Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: I want to ask a question... |
| Date | 2015-08-20 11:38 -0700 |
| Organization | None to speak of |
| Message-ID | <lna8tl3iwn.fsf@kst-u.example.com> (permalink) |
| References | (3 earlier) <d9ea783a-d784-4e13-8133-3cb8d7b42032@googlegroups.com> <lnmvxm3gwa.fsf@kst-u.example.com> <mr38rc$mbb$1@speranza.aioe.org> <lnio8a3akw.fsf@kst-u.example.com> <5620ecc0-0e7e-42a7-b389-4028db865a11@googlegroups.com> |
supercat@casperkitty.com writes:
> On Wednesday, August 19, 2015 at 9:33:27 PM UTC-5, Keith Thompson wrote:
>> What do you mean by "what the program says"? As far as the C standard
>> is concerned, the program has no meaning if condition1 and condition2
>> are both false.
>
> There certainly exist platforms where comparing the pointer to null could trap,
> and there may be some where it would cause non-deterministic behavior, and
> there may thus be usefulness to allowing a dialect of C to be used on such
> platforms. That does not imply, however, that there would not be just as much
> value in recognizing a dialect of C where pointer comparisons are guaranteed
> not to cause Undefined Behavior. I would consider the failure of the C
> Standard to recognize behavior which the vast majority of compilers' dialects
> have supported until recently to be a defect in the Standard.
The standard *does* recognize such behavior -- specifically by not
constraining the range of permitted behaviors in any way.
I'm not sure that the phrase "guaranteed not to cause Undefined
Behavior" makes sense. Undefined behavior is not some particular
kind of behavior, like a program crash. It means that the standard
refrains from specifying what the behavior is.
I think what you're actually suggesting is that a dialect of C
could define the behavior of accessing a pointer value after the
pointed-to object has reached the end of its lifetime. That still
leaves open the question of *how* it's to be defined. Would it
be implementation-defined or unspecified? If so, the standard
would have to specify a set of permitted behaviors from which
an implementation may choose, with or without a requirement to
document its choice. Or would some specific behavior be specified?
What would that behavior be?
I would argue that accessing the value of an indeterminate pointer:
free(ptr);
if (ptr == NULL) /* ... */
very probably indicates a bug in the program. I want that bug
diagnosed by the compiler if possible. Making the behavior undefined
permits compilers to, for example, generate code that causes the
program to die with a clear error message. Making the behavior
well-defined means that compilers cannot reasonably diagnose
the logical error (since, as far as the language is concerned,
it wouldn't be an error).
Suppose a CPU is designed so that loading an invalid pointer into a
register causes a trap. Defining the behavior of a accessing such
a pointer would mean that a compiler for such a CPU would have to
generate extra code to ensure that that trap doesn't happen.
There is one point on which I suspect we both agree: If a compiler
is clever enough to take advantage of undefined behavior to generate
*surprising* code (for example, code that executes unconditionally
even though it's protected by an "if"), it should issue a warning.
It may be difficult for the compiler to determine when such a
warning is appropriate -- and issuing too many warnings is almost
as bad as issuing too few. But I'm sure there's plenty of work to
be done in this area.
[...]
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Re: I want to ask a question... Gareth Owen <gwowen@gmail.com> - 2015-08-07 20:00 +0100
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 19:04 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 15:30 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 22:52 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 16:46 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-07 23:29 +0000
Re: I want to ask a question... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-08-08 01:37 +0100
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-07 18:55 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-08 05:51 +0000
Re: I want to ask a question... Robert Wessel <robertwessel2@yahoo.com> - 2015-08-08 01:44 -0500
Re: I want to ask a question... Ken Brody <kenbrody@spamcop.net> - 2015-08-10 12:46 -0400
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-19 16:43 -0700
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-19 18:09 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 01:00 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-19 20:25 -0700
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-20 08:15 -0700
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-20 11:38 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 18:47 +0000
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-20 14:01 -0700
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 16:43 +0000
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 13:02 -0400
Re: I want to ask a question... Tonton Th <tTh@nowhere.invalid> - 2015-08-20 17:40 +0000
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 15:27 -0400
Re: I want to ask a question... David Brown <david.brown@hesbynett.no> - 2015-08-20 22:58 +0200
Re: I want to ask a question... James Kuyper <jameskuyper@verizon.net> - 2015-08-20 23:24 -0400
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-21 06:19 +0000
Re: I want to ask a question... Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-08-21 09:20 -0600
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-22 12:25 -0700
Re: I want to ask a question... Robert Wessel <robertwessel2@yahoo.com> - 2015-08-23 01:33 -0500
Re: I want to ask a question... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-20 19:28 +0000
Re: I want to ask a question... Keith Thompson <kst-u@mib.org> - 2015-08-20 12:11 -0700
Re: I want to ask a question... David Brown <david.brown@hesbynett.no> - 2015-08-20 10:02 +0200
Re: I want to ask a question... raltbos@xs4all.nl (Richard Bos) - 2015-08-22 13:42 +0000
Re: I want to ask a question... supercat@casperkitty.com - 2015-08-22 12:23 -0700
csiph-web