Difference between Structure and Union in C Language

Structure:

A structure is a user-defined compound data type that consists of data members related to a person or an item. It has the following syntax:

[c]
struct tag_name
{
type data_member1;
type data_member2;
…………..
………..
};
[/c]

where type is the data type of data_members,
data_member1, data_member2 are available which are related to a person or item.
tag_name is the name of the structure which refers to the new compound data type.

Example :
[c]
struct employee
{
int id_no;
char ename[25];
int salary;
};
[/c]

Union:

A union also consists of data members but only one data member is active at a time. The union is useful in cases where the user selects any one data member for the application. It has the following syntax:

[c]
union tag_name
{
type data_member1;
type data_member2;
…………..
………..
};
[/c]
where type is the data type of data_members,
data_member1, data_member2 are the variable.
tag_name is the name of the union.

Structure vs Union:

Structure	Union
1. The keyword struct is used to declare a structure.	1. The keyword union is used to declare a union
2. All data members in a structure are active at time.	2. Only one data member is active at a time.
3. It is useful to declare a compound data type to group data members related to a person or item.	3. It is useful in certain cases where the user will select any one data member in the group of the data members.
4. It is commonly used in most of the applications.	It is not commonly used.