C Unions

In this tutorial, you'll learn about unions in C programming. More specifically, how to create unions, access its members and learn the differences between unions and structures.

A union is a user-defined type similar to structs in C programming. We recommend you to learn C structs before you check this tutorial.


How to define a union?

We use the union keyword to define unions. Here's an example:

union car
{
  char name[50];
  int price;
};

The above code defines a derived type union car.


Create union variables

When a union is defined, it creates a user-defined type. However, no memory is allocated. To allocate memory for a given union type and work with it, we need to create variables.

Here's how we create union variables.

union car
{
  char name[50];
  int price;
};

int main()
{
  union car car1, car2, *car3;
  return 0;
}

Another way of creating union variables is:

union car
{
  char name[50];
  int price;
} car1, car2, *car3;

In both cases, union variables car1, car2, and a union pointer car3 of union car type are created.


Access members of a union

We use the . operator to access members of a union. To access pointer variables, we use also use the -> operator.

In the above example,

  • To access price for car1, car1.price is used.
  • To access price using car3, either (*car3).price or car3->price can be used.

Difference between unions and structures

Let's take an example to demonstrate the difference between unions and structures:

#include <stdio.h>
union unionJob
{
   //defining a union
   char name[32];
   float salary;
   int workerNo;
} uJob;

struct structJob
{
   char name[32];
   float salary;
   int workerNo;
} sJob;

int main()
{
   printf("size of union = %d bytes", sizeof(uJob));
   printf("\nsize of structure = %d bytes", sizeof(sJob));
   return 0;
}

Output

size of union = 32
size of structure = 40

Why this difference in the size of union and structure variables?

Here, the size of sJob is 40 bytes because

  • the size of name[32] is 32 bytes
  • the size of salary is 4 bytes
  • the size of workerNo is 4 bytes

However, the size of uJob is 32 bytes. It's because the size of a union variable will always be the size of its largest element. In the above example, the size of its largest element, (name[32]), is 32 bytes.


Only one union member can be accessed at a time 

You can access all members of a structure at once as sufficient memory is allocated for all members. However, it's not the case in unions. You can only access a single member of a union at one time. Let's see an example.

#include <stdio.h>
union Job
{
   float salary;
   int workerNo;
} j;

int main()
{
   j.salary = 12.3;
   j.workerNo = 100;

   printf("Salary = %.1f\n", j.salary);
   printf("Number of workers = %d", j.workerNo);
   return 0;
}

Output

Salary = 0.0
Number of workers = 100

Notice that 12.3 was not stored in j.salary.