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Path: utzoo!utcsri!greg
From: greg@utcsri.UUCP (Gregory Smith)
Newsgroups: comp.lang.c
Subject: Re: draft ANSI standard: major, quiet, unnoticed change
Message-ID: <3722@utcsri.UUCP>
Date: Wed, 3-Dec-86 17:51:28 EST
Article-I.D.: utcsri.3722
Posted: Wed Dec  3 17:51:28 1986
Date-Received: Wed, 3-Dec-86 18:43:43 EST
References: <1379@hoptoad.uucp> <500@brl-sem.ARPA>
Reply-To: greg@utcsri.UUCP (Gregory Smith)
Organization: CSRI, University of Toronto
Lines: 66
Summary: 

>In article <1379@hoptoad.uucp>, gnu@hoptoad.uucp (John Gilmore) writes:
>> #pdefine sun
>> 
>> would cause #if sun or #if defined(sun) or #ifdef sun to be true, but
>> would not replace occurrances of "sun" in program text.
>
In article <500@brl-sem.ARPA> ron@brl-sem.ARPA (Ron Natalie <ron>) writes:
>Of course, no one said it had to be a macro definition.  You could
>just say
>	int	sun;
>and the #if defined(sun) would also work.  Of course, it takes up
>a few bytes.  Too bad the compiler won't allow me to declare voids.
>
Baloney, the preprocessor doesn't know that 'int sun' has been defined.

I like John's idea; it is not really a 'kludge' as he says; in fact
the concept of using string-substitution symbols to control conditional
compilation is the kludge, and the #pdefine would cure this.

How do you define these in the command line?
possibility:
	cc ... -Pfoo -Dbar -Dlimit=200
is equivalent to
	#pdefine foo
	#define bar 1
	#define limit 200
but I have always thought there should be an explicit mechanism for
importing stuff from the command line:

	cc -Dfoo -Dbar -Dlimit=200
combined with
	#conditional foo
	#parameter bar
	#parameter limit
in the source, would be equivalent to the above defines. the '-D'
symbols ( and any predefined stuff, 'unix' etc) would be waiting
in the wings, so to speak, until activated by the #parameter or
#conditional. It would be an error if a #parameter were applied
to a symbol that wasn't 'waiting in the wings' -- the advantage of
this is obvious. 

If you had a system that would compile on a vax or sun,
you would have to put "#conditional vax/#conditional sun" at the beginning.
If you compiled it on a 'widget', the definition of 'widget' would
not take effect, hopefully causing a compile error, and forcing the
user to take note that changes may be required, and to add "#conditional
widget" after making those changes. Conditional switches could be implemented
painlessly:
#conditional vax,sun,widget
could cause an error if none of the symbols are 'waiting'. ( if only
one symbol is listed, and is not waiting, nothing happens). If more
than one of the symbols is waiting, the first in the list *only* would
be activated. Thus the programmer would be certain that exactly one
of vax,sun,widget is defined.

The mechanism will not allow site-dependant or command line defines,
except those anticipated by the programmer, to have any effect on the
program. This is the main advantage.

Of course this breaks existing code; but it may be fixed by adding
#parameters, and #conditionals to the code. These parameters and
conditionals should be documented in comments anyhow, of course.
-- 
----------------------------------------------------------------------
Greg Smith     University of Toronto      UUCP: ..utzoo!utcsri!greg
Have vAX, will hack...