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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: the language (and the role of Undefined Behavior therein)
Date: Mon, 24 Aug 2015 18:21:07 -0700
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David Kleinecke <dkleinecke@gmail.com> writes:
> On Sunday, August 23, 2015 at 8:30:00 PM UTC-7, Keith Thompson wrote:
>> David Kleinecke <dkleinecke@gmail.com> writes:
[...]
>> > Signed arithmetic is also a version of modulo UINT_MAX+1 with a
>> > different way of writing some of the integers.
>> 
>> No, it isn't.  C doesn't specify wraparound behavior for signed
>> integers; signed integer overflow has undefined behavior.
>
> All the C90 standard says is "The result of the binary + operator is
> the sum of the the operands." "Sum" is nowhere defined. I haven't
> found any discussion of "overflow".

C90 6.3, 5th paragraph (C11 6.5p2 changes "exception" to "exceptional
condition"):

    If an *exception* occurs during the evaluation of an expression
    (that is, if the result is not mathematically defined or not
    representable), the behavior is undefined.

This applies to signed and floating-point types, but not to
unsigned types, since the standard explicitly defines the result
of an unsigned operation whose mathematical value is outside the
range of the type (C90 6.1.2.5, C11 6.2.5p9).

>> If by "signed arithmetic" you mean something other that what's
>> specified by C (such as, perhaps, the signed arithmetic operations
>> implemented by some particular CPU), you should make that clear.
>  
> I doubt that any CPU does other than arithmetic modulo-two-to-the-
> number-of-bits-power. Different CPU's set different sets of flags -
> so the interpretations can differ.

CPUs that use one's-complement or sign-and-magnitude almost certainly
do not operate that way.  I believe some CPUs can be configured
to trap on overflow, or to do other odd things like saturating.
I concede that such behavior is rare, but you did say "any CPU".

-- 
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"