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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: bit value of a negative number |
| Date | 2016-07-30 13:59 -0700 |
| Organization | None to speak of |
| Message-ID | <lntwf66dwh.fsf@kst-u.example.com> (permalink) |
| References | <159d5df7-d640-40b1-9866-2bb16d7769ee@googlegroups.com> <87a8gzxv8b.fsf@bsb.me.uk> <punning-20160730120244@ram.dialup.fu-berlin.de> <nniald$65o$1@dont-email.me> |
James Kuyper <jameskuyper@verizon.net> writes:
> On 07/30/2016 07:31 AM, Stefan Ram wrote:
[...]
> As I remember, the fact that it was technically problematic was due to
> an explicit statement to that effect in the C90 standard: reading the
> value of a member of a union that was not the same as the one last
> written to was problematic in some way - but I don't have a copy of C90
> that I can reference to confirm that. The relevant clause should be in
> the semantics section of the description of the member selection operator.
C90 6.3.2.3 says:
With one exception, if a member of a union object is accessed after
a value has been stored in a different member of the object, the
behavior is implementation-defined.
(The exception is the common initial sequence rule.) A footnote on that
sentence says:
The "byte orders" for scalar types are invisible to isolated
programs that do not indulge in type punning (for example, by
assigning to one member of a union and inspecting the storage by
accessing another member that is an appropriately sized array
of character type), but must be accounted for when conforming
to externally imposed storage layouts.
[...]
--
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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bit value of a negative number sinbad <sinbad.sinbad@gmail.com> - 2016-07-30 02:38 -0700
Re: bit value of a negative number Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-07-30 02:50 -0700
Re: bit value of a negative number Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-30 11:42 +0100
Re: bit value of a negative number James Kuyper <jameskuyper@verizon.net> - 2016-07-30 09:38 -0400
Re: bit value of a negative number "James R. Kuyper" <jameskuyper@verizon.net> - 2016-07-30 11:39 -0400
Re: bit value of a negative number Keith Thompson <kst-u@mib.org> - 2016-07-30 13:59 -0700
Re: bit value of a negative number Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-30 15:16 +0100
Re: bit value of a negative number Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-31 06:31 -0700
Re: bit value of a negative number Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-31 17:28 +0100
Re: bit value of a negative number supercat@casperkitty.com - 2016-07-31 13:19 -0700
Re: bit value of a negative number Ike Naar <ike@iceland.freeshell.org> - 2016-07-31 20:46 +0000
Re: bit value of a negative number Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-30 14:21 -0700
Re: bit value of a negative number supercat@casperkitty.com - 2016-07-31 13:37 -0700
Re: bit value of a negative number Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-31 16:59 -0700
Re: bit value of a negative number supercat@casperkitty.com - 2016-07-31 12:56 -0700
Re: bit value of a negative number Barry Schwarz <schwarzb@dqel.com> - 2016-07-30 10:15 -0700
Re: bit value of a negative number Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-30 10:25 -0700
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