C Foundation

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C Basics

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What is Operator C Comma Operator C Arithmetic Operators C Relational Operators C Logical Operators C Bitwise Operators C Conditional Operators C : : Operator C Operator Priority

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Control Statements

What is Control Statement C if Statement C if else Statement C Nested if Statement C Else if Statement C Break Statement C Continue Statement C Switch Statement C Goto Statement


What is Control Loop C for Loop C Nested for Loop C while Loop C Nested while Loop C do while Loop C Nested do while loop


What is Function C User Defined Functions C Recursion C Passing Parameters


Scope C Local Scope C Global Scope

Storage Classes

What is Storage Class C Auto C Extern C Static C Register


What is Array C One Dimensional Array C Two Dimensional Array C Multi Dimensional Array C Arrays Of Strings


What is String C String Functions


What is Pointer C Pointers Arithmetic C Pointer to Pointer C Pointers and Arrays C Pointers and Strings C Pointer to Functions Void Pointers Null Pointers C Null and Void Pointer


What is Structure C Struct within Struct C Array within Structure C Pointer to Structure C Structure and Function C Enum C Bitfield Structure C Type def


What is Union


What is File C read a file C write a file C File Handling C Error Handling C Low Level Disk I/O C Other file functions

Memory Allocation

What is Memory Allocation C Malloc() C Calloc() C Free() C Realloc() C Coreleft()

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The ones who are crazy enough to think they can change the world are the ones who do.
- Steve Jobs

C Union

Union is similar to the structures, it also contains number of members like structures. In C, union is identified by the keyword union The members of union share the same memory location within the computer, whereas each structure member has its own memory location within the computer. At any time of compilation, only one member of the union can reside in that memory location. The union require bytes that are equal to the bytes required by largest member within union. The union allocates fixed bytes. Thus, unions are used to conserve or save memory.

Declaration of Union

The declaration of union is similar to structure, except the keyword union instead of struct.

Syntax - Union

union unionname { 
data type member1;
data type member2;
}variable1, variable2;

C program - Difference Between Structure and Union

#include <stdio.h>
int main()
//structures declartion
struct sample{
    double d1;     //occupies 8 bytes in memory
    float f1;     //occupies 4 bytes in memory
//Union declartion
union samp{
    double d2;     //occupies 8 bytes in memory
    float f2;     //occupies 4 bytes in memory
printf("\nSize of Structure : %d ", sizeof(s1));
printf("\nSize of Union : %d ", sizeof(u1));
return 0;
  • Size of Structure : 16
  • Size of Union : 8


The program illustrates that the union allocate bytes, that is equal to its largest member. The size may vary from compilers.

Accessing a Union Member

A union member can be accessed similar to structure member, that by using (.)dot or period operator. The general format is as follows,

#include <stdio.h>
int main()
/structures declartion
struct sample{
    int a;
    int b;
//Union declartion
union samp{
    int c;
    int d;
s1.a = 10;
s1.b = 20;
printf("\nThe Structure member values a and b are : %d %d ", s1. a, s1.b);
u1.c = 30;
u1.d = 40;
printf("\nThe Union member values c and d are : %d %d ", u1. c, u1.d);
return 0;
  • The Structure member values a and b are : 10 20
  • The Union member values c and d are : 40 40


The program illustrates that the existing data of union member is replaced by new data, by this, unions can store one data at any time.

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