Syntax of Union in C

The syntax of the union in C can be divided into three steps which are as follows:

C Union Declaration

In this part, we only declare the template of the union, i.e., we only declare the members’ names and data types along with the name of the union. No memory is allocated to the union in the declaration.

union union_name {
    datatype member1;
    datatype member2;
    ...
};

Keep in mind that we have to always end the union declaration with a semi-colon.

Different Ways to Define a Union Variable

We need to define a variable of the union type to start using union members. There are two methods using which we can define a union variable.

1. With Union Declaration
2. After Union Declaration

1. Defining Union Variable with Declaration

union union_name {
    datatype member1;
    datatype member2;
    ...
} var1, var2, ...;

2. Defining Union Variable after Declaration

union union_name var1, var2, var3...;

where union_name is the name of an already declared union.

Access Union Members

We can access the members of a union by using the ( . ) dot operator just like structures.

var1.member1;

where var1 is the union variable and member1 is the member of the union.

The above method of accessing the members of the union also works for the nested unions.

var1.member1.memberA;

Here,

• var1 is a union variable.
• member1 is a member of the union.
• memberA is a member of member1.

Initialization of Union in C

The initialization of a union is the initialization of its members by simply assigning the value to it.

var1.member1 = some_value;

One important thing to note here is that only one member can contain some value at a given instance of time.

The Union is a user-defined data type in C language that can contain elements of the different data types just like structure. But unlike structures, all the members in the C union are stored in the same memory location. Due to this, only one member can store data at the given instance.