Path: csiph.com!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Portable 16 bit binary integer write
Date: Fri, 16 Sep 2016 19:06:05 -0700
Organization: None to speak of
Lines: 35
Message-ID: <lnfuozz3ea.fsf@kst-u.example.com>
References: <50a1e8f4-4fb3-4491-97d0-54768bf28685@googlegroups.com> <OtSBz.4346$g01.3583@fx33.iad> <eaa6c944-c151-4192-8857-1a1a194c1897@googlegroups.com> <nr91st$j82$1@dont-email.me> <kfnr38ni1ly.fsf@x-alumni2.alumni.caltech.edu> <nrdpih$k3q$1@dont-email.me> <4fde1133-61ff-4045-9cbe-cccff1408ad3@googlegroups.com> <nreism$b9v$1@dont-email.me> <b89ceb12-0c5b-4f0f-ad4a-a74d470b56de@googlegroups.com> <nrf2rf$5ha$1@dont-email.me> <0b757bbb-0494-44cb-bd5a-26f117aaa404@googlegroups.com> <nrgcuu$9hr$1@dont-email.me> <2cf666d7-0755-4c70-adad-07dbafc2b2e7@googlegroups.com> <nrhbt5$l7r$1@dont-email.me> <c9cb703c-db1b-4baa-9622-538aeaca5652@googlegroups.com> <lnk2ebzaxe.fsf@kst-u.example.com> <09cb4413-16a6-4b63-89d0-9dafe56562f6@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Injection-Info: mx02.eternal-september.org; posting-host="945944de09706c9b4e29b53c9d2efdc2"; logging-data="29564"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+yXA1mmCmKBe+CUfqYNZk8"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock: sha1:y2tpcBBqbsypQOzYQxOw5+p+9z0= sha1:Nrt9VCE8/ydcvJtLk/uBhSn6riY=
Xref: csiph.com comp.lang.c:93674

supercat@casperkitty.com writes:
> On Friday, September 16, 2016 at 4:17:31 PM UTC-5, Keith Thompson wrote:
>> supercat writes:
>> > I did read the standard.  From N1570 6.5.7:
>> >
>> > 4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits
>> >   are filled with zeros.... If E1 has a signed type and nonnegative value,
>> >   and E1 x 2E2 is representable in the result type, then that is the
>> >   resulting value; otherwise, the behavior is undefined.
>> 
>> Right.  Your statement was that "shifting a negative number
>> left is UB".  It's UB only if the result is outside the range of
>> the signed type.  Your statement implied (certainly to me, and
>> presumably to David) that left-shifting a negative number *always*
>> has undefined behavior.  It doesn't.
>
> The language of the Standard only specifies behavior if E1 is unsigned, or
> if *both* of the following conditions apply:
>
>   1. E1 is signed AND NON-NEGATIVE.
>
>   2. The result of multiplying E1 * 2^E2 would be representable in the
>      result type.
>
> The expression -5 << 3 satisfies the second criterion, but fails the first.
> Consequently, it invokes Undefined Behavior.

You're absolutely right.  Looks like I failed reading comprehension
for the day.  My apologies.

-- 
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"