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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: Portability and /**/
Message-ID: <24624@sun.uucp>
Date: Fri, 31-Jul-87 13:45:12 EDT
Article-I.D.: sun.24624
Posted: Fri Jul 31 13:45:12 1987
Date-Received: Sun, 2-Aug-87 02:56:10 EDT
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> You are right, the C pre-processor does (normally) strip out the comments.
> However, the exact semantics depend on the particular version (BSD or
> SYSV) of 'cpp'.  There *are* uses for this -- you note one.  The C++
> compiler (which also uses 'cpp') uses a similar mechanism to allow definition
> of generic classes.  The file <generic.h> has code something like:
> 
>     #ifdef BSD  /* BSD way */
>     #define name2(a,b) a\
>     b
>     #else       /* System V way */
>     #define name2(a,b) a/**/b
>     #endif

I don't understand this.  I have seen this several times, and I still
don't understand this.  Why is the second way labeled "System V
way", and the first way labeled "BSD way"?

	gorodish$ rlogin sunvax
	Password:
	Last login: Sat Jul 18 00:53:39 from gorodish
	4.3 BSD UNIX #1: Mon Jul  7 11:46:47 PDT 1986

	*** N.B.: Running straight 4.3bsd. ***
	% ed foo.c
	?foo.c
	a
	#define name2(a,b)      a/**/b

	name2(foo,bar)
	.
	w
	42
	q
	% cc E foo.c
	# 1 "foo.c"


	        foobar

The second way, mislabeled "System V way", works just fine on 4.3BSD;
the C preprocessor hasn't changed much at all between 4.1BSD and
4.3BSD, other than having the "-M" flag added.  The "System V way" is
used in several places by 4BSD Makefiles.

If you're using a Reiser preprocessor, the "System V way" will almost
certainly work.  If you're not using a Reiser preprocessor, *neither*
of the ways is guaranteed to work.  (If you're using an ANSI C
preprocessor, there is, of course, a standard way of doing this that
*is* guaranteed to work, unless they change that part of the standard
before it becomes final.)

Note, by the way, that due to the Reiser preprocessor's Principle of
Least Surprise-violating way of handling white space in macro calls,
the following call of "name2" will not work with *either* of the two
definitions above:

	name2(foo, bar)

It expands to "foo bar", because the second argument is " bar", not
"bar".
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com