Can anyone tell me what is the difference between undefined behavior
and unspecified behavior?
Though I've read what is given about them, in ISO standards, I'm still
not able to get the difference.
For example:
Consider the following code:
a[i++] = i;
We say that the above expression statement produces undefined
behavior. Why can't we call it as unspecified behavior, because
nothing is said about the order of completion of side effects, which
affects the output?
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
Thanks in advance for the reply.
12  3064 
On 17 Mar 2007 07:38:29 -0700, "Rajesh S R" <SR**********@g mail.com>
wrote:
>Can anyone tell me what is the difference between undefined behavior
and unspecified behavior?
Though I've read what is given about them, in ISO standards, I'm still
not able to get the difference.
For example:
Consider the following code:
a[i++] = i;
We say that the above expression statement produces undefined
behavior. Why can't we call it as unspecified behavior, because
nothing is said about the order of completion of side effects, which
affects the output?
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
Thanks in advance for the reply.
Undefined behavior requires the use of a "nonportabl e or erroneous
program construct or of erroneous data" (paragraph 3.4.3). Your
example above meets this condition because while syntactically correct
it violates the restriction in paragraph 6.5-2 and is therefore
erroneous.
Unspecified behavior is not erroneous. It is a slightly more general
case of implementation-defined behavior. In both cases, the standard
provides two or more possibilities. For implementation-defined
behavior, the chosen possibility must be documented by every
(conforming) implementation. For unspecified behavior, there are no
further requirements so it need not be documented.
Remove del for email
Rajesh S R wrote:
<snip>
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
It's just a humorous way of mentioning that anything can happen after
undefined behaviour has been invoked in a program. I've heard that
that particular phrase was coined in clc by a former regular called
Kaz Kyzhelku. Apparently it's a common response when undefined
behaviour is invoked in C programs running on the mythical DS9000.
On Mar 17, 2:38 pm, "Rajesh S R" <SRRajesh1...@g mail.comwrote:
Unspecified behavior: The C Standard gives more than one possibility,
and it is not specified which of these possibilities is chosen, but
you know that one of them will be picked.
Undefined behavior: Anything can happen, and I mean absolutely
anything. Don't even think about trying to figure out what could
happen and what couldn't, because _anything_ can happen. In your
example, if the unspecified order in which side effects happen where
the only problem, then it would be "unspecifie d" behavior. But it is
worse: _Anything_ can happen, including formatting your hard disk,
your computer starting to send spam all over the place, or reporting
your credit card numbers to some hackers.
"Rajesh S R" <SR**********@g mail.comwrites:
Can anyone tell me what is the difference between undefined behavior
and unspecified behavior?
Though I've read what is given about them, in ISO standards, I'm still
not able to get the difference.
For example:
Consider the following code:
a[i++] = i;
We say that the above expression statement produces undefined
behavior. Why can't we call it as unspecified behavior, because
nothing is said about the order of completion of side effects, which
affects the output?
In this case, you might assume that
a[i++] = i;
can only behave in one of a limited number of ways, simply because the
standard doesn't specify the order of evaluation; the "i" on the right
hand side of the assignment might be evaluated either before or after
"i" is incremented by the "++". (If that were the case, it would be
unspecified behavior.)
But in fact, the standard explicitly says that the behavior is
*undefined*, which means there are *no* constraints on the behavior.
It could quietly do exactly what you expect it to do, it could
increase i by 42 rather than by 1, it could clobber some unrelated
variable, it could crash the program, or it could crash your operating
system. Some of these outcomes are unlikely; the point is that the
standard doesn't preclude them.
The reason the standard allows this much freedom to the implementation
is basically to allow for optimizations. The compiler is allowed to
rearrange expressions to improve performance. To do so, it has to do
some compile-time analysis to make sure the rearrangement doesn't
break anything. The standard permits this analysis to *assume* that
no undefined behavior occurs. If that assumption is incorrect, it's
considered the programmer's fault that the code is broken.
Now in this particular case, the undefined behavior is something that
could fairly easily be detected at compile time. But pointers make
things more interesting. Rather than
a[i++] = i;
consider
a[(*p1)++] = (*p2)++;
The latter invokes undefined behavior only if p1 and p2 happen to
point to the same object, something the compiler can't necessarily
determine.
