Get post id in a function when edit/add a post

I am new to PHP and I am working in WordPress. I wrote a function in the functions.php file. My function is to update a custom field based on another field on the page when the user adds or edits a post. I am having trouble getting the post id in my function. How can I get the Post Id? What am I doing wrong?

Here is my function:

  function get_postid() {
    global $post;
    $id = $post->ID;
  }

  add_action( 'admin_notices', 'get_postid' );

  function set_post_sort_order() {
    /* Get post id */
    $post_id = get_postid();

    
    /* Does object exist */
    if ( !$post_id ):

        /* Get which product taxonomy is selected. */
        $terms = get_the_terms( $post_id, 'product' );
        $sort_order = 2;

        if ( ! empty( $terms ) ) :
          foreach( $terms as $term ) {
            if ( $term->name == 'blah blah' ) :
                $sort_order = 1;
            endif;

            update_post_meta( $post_id, 'post_order', $sort_order );
          } 
        endif;

    endif;
  }

Answers:

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Method 1

1. Your function get_postid() does not return anything. It should return the id:

function get_postid() {
    global $post;
    return $post->ID; 
}

2. If you are going to use the $post_id I guess your first if statement is wrong, because the $post_id should be evaluated to true not false. So it should be like this: if ( $post_id ):

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