Extending a WP_Query with a callback example.

wp-query-extensible-example-callback.php

<?php
/**
 * Changes the job query to show only 5 posts per page.
 *
 * @param  array $args The current query args.
 * @return array       The modified query args.
 */
function wpmark_alter_job_query_args( $args ) {
	
	// change the number of posts per page to 5.
	$args['posts_per_page'] = 5;
	
	// return the args.
	return $args;
	
}

add_filter( 'wpmark_job_query_args', 'wpmark_alter_job_query_args' );