Find the position of the first bit set in mask, searching
from (but not including) nth_bit downwards. Bits are numbered
from 0 (least significant) to sizeof(gulong) * 8 - 1 (31 or 63,
usually). To start searching from the last bit, set nth_bit to
-1 or GLIB_SIZEOF_LONG * 8.
Find the position of the first bit set in mask, searching from (but not including) nth_bit downwards. Bits are numbered from 0 (least significant) to sizeof(gulong) * 8 - 1 (31 or 63, usually). To start searching from the last bit, set nth_bit to -1 or GLIB_SIZEOF_LONG * 8.