#include 
void change(int,int);
int main()
{
    int a=10,b=20;
    change(a,b); //calling a function by passing the values of variables.
    printf("Value of a is: %d",a);
    printf("\n");
    printf("Value of b is: %d",b);
    return 0;
}
void change(int x,int y)
{
    x=13;
    y=17;
}

Value of a is: 10
Value of b is: 20

#include 
void change(int*,int*);
int main()
{
    int a=10,b=20;
    change(&a,&b); // calling a function by passing references of variables.
    printf("Value of a is: %d",a);
    printf("\n");
    printf("Value of b is: %d",b);
    return 0;
}
void change(int *x,int *y)
{
    *x=13;
    *y=17;
}

Value of a is: 13
Value of b is: 17

#include 
int calculate_fact(int);
int main()
{
 int n=5,f;
 f=calculate_fact(n); // calling a function
 printf("factorial of a number is %d",f);
  return 0;
}
int calculate_fact(int a)
{
  if(a==1)
  {
      return 1;
  }
  else
  return a*calculate_fact(a-1); //calling a function recursively.
   }

factorial of a number is 120

data_type array_name[size];

data_type array_name[size];

#include 
int main()
{
   int arr[5]={1,2,3,4,5}; //an array consists of five integer values.
   for(int i=0;i<5;i++)
   {
       printf("%d ",arr[i]);
   }
    return 0;
}

1 2 3 4 5

Data_type *p;

#include 
int main()
{
   int *p; //pointer of type integer.
   int a=5;
   p=&a;
   printf("Address value of 'a' variable is %u",p);
    return 0;
}

Address value of 'a' variable is 428781252

           #include
           void main()
           {
                   int *ptr = malloc(constant value); //allocating a memory space.
                   free(ptr); //ptr becomes a dangling pointer.
           }

     #include
           void main()
           {
                   int *ptr = malloc(constant value); //allocating a memory space.
                   free(ptr); //ptr becomes a dangling pointer.
                   ptr=NULL; //Now, ptr is no longer a dangling pointer.
           }

#include 
 int main()
{
    int a=10;
    int *ptr,**pptr; // *ptr is a pointer and **pptr is a double pointer.
    ptr=&a;
    pptr=&ptr;
    printf("value of a is:%d",a);
    printf("\n");
    printf("value of *ptr is : %d",*ptr);
    printf("\n");
    printf("value of **pptr is : %d",**pptr);
    return 0;
}

For example:
int a[10];

For example
int *p= malloc(sizeof(int)*10);

struct structure_name
{
  Member_variable1;
 Member_variable2
.
.
}[structure variables];

#include 
struct student
{
    char name[10];       // structure members declaration.
    int age;
}s1;      //structure variable
int main()
{
    printf("Enter the name");
    scanf("%s",s1.name);
    printf("\n");
    printf("Enter the age");
    scanf("%d",&s1.age);
    printf("\n");
    printf("Name and age of a student: %s,%d",s1.name,s1.age);
    return 0;
}

Enter the name shikha
Enter the age 26
Name and age of a student: shikha,26  

union union_name
{
Member_variable1;
Member_variable2;
.
.
Member_variable n;
}[union variables];

#include
union data
{
    int a;      //union members declaration.
    float b;
    char ch;
};
int main()
{
  union data d;       //union variable.
  d.a=3;
  d.b=5.6;
  d.ch='a';
  printf("value of a is %d",d.a);
  printf("\n");
  printf("value of b is %f",d.b);
  printf("\n");
  printf("value of ch is %c",d.ch);
  return 0;
}

value of a is 1085485921
value of b is 5.600022
value of ch is a

int sprintf ( char * str, const char * format, ... );

 #include
int main()
{
 char a[20];
 int n=sprintf(a,"javaToint");
 printf("value of n is %d",n);
 return 0;}

value of n is 9

#include  
#define start main  
void start() {  
   printf("Hello");  
}  

main(int count, char *args[]){
//code to  be executed
}

#include
#include
int main()
{
    
 char ch;
 printf("Enter a character ");
 ch=getch(); // taking an user input without printing the value.
 printf("\nvalue of ch is %c",ch);
 printf("\nEnter a character again ");
 ch=getche(); // taking an user input and then displaying it on the screen.
  printf("\nvalue of ch is %c",ch);
 return 0;
}

