Using add_filter to prepend content, result is out of order

Question

I'm using advanced custom fields to add a few fields to my posts. I want to prepend one of these fields to the content. I think I can do this using filters in wp, but I'm getting an odd result.

My filter to prepend a link to the content.

function add_fields_to_content($content)
{
    $acf_library_url = the_field('acf_library_url');
    $linkDisplay = '<a href="' . $acf_library_url . '">Link</a>';
    return $linkDisplay .= $content;
}
add_filter('the_content', 'add_fields_to_content');

When the above filter runs I'd expect it to prepend a link to my content. For example

<a href="linktoexternalsite.com">Link</a>
... the rest of the content.

Instead, the url is appearing outside of the <a> element and for some reason the URL of the posts is placed inside the href attribute. I think I'm misunderstanding something about how filters work in WP...

Answer 1

The problem not it wp hook,
the_field() displays the value of a specific field, so you need to use get_field() instead, which returns a value.