Man page 9 : bitmap_pos_to_ord

bitmap_pos_to_ord - find ordinal of set bit at given position in bitmap

int bitmap_pos_to_ord(const unsigned long *  buf , int  pos , int  bits );

buf
    pointer to a bitmap

pos
    a bit position in buf (0 <= pos < bits)

bits
    number of valid bit positions in buf

Map the bit at position pos in buf (of length bits) to the ordinal of which set bit it is. If it is not set or if pos is not a valid bit position, map to -1.

If for example, just bits 4 through 7 are set in buf, then pos values 4 through 7 will get mapped to 0 through 3, respectively, and other pos values will get mapped to 0. When pos value 7 gets mapped to (returns) ord value 3 in this example, that means that bit 7 is the 3rd (starting with 0th) set bit in buf.

The bit positions 0 through bits are valid positions in buf.