XmClipboardInquireLength(3) on Linux
The text 8683e6f6ec9f, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A clipboard function that returns the length of the stored data "XmClipboardInquireLength" "clipboard functions" "XmClipboardInquireLength"