10 Differences Between union and structure in c

Union and Structure in C: Understanding the Differences

C programming language provides two important constructs, unions and structures, that are used to define user-defined data types. While both unions and structures serve similar purposes, they have distinct characteristics and applied in different scenarios. In this article, we will explore the differences between union and structure in C, along with examples and use cases.

What is/are Union?

A union in C is a user-defined data type that allows storing different data types in the same memory location. It enables accessing the same memory location using different data types, based on the requirement. This approach helps in optimizing memory usage.

Examples of Union


union Data {
   int intValue;
   float floatValue;
   char stringValue[20];

int main() {
   union Data data;
   data.intValue = 10;
   printf("Int value: %d
", data.intValue);
   data.floatValue = 3.14;
   printf("Float value: %.2f
", data.floatValue);
   strcpy(data.stringValue, "Hello");
   printf("String value: %s
", data.stringValue);
   return 0;

In the above example, we define a union named “Data” that can hold an integer, float, or a string value using the members intValue, floatValue, and stringValue. By storing different values in the union, we can access the same memory location using different data types.

Uses of Union

Unions are often used in scenarios where multiple data types need to be stored in the same memory location, and the requirements demand accessing them interchangeably. Some common use cases of unions include:

  • Implementing variant data types
  • Handling type punning
  • Interchangeable data representation

What is/are Structure in C?

A structure in C is a user-defined data type that allows storing a collection of different data types together under a single name. It creates a composite data type by grouping variables of different types. Each variable within a structure is known as a member.

Examples of Structure in C


struct Person {
   char name[50];
   int age;
   float salary;

int main() {
   struct Person person1;
   strcpy(person1.name, "John");
   person1.age = 30;
   person1.salary = 5000.00;

   printf("Name: %s
", person1.name);
   printf("Age: %d
", person1.age);
   printf("Salary: %.2f
", person1.salary);
   return 0;

In the above example, we define a structure named “Person” that contains three members: name, age, and salary. We create an object of the structure and assign values to its members. The structure allows organizing related data together for easy access and manipulation.

Uses of Structure in C

Structures find various applications in programming, such as:

  • Representing records or entities
  • Defining complex data structures
  • Passing multiple arguments to functions

Differences between Union and Structure

Difference Area Union Structure
Memory Allocation Allocates the memory required for the largest member Allocates the memory required for each member
Memory Usage Memory overlap among members No memory overlap among members
Member Access Accessing one member at a time Accessing multiple members simultaneously
Size Size of the union is equal to the size of the largest member Size of the structure is the sum of sizes of all members
Memory Efficiency Memory efficient when multiple members are not simultaneously active Requires more memory as compared to unions
Initialization Only the first member can be explicitly initialized All members can be explicitly initialized
Member Modification Only one member can be modified at a time Multiple members can be modified simultaneously
Default Initialization Members are not automatically initialized Members are automatically initialized
Byte Alignment Follows byte alignment rules Follows byte alignment rules
Member Ordering The order of members may affect memory utilization Members are accessed according to their declaration order


To summarize, unions and structures provide distinct features and serve different purposes in C programming. Unions allow sharing memory among members, making them ideal for scenarios where different data types need to be stored interchangeably. On the other hand, structures are used to organize related data together and create composite data types.

People Also Ask

1. Can a structure contain a union?
Yes, a structure can contain a union as one of its members. This allows combining the advantages of both structures and unions, organizing related data and enabling different data types within the same structure.

2. How to access the members of a union or structure?
The members of a union or structure can be accessed using the dot (.) operator. For example, to access the “age” member of a structure named “person”, we can use the expression “person.age”.

3. What is the size of a union or structure?
The size of a union is determined by the size of its largest member. In contrast, the size of a structure is calculated as the sum of sizes of all its members, considering padding and alignment.

4. Can we nest unions and structures?
Yes, both unions and structures can be nested within each other. This allows creating complex data structures by combining multiple levels of encapsulation.

5. Are unions and structures platform-dependent?
Unions and structures follow the language rules and are not platform-dependent. However, the byte alignment and padding may vary based on the target architecture or compiler settings, leading to different memory layouts.

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