cpp reserved symbols

[Mike Goss]

Regarding the message:
> From: Alan Davis <davis at masig2.ocean.fsu.edu>
> Message-Id: <8908091305.AA01742 at masig2.ocean.fsu.edu>
> Subject: cpp reserved symbols
	
> I have been trying to compile a software package from a 3rd party which was
> designed to run on various machines including VAXes.  In the C code they have
> inserted #ifdef vax || #ifdef vms cpp directives (amongst others) so the code
> could be ported to various machines.  I was under the impression that these
> were reserved symbols that had already been defined to cpp ( 3xxx IRIS,
> running 3.6).  However I have just found out that vms and vax are NOT defined,
> contrary to the misleading statement in the cpp man page, which states upon
> closer inspection "The current list of these possibly reserved symbols
> includes:".  Does any one have a complete list of "possibly reserved symbols"
> that ARE currently defined?

It has been my experience that every C compiler vendor defines their own
set of symbols that you can test to see if you are using their compiler
or CPU.  The "#ifdef" ("#ifndef") directives test whether the symbol
is (is not) defined.  This is handy since C is not quite portable,
and also since the operating system is not always UNIX, and may not
support UNIX compatible system calls.

The VAX/VMS C compiler defines the
symbols "VMS" and "VAX" to allow you to test whether you are using
the VMS operating system and a VAX, respectively.  These symbols
should not (in theory) be defined on any other machine.  Most UNIX C
compilers define the symbol "unix" and/or "UNIX" to allow you to test
for the UNIX operating system.  On MS-DOS, Microsoft C defines the
symbols "MSC" and "MSDOS" to allow tests for this specific compiler
and operating system.  

Most compilers also allow tests for CPU type, as well.
On the MIPS systems (SGI 4D included), the preprocessor
defines the symbol "mips".  On some Motorola 68000 based systems, 
the symbol "M68000" is defined.
	
Unfortunately, there is no standard for these symbols.  Also, most
compiler manuals bury this information somewhere in the depths of the
appendices or worse, making it hard to track down.  I've often wished
that C compilers had some option which would cause the preprocessor
to spew out a list of all the predefined symbols.

Mike Goss
Merit Technology Inc.