XmGetXmDisplay(3) on Linux
The text c4c664cde7e2, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A Display function that returns the XmDisplay object ID for a specified display "XmGetXmDisplay" "Display functions" "XmGetXmDisplay"