Difference Between Structure and Union in C

In C programming, both structures and unions are used to group different types of data under a single name, but they behave in different ways. The main difference lies in how they store data.

The below table lists the primary differences between the C structures and unions:

Structures

A structure in C is a collection of variables, possibly of different types, under a single name. Each member of the structure is allocated its own memory space, and the size of the structure is the sum of the sizes of all its members.

Syntax

struct name {
member1 definition;
member2 definition;
...
memberN definition;
};

Example:

#include <stdio.h>

struct Student {
    char name[50];
    int age;
    float grade;
};

int main() {
  
    // Create a structure variable
    struct Student s1 = {"Geek", 20, 85.5};

    // Access structure members
    printf("%s\n", s1.name);
    printf("%d\n", s1.age);
    printf("%.2f\n", s1.grade);
  	printf("Size: %d bytes", sizeof(s1));

    return 0;
}

Geek
20
85.50
Size: 60 bytes

Explanation: In this example, we create a structure Student to store a student's name, age, and grade. Each of the members (name, age, grade) is stored in its own separate memory location, and we access them individually. The size is also the size of all members combined plus structure padding.

Unions

A union in C is similar to a structure, but with a key difference: all members of a union share the same memory location. This means only one member of the union can store a value at any given time. The size of a union is determined by the size of its largest member.

Syntax:

union name {
member1 definition;
member2 definition;
...
memberN definition;
};

Example:

//Driver Code Starts
#include <stdio.h>

//Driver Code Ends

union Data {
    int i;
    double d;
    char c;
};

int main() {
  
    // Create a union variable
    union Data data;

    // Store an integer in the union
    data.i = 100;
    printf("%d
", data.i);

    // Store a double in the union (this will
  	// overwrite the integer value)
    data.d = 99.99;
    printf("%.2f
", data.d);

    // Store a character in the union (this will
  	// overwrite the double value)
    data.c = 'A';
    printf("%c
", data.c);
  
  	printf("Size: %d", sizeof(data));

//Driver Code Starts

    return 0;
}

//Driver Code Ends

100
99.99
A
Size: 8

Similarities Between Structure and Union

Structures and unions are also similar in some aspects listed below:

• Both are user-defined data types used to store data of different types as a single unit.
• Their members can be objects of any type, including other structures and unions or arrays. A member can also consist of a bit field.
• Both structures and unions support only assignment = and sizeof operators. The two structures or unions in the assignment must have the same members and member types.
• A structure or a union can be passed by value to functions and returned by value by functions. The argument must have the same type as the function parameter. A structure or union is passed by value just like a scalar variable as a corresponding parameter.
• ‘.’ operator or selection operator, which has one of the highest precedences, is used for accessing member variables inside both the user-defined datatypes.