MPI_Info_get_valuelen(3) on Linux
The text 665078c5baf9, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 8 (Jessie), Debian 7 (Wheezy) shipped it.
Get the length of a value for a given key in an MPI_Info object