From: "Steven Smith" <sos22@cam.ac.uk>
To: Nasir Simbolon <nasir@orbisindonesia.com>
Cc: linux-c-programming@vger.kernel.org
Subject: Re: constant variable
Date: Thu, 11 Mar 2004 13:42:15 +0000 [thread overview]
Message-ID: <20040311134215.GA4714@archibold.chu.cam.ac.uk> (raw)
In-Reply-To: <200403111819.00752.nasir@orbisindonesia.com>
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> const int i=9;
> int* j=NULL;
> j = (int*) &i;
> *j = 3;
> when I compile using gcc version 3.3.2 (Debian) and run, I have the result :
>
> value of i = 9
> value of *j = 3
>
> Now, I notice that j point to address of i BUT they do not have the same
> value.
Probably because of compiler optimisations. If you've specified that
something is const, then the compiler is perfectly at liberty to
assume that it is. This means that it can assume the initial value of
i still applies, and so, rather than going to memory for i, it can
just substitute the constant 9 in. This is likely to be much quicker
at run time.
I can only reproduce this behaviour if I have compiler optimisations
turned on, which adds some more weight to the explanation.
Steven.
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next prev parent reply other threads:[~2004-03-11 13:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-03-11 11:19 constant variable Nasir Simbolon
2004-03-11 13:42 ` Steven Smith [this message]
-- strict thread matches above, loose matches on Subject: below --
2004-03-11 13:28 Sandro Dangui
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