Union in C Language – Unveil the Difference between Structures and Unions

Just like structures, union in C is a user-defined data type used to store data of different types that hold the same memory location.

In this tutorial, we will discuss the following-

• Union in C
• Difference between Structures and Unions in C
• Meaning of Pointers to a Union in C

Before we begin our discussion on unions, it is important to have a crystal clear understanding of structures in C.

1. Union in C Language

As the name itself suggests, a union refers to the grouping together of data members treated as a single entity. The keyword union is used to indicate the declaration of a union. The format to define a union is the same as that of structures.

A union can be defined in 2 ways, just like structures. They are:

1.1 When Union variables are created inside the main function

union union_name
{
data_type member1;
data_type member2;
data_type member3;
.
.
.
data_type membern;

};
int main()
{
union v1, v2, *v3; // Here v1, v2 and *v3 are variables to the union
/* Body of the main function */
return 0;
}

1.2 When Union variables are created outside the main function

union union_name
{
data_type member1;
data_type member2;
data_type member3;
.
.
.
data_type membern;

}v1, v2, *v3; // Here v1, v2 and *v3 are variables to the union
int main()
{
/* Body of the main function */
return 0;
}

Key takeaway: In order to access a data member through pointer *v3, the -> operator is used.

2. How to Implement Unions in C Programming?

Here is a code in C that illustrates the implementation of unions in a C program:

#include <stdio.h>
union DataFlair
{
char value1, value2;
};
int main()
{
printf("Welcome to DataFlair tutorials!\n\n");
union DataFlair d; // Here d is a variable of union DataFlair
d.value1 = 'a'; // Here d.value2 will automatically become 'a'
printf("value1 = %c, value2 = %c\n",d.value1, d.value2);
d.value2 = 'b'; // Here d.value1 will automatically become 'b'
printf("value1 = %c, value2 = %c\n",d.value1, d.value2);
return 0;
}

Code on Screen-

Output-

3. What is the Use of Union in C Programming?

Here is another code in C that illustrates the use of unions in a C program

#include <stdio.h>
union employee
{
int ID;
int age;
};
int main()
{

printf("Welcome to DataFlair tutorials!\n\n");

union employee e;
e.ID = 1001;
e.age = 22;

printf("The ID is: %d\n",e.ID);
printf("The age is: %d\n",e.age);
return 0;
}

Evidently, it is clear that only one member can be accessed at a time in a union.

Code on Screen-

Output-

4. Structures Vs Unions in C

There are a lot of similarities between structures and unions in C programming language, such as they have similar syntax and serve the same purpose used to store data members of different data types.

But, there are a couple of differences between the two. One of the striking differences between the two is that the data members in a structure have individual memory locations, whereas the data members of a union share the same memory location that gives rise to its distinction.

Here is a code in C that illustrates the difference between a structure and a union:

#include <stdio.h>
struct sample_structure
{
char name[30]; // size = 30
int emp_id; // size = 4
float salary; // size = 4
}s;
/* Total size of union = Largest size of data member = 30 */
union sample_union
{
char name[30]; // size = 30
int emp_id; // size = 4
float salary; // size = 4
/* Total size of structure = 30 + 4 + 4 = 38 */
}u;

int main()
{

printf("Welcome to DataFlair tutorials!\n\n");

printf("The size of the structure is : %ld bytes\n", sizeof(s));
printf("The size of the union is : %ld bytes\n", sizeof(u));
return 0;
}

Code on Screen- 

Output-

5. Pointers to Unions in C

Pointers to unions is a similar concept like pointers to structures in C.

Here is a code in C that illustrates the use of pointers to unions:

#include <stdio.h>
union sample
{
int number;
char character;
};

int main()
{

printf("Welcome to DataFlair tutorials!\n\n");

union sample s; // Here p is a pointer
s.number = 122;
union sample *p = &s;

printf("The ASCII value %d corresponds to %c\n", p->number, p->character); // Displaying the character corresponding to the ASCII value
return 0;
}

Code on Screen-

Output-

Quiz on Union in C

6. Summary

In this tutorial, we discussed what are unions, how to define them and various C programs associated with it. We saw the contrasting differences between a union and structure, although they seem to be quite similar to each other. Hope, you liked Union in C tutorial, but this is not the end, you may also like to know about Macros in C.

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