XmStringLength(3) on Linux
The text ab387821453c, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A compound string function that obtains the length of a compound string "XmStringLength" "compound string functions" "XmStringLength"