Multiple-Version-Support_Macro

   By default, the header files enable APIs in the C RTL
   provided by the version of the operating system on which the
   compilation occurs. This is accomplished by the predefined
   setting of the __VMS_VER macro, as described in the VSI C
   User's Guide for OpenVMS Systems. For example, compiling on
   OpenVMS Version 6.2 causes only C RTL APIs from Version
   6.2 and earlier to be made available.

   Another example of the use of the __VMS_VER macro is support
   for the 64-bit versions of C RTL functions available
   with OpenVMS Alpha Version 7.0 and higher. In all header files,
   functions that provide 64-bit support are conditionalized so that
   they are visible only if __VMS_VER indicates a version of OpenVMS
   that is greater than or equal to 7.0.

   To target an older version of the operating system, do the
   following:

   1. Define a logical DECC$SHR to point to the old version of
      DECC$SHR. The compiler uses a table from DECC$SHR to perform
      routine name prefixing.

   2. Define __VMS_VER appropriately, either with the /DEFINE
      qualifier or with a combination of the #undef and #define
      preprocessor directives. With /DEFINE, you may need to disable
      the warning regarding redefinition of a predefined macro.

   Targeting a newer version of the operating system might not
   always be possible. For some versions, you can expect that the
   new DECC$SHR.EXE will require new features of the operating
   system that are not present. For such versions, the defining
   if the logical DECC$SHR in Step 1 would cause the compilation to
   fail.

   To override the value of __VMS_VER, define __VMS_VER_OVERRIDE  on
   the compiler command line. Defining __VMS_VER_OVERRIDE without a
   value sets __VMS_VER to the maximum value.