The C program below does fairly general number base conversions. It doesn't handle bases bigger than 10, though that would be fairly easy to include.

/* writebase.c: convert between number bases in C
 *   This program does not handle bases > 10 nor
 *   does it handle negative numbers.
 *   Written by Neal R. Wagner, 7 Sept 1999
 */
#include <stdio.h>
#include <ctype.h>

void dumpout(int n, int base);
void writebase(int n, int base);
int readbase(int base);

void main(void) {
   int n, base;
   for( ; ; ) {
      printf("input base (in base 10)>>>");
      base = readbase(10);
      if (base <= 0) break;
      printf("Input number (in base ");
      writebase(base, 10);
      printf(")>>>");
      n = readbase(base);
      dumpout(n, 10);
      dumpout(n, 8);
      dumpout(n, 2);
      printf("--------------------------\n");
   }
}

void dumpout(int n, int base) {
   printf("Base "); writebase(base, 10);
   printf(": ");
   writebase(n, base); putchar('\n');
}

void writebase(int n, int base) {
   if (n != 0) {
      writebase(n/base, base);
      putchar((n%base) + '0');
   }
}

int readbase(int base) {
   char c;
   int i = 0;
   while (isdigit(c = getchar()))
      i = base*i + (c - '0');
   return i;
}

% cc -o writebase writebase.c
% writebase
input base (in base 10)>>>8
Input number (in base 8)>>>23456
Base 10: 10030
Base 8: 23456
Base 2: 10011100101110
--------------------------
input base (in base 10)>>>10
Input number (in base 10)>>>99999
Base 10: 99999
Base 8: 303237
Base 2: 11000011010011111
--------------------------
input base (in base 10)>>>0