XmTextGetSelectionPosition(3) on Linux
The text 7091a5f4c304, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A Text function that accesses the position of the primary selection "XmTextGetSelectionPosition" "Text functions" "XmTextGetSelectionPosition"