Structure, Union & Typedef

Structures in C

A structure in C is a collection of variables (called members) of different data types grouped together under one name.

Advantages of Structures:

• Handles complex data easily.
• Groups different data types logically.
• Useful in real-world applications like databases.

Disadvantages:

• Cannot directly compare two structures using ==.
• Takes more memory than arrays (because of padding).

Example

structure.c

#include <stdio.h>
#include <string.h>

struct Student {
    int roll;
    char name[50];
    float marks;
};

int main() {
    struct Student s;
    s.roll = 1;
    strcpy(s.name, "Akash");
    s.marks = 92.5;

    printf("Roll: %d\nName: %s\nMarks: %.2f\n", s.roll, s.name, s.marks);
    return 0;
}

Output

Roll: 1
Name: Akash
Marks: 92.50

Unions

A union is similar to a structure but uses shared memory for all its members.

• Only one member can hold a value at a time.
• Size of union = size of largest member.
• memory = size of largest member.

Advantages of Union:

• Memory efficient (used in embedded systems).
• Useful for interpreting the same memory differently.

Disadvantages:

• Only one member can be used at a time.
• Risk of data overwrite.

Example

union.c

#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 = 220.5; // overwrites i
    printf("f: %.2f\n", d.f);
    return 0;
}

Output

i: 10
f: 220.50

Memory Layout:

• Structure: memory = sum of all members (with padding).
• Union: memory = size of largest member.

typedef in C

typedef is used to create a new name (alias) for an existing data type.

Advantages of typedef:

• Makes code shorter and readable.
• Useful for portability (changing data types easily).

Example

typedef.c

#include <stdio.h>
typedef unsigned int uint;
typedef struct {
    int id;
    char name[50];
} Student;
int main() {
    uint age = 25;
    Student s = {101, "Shruti"};
    printf("Age: %u, Name: %s\n", age, s.name);
    return 0;
}

Output

Age: 25, Name: Shruti

Real-World Applications:

• Structures – Database records (students, employees).
• Unions – Device drivers, protocol headers, embedded systems.
• typedef – Portable code for operating systems and libraries.