User blog comment:Deedlit11/Bignum Bakeoff appetizer/@comment-5529393-20140916022609/@comment-5529393-20140916041745

Vell reminded me about the ? : operators, which allows the following improvement:

int i = 9 << 99999, b=1;

B(n)	{ int j=0; while (pow(b,++j) <= n); return n ? pow(b+1, B(--j)) + B(n - pow(b,j)) : 0; }

main 	{ while (i*++b) i = B(i)-1; return b;	}

129 characters.