XmAddProtocolCallback(3) on Linux
The text a9b4a422a467, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A VendorShell function that adds client callbacks for a protocol "XmAddProtocolCallback" "VendorShell functions" "XmAddProtocolCallback" "protocols"