Difference between Union and Structure in C in Tabular Form

Structure and Union are user defined data types, capable of holding data of various types.

The basic difference between structure and union is that structure provides a separate memory location for each member, whereas, Union provides a single memory location that is shared by all of its members.

Difference between Union and Structure in C in Tabular Form

Structure

In C, Structures can be defined as a collection of variables of different data types that can be accessed by a single name. Therefore, these elements are together considered as a single entity.

The main advantage of the structure is that it makes easier to handle a large set of data.

In terms of memory allocation, a compiler provides memory to each element and the total size of the structure variable is, therefore, equal to or greater than the sum of all members.

It is possible the size can be more than the sum of its member as machine architecture can be varied.

Points to remember about Structure:

• They are used to allocate sequentially
• Compiler allocates memory to every element of the structure separately
• The size of a structure variable is greater than or equal to the sum of its all elements
• The object of structure can handle all members at a time

Syntax:

structure structure_name{
Variable,
Variable2
}s;

Code example of structure in C:

#include <stdio.h> 
struct struct_name { 
int variable1; 
int variable2 
};
int main( ) {
struct struct_name str;
str.variable1 = 10;
printf( "Variable1 = %d \n",str.variable1);
printf( "Memory size of structure: %d\n", sizeof(str));
return 0;
}

Output:

Variable1 = 10
Memory size of structure : 8

Union

Union is capable of holding only an individual element’s value at a time. A compiler allocates only a single storage area that can be used to store a value of a specific type.

All elements use the same memory area in order to store value regardless of its type.

Points to remember about Union:

• A union can handle only a single member at a time
• The size of the union variable is always equivalent to the member with the largest datatype
• A compiler allocates the memory by considering the largest data element defined in the union

Syntax:

union union_name{
Variable,
Variable2
}u ;

Code example of Union in C :

#include <stdio.h>
 union union_name {
 int variable1;
 int variable2
 };
int main( ) {
union union_name un;
un.variable1 = 10;
printf( "Variable1 = %d \n",un.variable1);
printf( "Memory size of union : %d\n", sizeof(un));
return 0;
}

Output:

Variable1 = 10
Memory size of union : 4

C program to demonstrate the allocation of memory in case of Structure and Union

In case of Structure :

#include <stdio.h>

struct struct_name
{
  int variable1;
int variable2};

int main ()
{
  struct struct_name str;
  str.variable1 = 10;
  str.variable2 = 20;

  printf ("%p\n", &str.variable1);
  printf ("%p\n", &str.variable2);
  return 0;
}

Output:

0x7ffccf6e9260
0x7ffccf6e9264

In Case of Union:

#include <stdio.h>

union union_name
{
  int variable1;
int variable2};

int main ()
{
  union union_name str;
  str.variable1 = 10;
  str.variable2 = 20;

  printf ("%p\n", &str.variable1);
  printf ("%p\n", &str.variable2);
  return 0;
}

Output: 

0x7fff08cacd44
0x7fff08cacd44