runner% cat writebase.c
#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);
   }
}

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;
}

runner% writebase
input base (in base 10):10
Input number (in base 10):74
Base 10: 74
Base 8: 112
Base 2: 1001010
input base (in base 10):8
Input number (in base 8):112
Base 10: 74
Base 8: 112
Base 2: 1001010
input base (in base 10):2
Input number (in base 2):1001010
Base 10: 74
Base 8: 112
Base 2: 1001010
input base (in base 10):10
Input number (in base 10):93
Base 10: 93
Base 8: 135
Base 2: 1011101
input base (in base 10):10
Input number (in base 10):1000
Base 10: 1000
Base 8: 1750
Base 2: 1111101000
input base (in base 10):0
runner%