Question

I am trying to figure out how to print floating point numbers without using library functions. Printing the decimal part of a floating point number turned out to be quite easy. Printing the integral part is harder:

static const int base = 2;
static const char hex[] = "0123456789abcdef";

void print_integral_part(float value)
{
    assert(value >= 0);
    char a[129]; // worst case is 128 digits for base 2 plus NUL
    char * p = a + 128;
    *p = 0;
    do
    {
        int digit = fmod(value, base);
        value /= base;
        assert(p > a);
        *--p = hex[digit];
    } while (value >= 1);
    printf("%s", p);
}

Printing the integral part of FLT_MAX works flawlessly with base 2 and base 16:

11111111111111111111111100000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000 (base 2)

ffffff00000000000000000000000000 (base 16)

However, printing in base 10 results in errors after the first 7 digits:

340282368002860660002286082464244022240 (my own function)
340282346638528859811704183484516925440 (printf)

I assume this is a result of the division by 10. It gets better if I use double instead of float:

340282346638528986604286022844204804240 (my own function)
340282346638528859811704183484516925440 (printf)

(If you don’t believe printf, enter 2^128-2^104 into Wolfram Alpha. It is correct.)

Now, how does printf manage to print the correct result? Does it use some bigint facilities internally? Or is there some floating point trick I am missing?

Answer 1

It appears that the work horse for the float to string conversion is the dtoa() function. See dtoa.c in newlib for how they do it.

Now, how does printf manage to print the correct result?

I think it is close to magic. At least the source looks like some kind of dark incantation.

Does it use some bigint facilities internally?

Yes, search for _Bigint in the linked source file.

Or is there some floating point trick I am missing?

Likely.