Confusion with gmp_printf

[Bruce M. Axtens]

Torbjorn Granlund wrote:
> If I am right, you need to do the logarithm fraction, and compute the
> value of n from the number of bits you asked for when initializing f.
>
> log(2)/log(10) = 0.3010299957...
>   
Possibly one could use a call like this
    buff = mpf_get_str( NULL, &exp, 10, 0, res );

and do some arithmetic with exp and strlen( buff ) to calculate the 
number of digits either side of the decimal:
    gmp_printf ("%*.*Ff", digits_to_the_left, digits_to_the_right, res );

The troubles is, what do you do when you get a negative value in exp?

Kind regards,
Bruce.