"next_posts_link" and "previous_posts_link" display me NOTHING

Question

I'm trying to add links "Previous Post" and "Next Post" in my single.php file without success because previous_posts_link and next_posts_link dont return anything.

Honestly I do not know where in the loop put these buttons.

Thanks.

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

<div class="post-content">
<?php the_title(); the_content(); ?>
</div>

<?php endwhile; ?>

<ul class="nav-posts">
    <li class="prev-link"><?php next_posts_link('« Previous Post') ?></li>
    <li class="next-link"><?php previous_posts_link('Next Post»') ?></li>
</ul>

<?php endif; ?> 

Answer 1

next_posts_link and previous_posts_link (plural) are for archive pagination. For single posts you want next_post_link and previous_post_link (singular).

Answer 2

I was looking for this:

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

<div class="post-content">
<?php the_title(); the_content(); ?>
</div>

<?php endwhile; ?>

<ul class="nav-posts">
    <li class="prev-link"><?php previous_post_link( '%link', '« Previous Post' ); ?></li>
    <li class="next-link"><?php next_post_link( '%link', 'Next Post »' ); ?></li>
</ul>

<?php endif; ?> 

Thanks Milo for the aclaration (plural, singular)