Compatibility_Modes

   The following predefined macros are used to select header-file
   compatibility with previous versions of DEC C or the OpenVMS
   operating system:

   o  _DECC_V4_SOURCE

   o  _VMS_V6_SOURCE

   There are two types of incompatibilities that can be controlled
   in the header files:

   o  To conform to standards, some changes are source-code
      incompatible but binary compatible. To select DEC C Version
      4.0 source compatibility, use the _DECC_V4_SOURCE macro.

   o  Other changes to conform to standards introduce a binary or
      run-time incompatibility.

      In general, programs that recompile get new behaviors. In
      these cases, use the _VMS_V6_SOURCE feature test macro to
      retain previous behaviors.

      However, for the exit, kill, and wait functions, the OpenVMS
      Version 7.0 changes to make these routines ISO POSIX-1
      compliant were considered too incompatible to become the
      default. Therefore, in these cases the default behavior is
      the same as on pre-OpenVMS Version 7.0 systems. To access the
      versions of these routines that comply with ISO POSIX-1, use
      the _POSIX_EXIT feature test macro.

   The following examples help clarify the use of these macros:

   o  To conform to the ISO POSIX-1 standard, typedefs for the
      following have been added to <types.h>:

         dev_t         off_t
         gid_t         pid_t
         ino_t         size_t
         mode_t        ssize_t
         nlink_t       uid_t

      Previous development environments using a version of DEC C
      earlier than Version 5.2 may have compensated for the lack
      of these typedefs in <types.h> by adding them to another
      module. If this is the case on your system, then compiling
      with the <types.h> provided with DEC C Version 5.2 might cause
      compilation errors.

      To maintain your current environment and include the DEC C
      Version 5.2 <types.h>, compile with _DECC_V4_SOURCE defined.
      This will omit incompatible references from the DEC C Version
      5.2 headers. In <types.h>, for example, the previously listed
      typedefs will not be visible.

   o  As of OpenVMS Version 7.0, the C RTL getuid and
      geteuid functions are defined to return an OpenVMS UIC (user
      identification code) that contains both the group and member
      portions of the UIC. In previous versions of the DEC C RTL,
      these functions returned only the member number from the UIC
      code.

      Note that the prototypes for getuid and geteuid in <unistd.h>
      (as required by the ISO POSIX-1 standard) and in <unixlib.h>
      (for C RTL compatibility) have not changed. By default,
      newly compiled programs that call getuid and geteuid get
      the new definitions. That is, these functions will return
      an OpenVMS UIC.

      To let programs retain the pre-OpenVMS Version 7.0 behavior of
      getuid and geteuid, compile with the _VMS_V6_SOURCE feature-
      test macro defined.

   o  As of OpenVMS Version 7.0, the C RTL exit function is
      defined with ISO POSIX-1 semantics. As a result, the input
      status argument to exit takes a number between 0 and 255.
      (Prior to this, exit could take an OpenVMS condition code in
      its status parameter.)

      By default, the behavior for exit on OpenVMS systems is the
      same as before: exit accepts an OpenVMS condition code. To
      enable the ISO POSIX-1 compatible exit function, compile with
      the _POSIX_EXIT feature-test macro defined.