How do you declare a union in C:

C Union. Union is an user defined datatype in C programming language. It is a collection of variables of different datatypes in the same memory location. We can define a union with many members, but at a given point of time only one member can contain a value.

In union, all members share the same memory location. For example in the following C program, both x and y share the same location. If we change x, we can see the changes being reflected in y. #include <stdio.h> // Declaration of union is same as structures.

For example in the following C program, both x and y share the same location. If we change x, we can see the changes being reflected in y.

#include <stdio.h>

// Declaration of union is same as structures
union test {
	int x, y;
};

int main()
{
	// A union variable t
	union test t;

	t.x = 2; // t.y also gets value 2
	printf("After making x = 2:\n x = %d, y = %d\n\n",
		t.x, t.y);

	t.y = 10; // t.x is also updated to 10
	printf("After making y = 10:\n x = %d, y = %d\n\n",
		t.x, t.y);
	return 0;
}

output:

After making x = 2:
 x = 2, y = 2

After making y = 10:
 x = 10, y = 10

Pointers to unions?
Like structures, we can have pointers to unions and can access members using the arrow operator (->). The following example demonstrates the same.

#include <stdio.h>

union test {
	int x;
	char y;
};

int main()
{
	union test p1;
	p1.x = 65;

	// p2 is a pointer to union p1
	union test* p2 = &p1;

	// Accessing union members using pointer
	printf("%d %c", p2->x, p2->y);
	return 0;
}

output:

65 A

Let’s have a look at the pictorial representation of the memory allocation.

The below figure shows the pictorial representation of the structure. The structure has two members; i.e., one is of integer type, and the another one is of character type. Since 1 block is equal to 1 byte; therefore, ‘a’ variable will be allocated 4 blocks of memory while ‘b’ variable will be allocated 1 block of memory.

The below figure shows the pictorial representation of union members. Both the variables are sharing the same memory location and having the same initial address.