C program to convert Decimal number to Octal number:
If you are using any software then below program will not give an error, but if you are using TURBO C then you have to make some changes like: void main() and some function like clrscr() or getch().

#include<stdio.h>
#include<math.h>

// Function For Calculating OCTAL Number
void dec_oct(long int num)
{
  long int rem[50],i=0,length=0;
  while(num>0)
  {
   rem[i]=num%8;
   num=num/8;
   i++;
   length++;
  }
  printf("Octal number : "); //Printing Octal Number
     for(i=length-1;i>=0;i--)
        printf("%ld",rem[i]);
}

//Main Function
main()
{
long int num;

 printf("Enter the decimal number : ");
 scanf("%ld",&num);

    dec_oct(num);   //Function Call

}

OUTPUT:

Enter the decimal number : 45
Octal number : 55