Difference Between Structure and Union in C

Last Updated : 18 Jul, 2026

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.

Structures

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

  • Each structure member has its own separate memory location.
  • The size of a structure is approximately the sum of the sizes of all its members (including any padding added by the compiler).
C 
#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;
}

Output
Geek
20
85.50
Size: 60 bytes

Syntax

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

Unions

A union is a user-defined data type in which all members share the same memory location, allowing only one member to hold a valid value at a time.

  • The size of a union is equal to the size of its largest data member.
  • Unions help save memory by storing different data types in the same memory location.
C 
//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

Output
100
99.99
A
Size: 8

Syntax

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

Structure Vs Union

StructureUnion
A user-defined data type that groups different data types into a single entity.A user-defined data type that stores different data types in the same memory location.
Defined using the struct keyword.Defined using the union keyword.
Size is the sum of all member sizes (including padding if required).Size is equal to the size of the largest member (including padding if required).
Each member has its own separate memory location.All members share the same memory location.
Members do not overlap in memory.Members completely overlap in memory.
All members can store and retain their values simultaneously.Only one member holds a valid value at a time.

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.
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