A Very Basic yet important question in a life of a software developer

     Convert Binary to Decimal,Octal,Hexadecimal
   #include<stdio.h>
   #include<string.h>
   void hexadecimal();
   void main()
   {
     int num, bnum, dec = 0, base = 1, rem ,dec1=0,oct[25],dec2=0,flag=0,i=0,counter=0,j;
     printf("Enter the binary number(1s and 0s)\n");
     scanf("%d", &num);
     bnum = num;
     while( num > 0)
     {
       rem = num % 10;
       if((rem==0) || (rem==1))
       {
         dec = dec + rem * base;
         num = num / 10 ;
         base = base * 2;
         flag=1;
       }
       else
       {
         flag=0;
         printf("\n Enter binary number \n");
         break;
       }
     }
     if(flag==1)
     {
       printf("The Binary number is = %d\n", bnum);
       printf("Its decimal equivalent is =%d\n", dec);
       dec1=dec;
       dec2=dec1;
       while(dec>0)
       {
         rem=dec%8;
         oct[i]=rem;
         dec=dec/8;
         i++;
         counter++;
       }
       counter--;
       printf("\n Its octal equivalent is:");
       while(counter>=0)
       {
         printf("%d" ,oct[counter]);
         counter--;
       }
       printf("\nIts Hexa Decimal equivalant is: ");
       hexadecimal(dec2);
     }
   }
   void hexadecimal(long n)
   {
     long i;
     if(n>0)
     {
       i=n%16;
       n=n/16;
       hexadecimal(n);
       if(i>=10)
       {
         switch(i)
         {
           case 10:
             printf("A");
             break;
           case 11:
             printf("B");
             break;
           case 12:
             printf("C");
             break;
           case 13:
             printf("D");
             break;
           case 14:
             printf("E");
             break;
           case 15:
             printf("F");
             break;
         }
       }
       else
         printf("%ld",i);
     }
   }