manual tco :)

tail_recursion_optimisation.c

#include <stdio.h>
#include <stdlib.h>

static int
factorial_helper (int n, int a)
{
 start:
  a *= n;

  if (n-- > 1)
    goto start;

  return a;
}

int factorial (int n) { return factorial_helper (n, 1); }

int
main (int argc, char **argv)
{
  if (argc < 2)
    return 1;

  printf ("%d\n", factorial (atoi (argv[1])));
  return 0;
}