What is union of structure in C language?

A union is a memory location that is shared by several variables of different data types in C programming language.

Syntax

The syntax for union of structure is as follows −

union uniontag{
   datatype member 1;
   datatype member 2;
   ----
   ----
   datatype member n;
};

Example

The following example shows the usage of union of structure −

union sample{
   int a;
   float b;
   char c;
};

Declaration of union variable

Following is the declaration for union variable. It is of three types as follows −

Type 1

union sample{
   int a;
   float b;
   char c;
}s;

Type 2

union{
   int a;
   float b;
   char c;
}s;

Type 3

union sample{
   int a;
   float b;
   char c;
};
union sample s;

• When union is declared, the compiler automatically creates largest size variable type to hold variables in the union.

• At any time, only one variable can be referred.

Initialization and accessing

• Same syntax of structure is used to access a union member.

• The dot operator is for accessing members.

• The arrow operator ( ->) is used for accessing the members using pointer.

Sample program 1

The following program shows the usage of union of structure.

union sample{
   int a;
   float b;
   char c;
}
main ( ){
   union sample s = {10, 20.5, "A"};
   printf("a=%d",s.a);
   printf("b=%f",s.b);
   printf("c=%c",s.c);
}

Output

When the above program is executed, it produces the following result −

a = garbage value
b = garbage value
c = A

Union of structures

• A structure can be nested inside a union and it is called union of structures.

• It is possible to create a union inside a structure.

Sample Program 2

An another C program which shows the usage of union of structure is given below −

struct x{
   int a;
   float b;
};
union z{
   struct x s;
};
main ( ){
   union z u;
   u.s.a = 10;
   u.s.b = 30.5;
   printf("a=%d", u.s.a);
   printf("b=%f", u.s.b);
}

Output

When the above program is executed, it produces the following result −

a= 10
b = 30.5