C Unions

A union is a user-defined type similar to structs in C except for one key difference.

Structures allocate enough space to store all their members, whereas unions can only hold one member value at a time.

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. And to access pointer variables, we 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.

With a union, all members share the same memory.

Example: Accessing Union Members

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

int main() {
   j.salary = 12.3;

   // when j.workerNo is assigned a value,
   // j.salary will no longer hold 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

To learn where unions are used, visit Why do we need C Unions?