Difference Between Structure and Union in C

Structure and Union both are used to create a custom data type. As C language is a general-purpose, procedural computer programming language we often need functionality similar to OO language like C++. This is obtained through structure and interfaces.

Structure in C

Structure is used to create a custom data type that groups a number of primitive ones. It is more like several data type elements are packed and created into one data type.

Address of all members is different in memory.

Declaration

struct address
{
   char name;
   char street;
   int city_id;
   int state_id;
   long description;
};

Size of structure

The size of the structure element is the combined size of all elements inside it. That will come to 18 bytes. You can test with the compiler.

#include<stdio.h>

struct address
{
   char name;
   char street;
   int city_id;
   int state_id;
   long description;
};
int main(void) {
    struct address addr;
    printf("%d\n",sizeof(address));
    return 0;
}

Success — 24 bytes output
18 byte + 6 unused bytes added by compiler for alignment in memory.

Union in C

Union is used to create a custom data type that is a union of all members declared into it. Basically a data type of minimum size that can hold a value of any kind of its member.

Address of all members is always the same in memory.

Declaration

union address
{
   char name;
   char street;
   int city_id;
   int state_id;
   long description;
};

Size of Union

Size of union is the size of the largest member.

#include<stdio.h>

union address
{
   char name;
   char street;
   int city_id;
   int state_id;
   long description;
};
int main(void) {
    union address addr;
    printf("%d\n",sizeof(addr));
    return 0;
}

Success — 8 bytes output
8 byte = long size (largest member)