Union in C

Union in c language is a user defined datatype that is used to hold different type of elements.
But it doesn't occupy sum of all members size. It occupies the memory of largest member only. It shares memory of largest member.

Advantage of union over structure

it occupies less memory because it occupies the memory of largest member only.

Disadvantage of union over structuree

It can store data in one member only.

Defining union

The union keyword is used to define union. Let's see the syntax to define union in c. 
union union_name   
{  
data_type member1;  
data_type member2;  
  .  
   .  
data_type memeberN;  
};
see the example to define union for student. 
union student {
int id;  
char name[60];  
float balance;  
 };

Union example-1

see a simple example of union . 
#include <stdio.h>
#include <string.h>
union student {  
int id;    
char name[60];    
  } s1;   //declaring e1 variable for union  
int main( )  
  {  
  //store first student information  
s1.id=110;  
strcpy(s1.name, "Pramesh gupta");   //copying string into char array  
    //printing first student information  
printf( "student 1 id : %d\n", s1.id);  
printf( "student 1 name : %s\n", s1.name);  
 return 0;  
   }  
Output:
student 1 id : 1835102800
student 1 name : Pramesh gupta

As you can see, id gets garbage value because name has large memory size. So only name will have actual value.

Union example-2

#include <stdio.h>
#include <string.h>
union student {  
int id;    
char name[60];    
  } s1;   //declaring e1 variable for union  
int main( )  
  {  
  //store first student information  
s1.id=110;  
printf( "student 1 id : %d\n", s1.id);  
strcpy(s1.name, "Pramesh gupta");   //copying string into char array   

printf( "student 1 name : %s\n", s1.name);  
 return 0;  
   }  
Output:
student 1 id : 110
student 1 name : Pramesh gupta

As you can see, id gets 110 value and name.