Adding a file to a node programmatically

/**
 * @param node the node to which you want to add the file
 * @param file the path to the file: e.g.: /tmp/image.jpg
 * @return TRUE if success, FALSE if not
 */
function store_file($node, $file) {
    if (!isset($file) || !file_exists($file)) {
        drupal_set_message('File does not exists');
        return FALSE;
    }
 
    $details = stat($file);
    $filesize = $details['size'];
    $dest = file_directory_path();
    if(!file_copy($file,$dest)) {
        drupal_set_message("Failed to move file: $file");
        return FALSE;
    } else {
        $name = basename($file);
    }
 
    // build the file object
    $file_obj = array();
    $file_obj['filename'] = $name;
    $file_obj['filepath'] = $file;
    $file_obj['filemime'] =  file_get_mimetype($name);
    $file_obj['filesize'] = $filesize;
    $file_obj['status'] = FILE_STATUS_TEMPORARY;
    $file_obj['timestamp'] = time();
    $file_obj['list'] = 1;
    $file_obj['new'] = TRUE;
    $file_obj['data']['alt'] = 'foo';
    $file_obj['data']['title'] = 'bar';
    $file_obj['uid'] = '1'; // 1 = admin, but you can do here whatever you want ...
 
    // save file to files table, fid will be set
    drupal_write_record('files', $file_obj);
 
    // change file status to permanent
    file_set_status($file_obj, 1);
 
    // attach the file object to your node
    $node->field_main_picture[0] = $file_obj; // property can be different 
    return TRUE;
}