilers, and system-level programming.

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1. Introduction to Structures in C

A structure is a user-defined data type that groups variables of different data types under a single name.

For example, storing student data (name, roll number, marks) is easy using structures.

Why Structures?

• Combine different data types
• Easy to manage complex records
• Used in real-world systems (employee records, databases, etc.)

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2. Defining a Structure

Syntax

struct structure_name {
    data_type member1;
    data_type member2;
    ...
};

Example: Student Structure

struct Student {
    int roll;
    char name[20];
    float marks;
};

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3. Declaring Structure Variables

Method 1: After structure definition

struct Student s1, s2;

Method 2: Along with definition

struct Student {
    int roll;
    char name[20];
    float marks;
} s1, s2;

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4. Accessing & Processing Structure Members

Use the dot operator (.).

Example

#include <stdio.h>

struct Student {
    int roll;
    char name[20];
    float marks;
};

int main() {
    struct Student s;

    printf("Enter roll, name, marks: ");
    scanf("%d %s %f", &s.roll, s.name, &s.marks);

    printf("Roll: %d\nName: %s\nMarks: %.2f\n",
           s.roll, s.name, s.marks);

    return 0;
}

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5. User-Defined Data Types (typedef)

typedef allows you to define an alias for a structure to simplify usage.

Without typedef

struct Student s1;

With typedef

typedef struct {
    int roll;
    char name[20];
} Student;

Student s1;   // No need to write 'struct'

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6. Structures and Pointers

Pointers can be used to access structure variables using the arrow operator (->).

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Pointer to a Structure

struct Student {
    int roll;
    char name[20];
    float marks;
};

struct Student s;
struct Student *ptr = &s;

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Accessing Using Pointer

ptr->roll = 10;
ptr->marks = 88.5;

Equivalent to:

s.roll = 10;

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7. Passing Structures to Functions

You can pass structures in three ways:

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7.1 Pass by Value

Copies the entire structure.

void display(struct Student s);

Example

void display(struct Student s) {
    printf("%d %s %.2f", s.roll, s.name, s.marks);
}

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7.2 Pass by Reference (Efficient)

Using pointers:

void display(struct Student *s);

Example

void display(struct Student *s) {
    printf("%d %s %.2f", s->roll, s->name, s->marks);
}

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7.3 Returning Structure from Function

struct Student input() {
    struct Student s;
    scanf("%d %s %f", &s.roll, s.name, &s.marks);
    return s;
}

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8. Introduction to Unions in C

A union is similar to a structure but with one major difference:

💡 Key Difference:

• Structure → Allocates memory for all members.
• Union → Allocates memory for the largest member only.
  • All members share the same memory location.

Unions are memory-efficient and useful when only one member is used at a time.

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9. Defining a Union

Syntax

union union_name {
    data_type member1;
    data_type member2;
};

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Example

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

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10. Working with Unions

#include <stdio.h>

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

int main() {
    union Data d;

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

    d.f = 22.5;
    printf("f = %.2f\n", d.f);

    // Now d.i value becomes corrupted
    return 0;
}

Why?
All members share the same memory. Assigning f overwrites i.

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11. Structure vs Union (Comparison)

Feature	Structure	Union
Memory	Sum of all members	Size of the largest member
Multiple members at once	✔ Yes	✖ No
Usage	Records	Memory-saving scenarios
Data Integrity	High	Low (overwritten)

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Conclusion

Structures and unions are powerful tools in C for creating custom data types. Structures help organize different data types into one composite unit, while unions are useful when memory optimization is required.

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📚 Citations

🔗 View other articles about C Programming:
https://savanka.com/category/learn/c-programming/

🔗 External C Documentation:
https://www.w3schools.com/c/