Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!unisoft!greywolf
From: greywolf@unisoft.UUCP (The Grey Wolf)
Newsgroups: comp.lang.c
Subject: Re: When do you use const
Keywords: const, #define, C
Message-ID: <3344@unisoft.UUCP>
Date: 5 Feb 91 22:52:49 GMT
References: <1220@tredysvr.Tredydev.Unisys.COM> <1991Feb2.181948.2147@demott.com>
Reply-To: greywolf@unisoft.UUCP (The Grey Wolf)
Organization: Foo Bar and Grill
Lines: 56

In article <1991Feb2.181948.2147@demott.com> kdq@demott.com (Kevin D. Quitt) writes:
[ const double x; example deleted ]
>    Your variable x above is pretty useless, since it has no initialized
>value (and is therefore zero).  How about:
>
>const char	*foo	= "some really long string";
>
>    If this is #defined, and is accessed in several places, your
>compiler may save multiple copies of the string.  Even if your compiler
>is smart enough to make it a single instance, it won't do this over
>several modules. 

I know, I know, not ALL Unices are BSD, and not everyone can do this (yet),
but there is something called xstr on most sensible systems which solves the
#define problem quite nicely across multiple modules.

for module in modules
do	cc -E $module.c | xstr -
	cc -c x.c
	mv x.o module.o
	mv x.c module.cx 	# (if you want to see what it's doing)
done
cc -c xs.c
mv xs.o strings.o

>
>  Another advantage is in the optimization phases, where the const keyword
>provides more information to the optimizer.

This does not get solved by xstr :-)...

Where possible, the old method was to use cc -R on the resulting strings
object (xs.o) to make the strings shared/read-only[*].  The C shell is
compiled like this.  What with the advent of ANSI C, it appears that the
need for such a compiler flag has been obsoleted by the presence of the
"const" qualifier.

xstr still has its uses -- in places where quoted phrases might be used more
than once (such as {fprintf(stderr,"ioctl: "); perror(arg);} sequences)
all references to the quoted phrase would get changed to be pointers to
the master string array.

> 
> 
>-- 
> _ 
>Kevin D. Quitt demott!kdq kdq@demott.com 
>DeMott Electronics Co. 14707 Keswick St.  Van Nuys, CA 91405-1266 
>VOICE (818) 988-4975 FAX (818) 997-1190 MODEM (818) 997-4496 PEP last


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