How to convert a 128-bit integer to a decimal ascii string in C?

Question

I\'m trying to convert a 128-bit unsigned integer stored as an array of 4 unsigned ints to the decimal string representation in C:

unsigned int src[] = { 0x1         


        

Answer 1

Straightforward division base 2^32, prints decimal digits in reverse order, uses 64-bit arithmetic, complexity O(n) where n is the number of decimal digits in the representation:

#include 

unsigned int a [] = { 0x12345678, 0x12345678, 0x12345678, 0x12345678 };

/* 24197857161011715162171839636988778104 */

int
main ()
{
  unsigned long long d, r;

  do
    {
      r = a [0];

      d = r / 10;
      r = ((r - d * 10) << 32) + a [1];
      a [0] = d;

      d = r / 10;
      r = ((r - d * 10) << 32) + a [2];
      a [1] = d;

      d = r / 10;
      r = ((r - d * 10) << 32) + a [3];
      a [2] = d;

      d = r / 10;
      r = r - d * 10;
      a [3] = d;

      printf ("%d\n", (unsigned int) r);
    }
  while (a[0] || a[1] || a[2] || a[3]);

  return 0;
}

EDIT: Corrected the loop so it displays a 0 if the array a contains only zeros. Also, the array is read left to right, a[0] is most-significant, a[3] is least significant digits.