Enter a character
value of ch is a
Enter a character again a
value of ch is a

(type_name) expression;

for(;;){
//code to be executed
}

while(1){
//code to be executed
}

do{
//code to be executed
}while(1);

#include    
void main(){    
 if(printf("hello world")){} // It prints the ?hello world? on the screen.
}   

#include    
#include    
main()    
{    
int a=10, b=20;    //declaration of variables.
clrscr();        //It clears the screen.
printf("Before swap a=%d b=%d",a,b);      
    
a=a+b;//a=30 (10+20)     
b=a-b;//b=10 (30-20)    
a=a-b;//a=20 (30-10)    
    
printf("\nAfter swap a=%d b=%d",a,b);    
getch();    
}

#include  
#include  
void main()  
{  
 int n1=0,n2=1,n3,i,number;  
 clrscr();  
 printf("Enter the number of elements:");  
 scanf("%d",&number);  
 printf("\n%d %d",n1,n2);//printing 0 and 1  
  
 for(i=2;i

#include    
#include    
void printFibonacci(int n) // function to calculate the fibonacci series of a given number.
{    
static int n1=0,n2=1,n3;    // declaration of static variables.
    if(n>0){    
         n3 = n1 + n2;    
         n1 = n2;    
        n2 = n3;    
         printf("%d ",n3);    
         printFibonacci(n-1);    //calling the function recursively.
    }    
}    
void main(){    
    int n;    
    clrscr();    
    printf("Enter the number of elements: ");    
    scanf("%d",&n);    
    printf("Fibonacci Series: ");    
    printf("%d %d ",0,1);    
    printFibonacci(n-2);//n-2 because 2 numbers are already printed    
    getch();    
}    

#include    
#include    
void main()    
{    
int n,i,m=0,flag=0;    //declaration of variables.
clrscr();    //It clears the screen.
printf("Enter the number to check prime:");    
scanf("%d",&n);    
m=n/2;    
for(i=2;i<=m;i++)    
{    
if(n%i==0)    
{    
printf("Number is not prime");    
flag=1;    
break;    //break keyword used to terminate from the loop.
}    
}    
if(flag==0)    
printf("Number is prime");    
getch();    //It reads a character from the keyword.
}

#include  
#include  
main()  
{  
int n,r,sum=0,temp;  
clrscr();  
printf("enter the number=");  
scanf("%d",&n);  
temp=n;  
while(n>0)  
{  
r=n%10;  
sum=(sum*10)+r;  
n=n/10;  
}  
if(temp==sum)  
printf("palindrome number ");  
else  
printf("not palindrome");  
getch();  
}  

#include  
#include  
void main(){  
  int i,fact=1,number;  
  clrscr();  
  printf("Enter a number: ");  
  scanf("%d",&number);  
  
  for(i=1;i<=number;i++){  
      fact=fact*i;  
  }  
  printf("Factorial of %d is: %d",number,fact);  
  getch();  
}  

#include    
#include    
 long factorial(int n)    // function to calculate the factorial of a given number. 
{    
  if (n == 0)    
    return 1;    
else    
return(n * factorial(n-1));    //calling the function recursively.
}    
 void main()    
{    
  int number;    //declaration of variables.
  long fact;    
 clrscr();    
  printf("Enter a number: ");    
scanf("%d", &number);     
 fact = factorial(number);    //calling a function.
printf("Factorial of %d is %ld\n", number, fact);    
 getch();   //It reads a character from the keyword. 
}

#include    
#include    
main()    
{    
int n,r,sum=0,temp;    //declaration of variables.
clrscr(); //It clears the screen.   
printf("enter the number=");    
scanf("%d",&n);    
temp=n;    
while(n>0)    
{    
r=n%10;    
sum=sum+(r*r*r);    
n=n/10;    
}    
if(temp==sum)    
printf("armstrong  number ");    
else    
printf("not armstrong number");    
getch();  //It reads a character from the keyword.
}  

#include    
#include    
main()    
{    
int n, reverse=0, rem;    //declaration of variables.
clrscr(); // It clears the screen.   
printf("Enter a number: ");    
scanf("%d", &n);    
while(n!=0)    
{    
     rem=n%10;    
     reverse=reverse*10+rem;    
     n/=10;    
}    
printf("Reversed Number: %d",reverse);    
getch();  // It reads a character from the keyword.  
}