Man page 9 : bitmap_ord_to_pos

bitmap_ord_to_pos - find position of n-th set bit in bitmap

int bitmap_ord_to_pos(const unsigned long *  buf , int  ord , int  bits );

buf
    pointer to bitmap

ord
    ordinal bit position (n-th set bit, n >= 0)

bits
    number of valid bit positions in buf

Map the ordinal offset of bit ord in buf to its position in buf. Value of ord should be in range 0 <= ord < weight(buf), else results are undefined.

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

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