Union in C programming

         Unions are similar to structure. Its syntax and use is similar to structure. It also contains members whose individual data types may differ from one another. The distinction is that all members within union share the same storage area of computer memory, whereas each member within a structure is assigned its own unique storage. Thus, unions are used to conserve memory. Since same memory is shared by all members, one variable can reside into memory at a time. When another variable is set into memory, the previous is replaced i.e. previous can not persist. Thus, unions are useful for applications involving multiple members where values need to be assigned to all of the members at any one time. Therefore, although a union may contain many members of different types, it can handle only one member at a time.

             The compiler allocates a piece of storage that is large enough to hold the largest variable type in the union. For example

         The union is created as

                     union student

                               {

                                 int roll;

                                 float marks;

                              } ;

Here, the union student has members roll and marks. The data type of roll is integer which contains 2 bytes in memory and the data type of marks is float which contains 4 bytes in memory. As all union members share same memory, the compiler allocates larger memory (i.e. 4 bytes in this case).

         The declaration of union and its variable is similar to that of structure. The union variable can be passed to function and it can be member of any other structure. The union can be illustrated as following examples;

Example:

             Create an union named student that has roll and marks as member. Assign some values to these members one at a time and display the result one at a time.

 void main()

  {

   union student

            {   int roll;

                float marks;

             };

 union student st;

  st.roll=455;

 printf("\nRoll=%d",st.roll);

 st.marks=78;

 printf("\nMarks=\t%f",st.marks);

   }

output:

Roll=455

Marks=  78.000000

            If two members are used simultaneously, the output is unexpected as following

void main()

  {

   union student

        {     int roll;

             float marks;

         };

 union student st;

  st.roll=455;

  st.marks=78;

printf("\nRoll=%d",st.roll);

printf("\nMarks=\t%f",st.marks);

   }

output:

Roll=0

Marks=  78.000000