VMS Help CC, /STANDARD  *Conan The Librarian (sorry for the slow response - running on an old VAX)

     /STANDARD=(option)
     /STANDARD=RELAXED_ANSI89 (equivalent to /NOSTANDARD) (D)
     /NOSTANDARD (D)

  Defines the compilation mode.  You can select the following
  options:

  ANSI89            Places the compiler in strict ANSI C Standard 89
                    (C89) mode.  This mode compiles the C language as
                    defined by the American National Standard for C,
                    along with any extensions not prohibited by that
                    standard.

  C99               On OpenVMS Alpha systems, places the compiler in
                    strict ISO/IEC C Standard 99 (C99) mode.  This
                    mode accepts just the C99 language without
                    extensions, and diagnoses violations of the C99
                    standard.

                    On OpenVMS VAX systems, produces a warning and
                    places the compiler in /STANDARD=REALXED ANSI89
                    mode.

  RELAXED_ANSI89    Places the compiler in relaxed ANSI C Standard
                    mode.  The compiler accepts ANSI/ISO C Standard
                    C89 and C99 features, as well as nearly all
                    language extensions (such as additional Compaq C
                    keywords and predefined macros that do not begin
                    with an underscore).  It excludes only K&R
                    (COMMON mode), VAX C, and Microsoft features that
                    conflict with standard C.

                    This is the default mode of the compiler, and is
                    equivalent to /NOSTANDARD.

  MS                Places the compiler in Microsoft compatibility
                    mode, which interprets source programs according
                    to certain language rules followed by the C
                    compiler provided with the Microsoft Visual C++
                    compiler product.

  ISOC94            Places the compiler in ISO C 94 mode, which
                    enables digraph processing and defines the
                    predefined macro __STDC_VERSION__=199409.

                    This option can be specified alone or with any
                    other /STANDARD option except VAXC.  If it is
                    specified alone, the default major mode is
                    RELAXED_ANSI89.

  COMMON            Places the compiler in K & R language mode; that
                    is, compatibility with older UNIX compilers such
                    as pcc and gcc.  This mode is close to a subset
                    of /STANDARD=VAXC mode.

  VAXC              Places the compiler in VAX C mode.  There are
                    differences in the C language as implemented in
                    previous versions of VAX C and the C language as
                    defined by ANSI (the differences are primarily
                    concerned with how the preprocessor works).  This
                    mode provides compatibility for programs that
                    depend on old VAX C behavior.

  PORTABLE          Places the compiler in RELAXED_ANSI89 mode, and
                    enables the issuance of diagnostics that warn of
                    any nonportable usages encountered.

                    Note that /STANDARD=PORTABLE is supported for
                    VAX C compatibility only.  It is equivalent to
                    the recommended combination of qualifiers
                    /STANDARD=RELAXED_ANSI89
                    /WARNINGS=ENABLE=PORTABLE.

  MIA               Places the compiler in strict ANSI C mode with
                    the following behavior differences to conform to
                    the Multivendor Integration Architecture (MIA)
                    standard:

                    o  On OpenVMS VAX systems, G_FLOAT becomes the
                       default floating-point format for double
                       variables.  (On OpenVMS Alpha systems, G_FLOAT
                       is already the default.)

                    o  In structures, zero-length bit fields cause
                       the next bit field to start on an integer
                       boundary, rather than on a character boundary.

  The default is /NOSTANDARD, which is equivalent to
  /STANDARD=RELAXED_ANSI89.

  With one exception, the /STANDARD qualifier options are mutually
  exclusive.  Do not combine them.  The exception is that you can
  specify /STANDARD=ISOC94 with any other option except VAXC.

  Compaq C modules compiled in different modes can be linked and
  executed together.