Union in c

What is union in C?

Union in c is user-defined data type that stores different type of elements.

At once only one member can occupy memory.

Thus the size of a union at any given time is equal to the size of its largest element.

Syntax

union [union tag]{
 member definition;
 member definition;
 ...
}[member variable(s)];

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Example

union student{
 char name[30];
 int roll_number;
}s1;

In the above example, student is union type and it contains 2 members i.e. name and roll_number. The size of the union is its largest member therefore in the above example, the size of student datatype is 30 bytes.

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Accessing union members

Union members can be accessed through .(dot) operator.

For pointers using arrow operator (->).

Example

// C program to illustrate union
#include<stdio.h>
#include<string.h>
union student{
   char name[30];
   int roll_number;
  }s1;
int main()
{
  strcpy(s1.name,"Ram");
  s1.roll_number=20;
  printf("Name of the student %s \n",s1.name);
  printf("Roll number is %d \n",s1.roll_number);
  return 0;
}  

Output

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Pointer to union

Pointer to union can be created just like pointer to other primitive datatypes.

Example

// C program to illustrate pointer to union
#include<stdio.h>
int main(){
  union unit{
     int m;
     int n;
  };
  union unit a={100};
  union unit* pointer=&a;
  printf("Value of m is %d \n",pointer->m);
  pointer->m=200;
  printf("Value of m is %d \n",pointer->m);
  return 0;
}

Output

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Applications of union

• Hardware input/output access
• Bitfield sharing
• word sharing
• Low level polymorphism

Checkout how to cast to union in GNU documentation.

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Advantage and disadvantage of union

Advantages of union

• Space efficiency: It takes less memory space as compared to the structure.
• It allocates memory size to all its members to the size of its largest member.

Disadvantage of union

• It allows to access only one of it’s member at a time.

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