Retrieve the logarithm of number in a specified base ?

[Evadream]

Hello everybody,

I'm discovering this fantastic library, and begin to play with it. 

Here is my problem, maybe there is a better way to do it, so please tell me
if you know it :)

I've got one mpz_t (an integer, for exemple i ), and i would like to find a
to have :

B^a < i < B^(a+1).

My (bad ?) idea is to compute the log of i in the base B (ln(i)/ln(B)) and
to take the integer part, but i don't see any function to deal with log in
a specified base (only natural log, log2 and log10 using mpfr).

Maybe you can help me ?

Thanx !