using memmapfile()

Hi!
I have some difficulties using the new memmapfile function.
Data is stored in an ASCII file (3gb large) consisting of m lines.
each line holds n double values. ind is an array holding line
numbers.
I would like to retrieve all lines defined by ind to a double array.
my efforts with memmapfile returned my some struct which I couldn´t
use...I looked through the docs for the 'format' property, but what I
figured out doesn´t seem to work...
thanks for any help!
felix.
.