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| From | Nobody <nobody@nowhere.com> |
|---|---|
| Subject | Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? |
| Date | 2012-05-03 19:14 +0100 |
| Message-Id | <pan.2012.05.03.18.14.50.178000@nowhere.com> |
| Newsgroups | comp.lang.c |
| References | <37f36566-0b91-4ee4-9135-6890c7c8de7a@12g2000vba.googlegroups.com> <4FA16280.6040703@verizon.net> <bf0df0b2-c9e5-4306-8c7e-ea99f5346364@v22g2000yqm.googlegroups.com> |
| Organization | Zen Internet |
On Thu, 03 May 2012 09:20:24 -0700, John Reye wrote:
>> tmp.c1 = 1;
>> printf("%d\n", ((struct b*)&tmp)->c1);
>>
>> An implementation is not required to notice that the printf() is
>> referring to the same object as the assignment statement.
>
> Hmmm... I think that would be one heck of a rubbish compiler (or more
> precisely *optimizing* compiler)! ;)
>
> If the standard allows that kind of stuff, then it simply is not
> bullet-proof enough.
>
> Because tmp occurs in both lines. Every compiler should notice that!
The compiler is free to lose track of that information during
optimisation, and may well do so.
> Why? Because it's an optimizer BUG.
A bug is a failure to behave as documented, not a failure to behave
according to the intuition of some guy on usenet.
If it were otherwise, all compilers would have bugs, and those bugs would
be impossible to fix, as different people's intutions are different and
often contradictory.
> Why?
> Because any simple compiler, that does not optimize... get's it right!
> And if any simple compiler get's it right, then any optimization must
> guarantee to get it right as well.
The behaviour of a "simple" compiler does not form a part of the standard.
> I mean: if the C standard allows one to create optimizers that result
> in such ... ummm... surprises (read: "rubbish"), then the standard is
> faulty in my eyes.
Whether or not it is faulty "in your eyes" is irrelevant. The standard
is what it is.
The standard explicitly and intentionally facilitates optimisation, rather
than forcing the majority of real-world code to be suboptimal for the sake
of pathological cases. This is also why "const" and "volatile" were added
to C89 and "restrict" to C99, in spite of being completely unnecessary for
"simple" (i.e. non-optimising) compilers.
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-02 09:06 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? "In a very dark China Blue, China Blue condition." <chine.bleu@yahoo.com> - 2012-05-02 09:33 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 12:36 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-02 10:04 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 13:23 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:20 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:25 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 10:16 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 11:05 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:21 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:46 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Nobody <nobody@nowhere.com> - 2012-05-03 19:14 +0100
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 11:45 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 21:05 +0200
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:14 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:16 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Lanarcam <lanarcam1@yahoo.fr> - 2012-05-03 21:56 +0200
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 16:12 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 13:45 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Nobody <nobody@nowhere.com> - 2012-05-04 01:34 +0100
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 22:32 +0200
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:10 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Keith Thompson <kst-u@mib.org> - 2012-05-03 16:14 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:24 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:19 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:25 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 17:44 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Barry Schwarz <schwarzb@dqel.com> - 2012-05-02 12:35 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-05-02 14:40 -0700
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 17:51 -0400
Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:16 -0700
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