Question

i tried this :

<?php $prev=get_previous_posts_link();
if  (strlen($prev) == 0)
{
    echo "no posts";
}
else
{
    echo $prev;
}
?>

it always returns no posts!

Answer 1

The code you use should be placed within the loop.
ie. inside,

<?php while ( have_posts() ) : the_post(); ?>
.......
.......
<?php endwhile; // end of the loop. ?>

Your code might not be inside the loop and it could be the only reason why it always return no posts!