DECC$DISABLE_POSIX_ROOT

   With DECC$DISABLE_POSIX_ROOT enabled, support for the POSIX root
   directory defined by SYS$POSIX_ROOT is disabled.

   With DECC$DISABLE_POSIX_ROOT disabled, the SYS$POSIX_ROOT logical
   name is interpreted as the equivalent of the file path "/". If
   a UNIX path starting with a slash (/)  is given and the value
   after the leading slash cannot be translated as a logical name,
   SYS$POSIX_ROOT is used as the parent directory for the specified
   UNIX file path.

   The C RTL supports a UNIX style root that behaves like a real
   directory. This allows such actions as:

   % cd /
   % mkdir /dirname
   % tar -xvf tarfile.tar /dirname
   % ls /

   Previously, the C RTL did not recognize "/" as a directory name.
   The normal processing for a file path starting with "/" was to
   interpret the first element as a logical name or device name. If
   this failed, there was special processing for the name /dev/null
   and names starting with /bin and /tmp:

   /dev/null       NLA0:
   /bin            SYS$SYSTEM:
   /tmp            SYS$SCRATCH:

   These behaviors are retained for compatibility purposes. In
   addition, support has been added to the C RTL for the logical
   name SYS$POSIX_ROOT as an equivalent to "/".

   To enable this feature for use by the C RTL, define SYS$POSIX_
   ROOT as a concealed logical name. For example:

   $ DEFINE/TRANSLATION=(CONCEALED,TERMINAL) SYS$POSIX_ROOT -
                                       "$1$DKA0:[SYS0.abc.]"

   To disable this feature:

   $ DEFINE DECC$DISABLE_POSIX_ROOT DISABLE

   Enabling SYS$POSIX_ROOT results in the following behavior:

   o  If the existing translation of a UNIX path starting with "/"
      fails and SYS$POSIX_ROOT is defined, the name is interpreted
      as if it starts with /sys$posix_root.

   o  When converting from an OpenVMS to a UNIX style filename,
      and the OpenVMS name starts with "SYS$POSIX_ROOT:", then
      the "SYS$POSIX_ROOT:" is removed. For example, SYS$POSIX_
      ROOT:[dirname] becomes /dirname. If the resulting name
      could be interpreted as a logical name or one of the special
      cases previously listed, the result is /./dirname instead of
      /dirname.