In certain situations, this function attempts to perform a
    logical name translation on the user-specified argument:

    1. If the argument to getenv does not match any of the
       environment strings present in your environment array, getenv
       attempts to translate your argument as a logical name by
       searching the logical name tables indicated by the LNM$FILE_
       DEV logical, as is done for file processing.

       getenv first does a case-sensitive lookup. If that fails, it
       does a case-insensitive lookup. In most instances, logical
       names are defined in uppercase, but getenv can also find
       logical names that include lowercase letters.

       getenv does not perform iterative logical name translation.

    2. If no logical name exists, getenv attempts to translate
       the argument string as a command-language interpreter (CLI)
       symbol. If it succeeds, it returns the translated symbol text.
       If it fails, the return value is NULL.

       getenv does not perform iterative CLI translation.

    If your CLI is the DEC/Shell, the function does not attempt a
    logical name translation since Shell environment symbols are
    implemented as DCL symbols.

                                   NOTE

       In OpenVMS Version 7.1, a cache of VMS environment variables
       (that is, logical names and DCL symbols) has been added to
       the getenv function to avoid the library making repeated
       calls to translate a logical name or to obtain the value
       of a DCL symbol. By default, the cache is disabled. If
       your application does not need to track changes in OpenVMS
       environment variables that can occur during its execution,
       the cache can be enabled by setting the DECC$ENABLE_
       GETENV_CACHE logical before invoking the application (any
       equivalence string).