How copy data to GstMapInfo user_data?

hi, how to copy data to GstMapInfo user_data? my code as follow:

buffer = gst_buffer_new_allocate (NULL, (guint)(imageData.size + imageID.length()), NULL);
gst_buffer_map (buffer, &map, GST_MAP_WRITE);
memcpy( map.data, imageData.data, imageData.size);
memcpy(map.user_data, imageID.c_str(), imageID.length() );

but run error.

Hi,
You may refer to this sample: