Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!husc6!bu-cs!polygen!pablo
From: pablo@polygen.uucp (Pablo Halpern)
Newsgroups: comp.lang.c
Subject: Re: Does extern "const" allocate storage?
Message-ID: <136@polygen.UUCP>
Date: 30 Mar 88 01:49:37 GMT
References: <3117@haddock.ISC.COM>
Organization: Polygen Corporation, Waltham, MA
Lines: 41

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."  The
lack of the volatile modifier simply indicates that the object will
not be modified "behind the function's back" and therefore can take part
in certain optimizations.  This does bring up a problem, though.
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 look as follows:

	volatile int clock;

	void update_clock()	/* called on clock interupt */
	{
		clock++;
	}

Here, clock is defined with two different types (const volatile int verses
volatile int) in two different scopes.  Wouldn't a compiler (or lint) that
"sees" both scopes be allowed to complain about this?  If so, how could
you ever update a const volatile type.  (Don't tell me you should do it
in assembly language.  Avoiding the compiler's type checking is not the
issue.  The meaning of type modifiers in C is the issue.)  In general,
are you allowed to declare the same object with different type qualifiers
provided that the scopes of the declarations do not overlap?  If so,
please direct me to the section in the standard that says so.

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