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
It's a humorous expression of the idea that the standard "places no
requirements" on the results of undefined behavior. Obviously a C
implementation can't literally make demons fly out of your nose -- but
if executing "a[i++] = i;" actually *did* make demons fly out of your
nose, that wouldn't tell you that the implementation is
non-conforming.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Rajesh S R wrote:
Can anyone tell me what is the difference between undefined behavior
and unspecified behavior?
Though I've read what is given about them, in ISO standards, I'm still
not able to get the difference.
From N869:
3.19
[#1] unspecified behavior
behavior where this International Standard provides two or
more possibilities and imposes no requirements on which is
chosen in any instance
There are specified bounds on the results of unspecified behavior. It
might be an integer with some value.
Undefined behavior has no specified bounds, so may cause the computer to
crash, for example, which would not be permitted for unspecified behavior.
For example:
Consider the following code:
a[i++] = i;
We say that the above expression statement produces undefined
behavior. Why can't we call it as unspecified behavior, because
nothing is said about the order of completion of side effects, which
affects the output?
I can't give a good reason.
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
It is a fanciful image of something very unlikely. The point is that
since /that/ is permitted, any conceivable failure is permitted by the
standard (such as crashing, clobbering variables, restarting, you name
it). The shorthand phrase, then, covers all those and more!
--
Thad
santosh wrote:
Rajesh S R wrote:
<snip>
>Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
It's just a humorous way of mentioning that anything can happen after
undefined behaviour has been invoked in a program. I've heard that
that particular phrase was coined in clc by a former regular called
Kaz Kyzhelku. Apparently it's a common response when undefined
behaviour is invoked in C programs running on the mythical DS9000.
What mythical? Go here: http://dialspace.dial.pipex.com/town/green/gfd34/art/
--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>
--
Posted via a free Usenet account from http://www.teranews.com
santosh wrote:
Rajesh S R wrote:
<snip>
Furthermore, I've found this expression "demons out of your nose",
used by many, while talking about undefined behavior. Please tell me
what that expression means, in this context.
It's just a humorous way of mentioning that anything can happen after
undefined behaviour has been invoked in a program. I've heard that
that particular phrase was coined in clc by a former regular called
Kaz Kyzhelku. Apparently it's a common response when undefined
behaviour is invoked in C programs running on the mythical DS9000.
I've usually seen a fellow name John Woods (who I don't recall
personally) credited with coining it. This may be it:
<http://groups.google.c om/group/comp.std.c/msg/dfe1ef367547684 b>
Kaz famously had said something like, "I ran it on the Deathstation
9000, and demons flew out of my nose." That became a .sig for one or
more regulars over the years.
I can't find Kaz's original post (hey, it's Saturday) only follow-ups.
<http://groups.google.c om/group/comp.arch/msg/3095a0d47c235c8 6>
The more you dig, the more you realize how many messages aren't in the
archives.
Brian
Default User wrote:
>
.... snip ...
>
The more you dig, the more you realize how many messages aren't
in the archives.
Assuming you mean Google by archives, that may be because of
anti-social x-noarchive headers, or because the originator
specifically asked that it be removed. The latter is a perfectly
respectable means of retracting foolishness.
--
Some informative links:
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html >
<http://www.netmeister. org/news/learn2quote.htm l>
<http://cfaj.freeshell. org/google/ (taming google)
<http://members.fortune city.com/nnqweb/ (newusers)
--
Posted via a free Usenet account from http://www.teranews.com
CBFalconer wrote:
Default User wrote:
... snip ...
The more you dig, the more you realize how many messages aren't
in the archives.
Assuming you mean Google by archives,
Yes. I know there are other archives, but I don't of any that are
publicly searchable.
that may be because of
anti-social x-noarchive headers, or because the originator
specifically asked that it be removed.
Kaz has many posts in Google, including many with that email address. I
don't know much about Mr. Woods.
There are posts of mine that are missing, that neither of the above
apply to.
The latter is a perfectly
respectable means of retracting foolishness.
Neither post that I found referenced seems to fit that category, but
you never know.
Brian
--
Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or:
<http://www.caliburn.nl/topposting.html > This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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