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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: cpp macro expansion
Message-ID: <16909@sun.uucp>
Date: Fri, 17-Apr-87 12:34:51 EST
Article-I.D.: sun.16909
Posted: Fri Apr 17 12:34:51 1987
Date-Received: Sun, 19-Apr-87 12:09:06 EST
References: <2857@linus.UUCP>
Sender: news@sun.uucp
Reply-To: guy@sun.UUCP (Guy Harris)
Organization: Sun Microsystems, Mountain View
Lines: 51
Keywords: cpp, #define, macro

>#define	MACRO(first,last)	(\
>first\
>_\
>last)
>

>Which is the correct expansion, or is it left to the cpp implementors?

The ANSI C standard indicates that backslash-newline should be
completely stripped from source code fairly early in the translation
process.  This means that inserting blanks is incorrect; however, it
also indicates that substituting for "first" and "last" is incorrect,
because this macro definition should be treated identically to

	#define	MACRO(first,last)	(first_last)

>Is there a problem with the (our?) Sun version 3.2 cpp?

There is a problem with the System V "cpp", from which the 3.2 "cpp"
is derived.  There is a technique more likely to work on various
versions of UNIX:

	#define	MACRO(first,last)	(first/**/_/**/last)

However, *this* is not guaranteed to work on all C implementations,
either.

Also note that *neither* technique will work with the version of the
preprocessor used in many UNIX C implementations if you call the macro as

	MACRO(foo, bar)

since the blank in front of "bar" is considered part of the
argument.

Both of these facts argue against widespread use of this technique,
since it isn't guaranteed to work and since it breaks if you make
changes to the source code that one would think safe.

>What (if anything) does the new standard say about this?

It says you should write the macro like:

	#define	MACRO(first,last)	(first##_##last)

which will cause the "first", the "_", and the "last" to be glued
together into one token.  It also says (see the "debug" macro in the
example on pages 80 and 81 of the October 1, 1986 draft) that blanks
in the argument list should not be considered part of the argument.

Of course, this is a draft standard, and is subject to change.