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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: the language |
| Date | 2014-12-20 00:37 -0800 |
| Organization | None to speak of |
| Message-ID | <lntx0qaf9m.fsf@nuthaus.mib.org> (permalink) |
| References | (14 earlier) <LjAkw.152595$Sn4.106292@fx10.iad> <m6uksd$7t3$1@dont-email.me> <rsOkw.818947$DX.276445@fx06.iad> <lnppbfbneg.fsf@nuthaus.mib.org> <gW6lw.1032051$412.1025840@fx30.iad> |
Richard Damon <Richard@Damon-Family.org> writes:
> Keith,
> I kind of feel you are being a bit deliberately obtuse,
No, not at all.
> but let me give
> an alternate simple case and you can tell me where there is a
> significant difference.
>
> A compiler claims to be a "Windows" compiler.
> A program that is run through it that includes "windows.h" and calls a
> function defined therein.
>
> By the C standard, this is "undefined behavior", as the C standard does
> not define this behavior, and if windows.h were to be considered as
> "user code" it contains elements that are reserved for the
> implementation. (And there is no formal standard that can be appealed to
> which requires the specified behavior)
I'd say that <windows.h> is probably part of the implementation.
> If the compiler generated code that did anything different than to call
> the name function, it would be considered a defect in the compiler.
The semantics of a function call are defined by the C standard. But it
doesn't matter whether the compiler emits a CALL instruction (or
whatever it's called), as long as the behavior is consistent with an
actual call. For example, the call could be inlined, even if it
isn't marked as inline.
> Second case,
> The compiler claims to be a compiler targeting the 80386 (going back old
> school so I can get 16 bit ints for simplisity).
>
> The code computes the value of 32767 + 1
>
> Intel, in the documentation for the machine, defines that on a 80386,
> the signed addition of the two 16 bit signed numbers 32767 and 1 will
> yield the signed value -32768 when taken as a 16 bit signed value
> (irrespective of the exact operation code used).
I presume Intel's documentation describes the behavior of the ADD
instruction.
> Any other result could be seen as a lack of conformance to the
> specification of targeting an 80386.
>
> Yes, the C standard, BY ITSELF, doesn't define the behavior, but the C
> standard isn't by itself here, it has claimed a target processor, and
> not acted like that target processor.
>
> Other cases mentioned talk about "what if the processor has an
> instruction that does X", well, if the processor has such an
> instruction, then doing X in that situation conforms to the processors
> definition.
>
> The x86 family has NO instruction (that I know of) that would process a
> sequence like:
>
> if (i+1 > i) {
> j = 5;
> } else {
> j = 4;
> }
>
> and execute the assignment of j=5 when i has the value of INT_MAX. Thus
> to do so, "because the program has engaged undefined behavior" has
> violated the promise of acting like an x86 processor.
A C compiler is not required implement the "+" operator in a manner
that's consistent with the ADD instruction for the target processor,
and actual C compilers do not behave that way in all cases.
If you think they should, that's a perfectly valid opinion, though not
one that I share. But neither the C standard nor typical C compilers
work that way.
If you want your program to conform to the behavior specified for the
ADD instruction, you'll just have to write in assembly language, or in
some non-C language that makes the guarantees you want. (Or find a C
compiler that operates the way you want.)
> Note, that "undefined behavior" by the C standard does NOT remove the
> requirement to follow other defined behavior (like the windows.h case
> showed).
Agreed -- and a compiler can implement that behavior any way it likes.
> The biggest part of my complaint is that by the time the compiler has
> gone through enough work to determine that by using freedom in the
> presence of undefined behavior it can remove the comparison and "unused"
> code branch, it also has enough data to issue a warning that the code
> seems to be expecting undefined behavior (or has dead code), but it
> appears that it doesn't (at least by default).
Most compilers have options to increase the number of warnings they
issue. Sometimes excessive warnings can be distracting. And yes,
sometimes compilers fail to issue warnings when they should.
--
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"
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Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-01 00:55 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-02 05:42 +0000
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Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 13:57 -0500
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-19 14:31 -0800
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-19 20:29 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 20:36 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-17 07:18 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-17 15:45 +0000
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-17 16:50 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-17 17:24 +0000
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-18 02:07 -0600
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Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-18 08:06 -0500
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-18 08:30 -0500
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-19 00:11 -0500
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Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-19 08:43 -0800
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-19 23:29 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 00:37 -0800
Re: the language gazelle@shell.xmission.com (Kenny McCormack) - 2014-12-20 08:45 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-20 09:45 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-20 03:17 -0800
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Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-18 14:30 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-18 14:23 +0000
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-18 08:18 -0800
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-20 12:22 -0500
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-20 13:36 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 11:43 -0800
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 11:37 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-20 20:10 +0000
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-21 15:02 -0600
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-12-21 22:02 +0000
Re: the language Stephen Sprunk <stephen@sprunk.org> - 2014-12-21 23:49 -0600
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-21 17:45 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-18 10:29 -0800
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-20 05:29 -0600
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-20 11:32 -0800
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-21 15:23 -0600
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-22 23:39 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-20 20:22 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-18 10:24 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-18 18:36 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-19 09:24 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 17:53 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-20 11:48 -0800
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-19 02:20 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 08:31 +0000
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-19 09:31 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 10:10 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-19 07:08 -0800
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-20 06:39 +0000
Re: the language John Forkosh <forkosh@panix.com> - 2014-12-20 07:32 +0000
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-04 17:02 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-04 12:40 -0500
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-04 18:02 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-05 09:47 -0500
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-05 15:14 +0000
Re: the language Keith Thompson <kst-u@mib.org> - 2015-01-05 08:42 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-01-06 00:14 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-05 13:19 -0500
Re: the language Richard Damon <Richard@Damon-Family.org> - 2015-01-04 15:59 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2015-01-04 14:30 -0800
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-01-04 15:44 -0800
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Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-04 04:49 -0800
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Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-04 10:08 -0800
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Re: the language "ocq (an evil corporate man)" <ocq@maili.ng> - 2014-12-19 02:34 +0100
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