Unions in C : Understanding Memory Allocation

In C, a union is a type of data structure that allows you to store different data types in the same memory location. A union is similar to a structure, but it only allows one member to be active at a time. The size of a union is the size of its largest member.

   Syntax :
        union union_name
        {
           datatype member_1 ;
           datatype member_2 ;
           datatype member_n ;
        }

This program demonstrates the use of union in C programming language.

The program defines two data structures, one is a struct called "demo" and the other is a union called "udemo". Both of them have the same two members, one is an integer "a" and the other is a character "b".
In the main function, an instance of the union "udemo" is created and named as "o". The program assigns a value 65 to the member "a" of union "o".
The program then uses the printf function to display the value of member "a" and "b" of the union "o".
It's worth noting that the size of the union "udemo" is equal to the size of its largest member, which is 4 bytes for an integer. On the other hand, the size of the struct "dememo" is equal to the sum of its members' size, which is 4 bytes for an integer and 1 byte for a character, a total of 5 bytes.
You can also see the commented lines inside the main function, those lines demonstrate how to find the size of different data types in C and the size of the union and struct defined in the program.
It's also important to note that when you assign a value to one member of the union, the previous values of the other members are overwritten and may not be accessible.
In this program, when you assign 65 to the member a of union, it will be stored in the memory location as an integer and when you try to access it using the member b, it will be interpreted as a character, which is the ASCII value of 65 which is 'A'.

This program is a simple example of how to use union in C and how to access the members of a union, it also demonstrates the difference in memory usage between union and struct.

Source Code

//Union in C Programming
#include<stdio.h>
 
/*
    1.User Define Data Type
    2.Union Members Share the Same Memory Location
    3.Union Size is based in biggest size of a data type
 
*/
 
struct demo
{
    int a;//4
    char b;
};
union udemo
{
    int a;//4
    char b;
}o;
 
int main()
{
   /* printf("\nInteger : %d",sizeof(int));
    printf("\nchar : %d",sizeof(char));
    printf("\nUnion Size : %d",sizeof(union udemo));
    printf("\nStruct Size : %d",sizeof(struct demo));*/
    o.a=65;
    printf("\nUnion A : %d",o.a);
    printf("\nUnion B : %c",o.b);
    return 0;
}

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Output

Union A : 65
Union B : A