Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!necntc!ima!haddock!karl
From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Does extern "const" allocate storage?
Message-ID: <3288@haddock.ISC.COM>
Date: 1 Apr 88 20:12:25 GMT
References: <3117@haddock.ISC.COM> <136@polygen.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 29

In article <136@polygen.UUCP> pablo@polygen.uucp (Pablo Halpern) writes:
>From article <3117@haddock.ISC.COM>, by karl@haddock.ISC.COM (Karl Heuer):
>>An object which is declared const but not volatile can never be modified by
>>a correct program.  A conforming implementation is allowed to take advantage
>>of this knowledge, by putting it in read-only memory and/or by inlining it.
>
>I disagree.  An object that is declared const but not volatile can not
>be modified within the scope in which the declaration holds.  Nowhere
>does the standard say that such an object is "really constant."

Footnote 50 certainly implies it.  If the body of the standard doesn't say
that modifying a const object through a non-const handle is undefined, I'm
sure the Committee would like to correct the oversight.

(Note: I'm talking about actual objects declared const.  Your statement is
true for a dereferenced pointer-to-const.)

>In the standard, a clock register is used as an example of a volatile const.
>The declaration looked something like:
>	extern const volatile int clock;
>The meaning of this is explaned that the clock register may not be written to
>by the program and that if it is read once, the value may be different if it
>is read again.  The question is, doesn't this imply that somewhere there is a
>function that might [modify it through a non-const handle]

No, the dpANS said that this may be "modifiable by hardware".  A memory-mapped
address could be so declared.  (Presumably the linker will bind the unresolved
extern to the appropriate magic location.)

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint