Structure and Union in C

Posted by yanib on 2025-03-22 09:25:14 |

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In C, structure and union are user-defined data types that allow grouping variables of different data types under a single name. However, they work differently in memory allocation.

Structure (struct)

• A structure is a collection of variables (members) of different types grouped together.

• Each member has its own memory location.

• The total memory occupied by a structure is the sum of the memory sizes of all its members.

Example of Structure:

c
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#include <stdio.h>

// Define a structure
struct Student {
    char name[20];
    int age;
    float marks;
};

int main() {
    struct Student s1 = {"Ram", 20, 85.5};

    // Accessing structure members
    printf("Name: %s\n", s1.name);
    printf("Age: %d\n", s1.age);
    printf("Marks: %.2f\n", s1.marks);

    return 0;
}

Memory Allocation for Structure:

If char[20] takes 20 bytes, int takes 4 bytes, and float takes 4 bytes, then total memory used = 28 bytes.

Union (union)

• A union is similar to a structure, but all members share the same memory.

• The memory allocated to a union is equal to the size of its largest member.

• Only one member can store a value at a time.

Example of Union:

#include <stdio.h>

// Define a union
union Data {
    int num;
    float decimal;
    char ch;
};

int main() {
    union Data d;
    
    d.num = 10;
    printf("Num: %d\n", d.num);

    d.decimal = 5.5;
    printf("Decimal: %.1f\n", d.decimal);

    d.ch = 'A';
    printf("Character: %c\n", d.ch);

    return 0;
}

Memory Allocation for Union:

If int takes 4 bytes, float takes 4 bytes, and char takes 1 byte, then the memory allocated = 4 bytes (size of the largest member).

Key Differences Between Structure and Union

When to Use?

• Use struct when you need to store multiple values at the same time.

• Use union when you need to save memory and only one value is used at a time.