The C RTL supplements the list of environment variables in the
    environ table with all logical names and DCL symbols available to
    the process.

    By default, whenever getenv is called for a name not in the
    environ table, an attempt is made to resolve this as a logical
    name and, if this fails, as a DCL symbol.

    With DECC$ENABLE_GETENV_CACHE enabled, once a logical name or
    DCL name has been successfully translated, its value is stored
    in a cache. When the same name is requested in a future call
    to getenv, the value is returned from the cache instead of
    reevaluating the logical name or DCL symbol.