Re: Is this defined behavior?

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Is this defined behavior?
Date	2024-02-04 15:27 +0000
Organization	A noiseless patient Spider
Message-ID	<87cytcp7vz.fsf@bsb.me.uk> (permalink)
References	<7h8vN.323427$xHn7.112429@fx14.iad> <8734uaq19p.fsf@nosuchdomain.example.com> <8634ua9bts.fsf@linuxsc.com> <uploc3$36iui$1@dont-email.me> <upnvla$3l4rm$1@dont-email.me>

Show all headers | View raw

David Brown <david.brown@hesbynett.no> writes:

> On 03/02/2024 17:07, Malcolm McLean wrote:
>> On 03/02/2024 08:44, Tim Rentsch wrote:
>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>>
>>>> Anthony Cuozzo <anthony@cuozzo.us> writes:
>>>>
>>>>> Time T:  Add N to a uintptr_t
>>>>>
>>>>> Time T+1:  Subtract N from that same uintptr_t
>>>>>
>>>>> Questions:
>>>>>
>>>>> 1.  Is this behavior defined?
>>>>
>>>> Sure, why wouldn't it be?  uintptr_t is just an unsigned integer
>>>> type.  The guarantees about converting to and from void* don't
>>>> affect its arithmetic behavior.  Adding and then subtracting N
>>>> yields the original value.
>>>
>>> Extremely likely to hold.  A perverse implementation could choose
>>> (IIANM) a uintptr_t whose integer conversion rank is less than
>>> that of int, and that could mess things up.  In practical terms
>>> though what you say is spot on.
>
> An 8-bit implementation with very limited RAM might use 8-bit pointers, at
> least for "normal" pointers.  (Typically such systems also support
> extensions such as "far" pointers, or "code memory" pointers.)  I haven't
> seen such an implementation, but it is certainly conceivable. ("int" would
> be 16-bit.)

I think you are suggesting that uintptr_t might, on such a system, be
only 8 bits wide, but that's no permitted.  UINTPTR_MAX can be no less
than 2**16 - 1.  Of course that alone does not prevent its rank being
less that that of int, but the system you describe would not be an
example.

-- 
Ben.

Thread

Is this defined behavior? Anthony Cuozzo <anthony@cuozzo.us> - 2024-02-02 10:56 -0500
  Re: Is this defined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-02 08:28 -0800
    Re: Is this defined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-02-03 00:44 -0800
      Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-03 16:07 +0000
        Re: Is this defined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-03 13:41 -0800
          Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-04 03:18 +0000
            Re: Is this defined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-03 19:48 -0800
              Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-04 04:34 +0000
        Re: Is this defined behavior? David Brown <david.brown@hesbynett.no> - 2024-02-04 13:24 +0100
          Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-04 15:16 +0000
            Re: Is this defined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-04 13:31 -0800
              Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-04 23:18 +0000
                Re: Is this defined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-04 15:56 -0800
                Re: Is this defined behavior? gazelle@shell.xmission.com (Kenny McCormack) - 2024-02-05 00:05 +0000
                Re: Is this defined behavior? Poprocks <please@replytogroup.com> - 2024-02-05 13:35 -0500
                Re: Is this defined behavior? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-06 13:10 +0000
                Is this sane behavior? (Was: Is this defined behavior?) gazelle@shell.xmission.com (Kenny McCormack) - 2024-02-06 14:52 +0000
                Re: Is this sane behavior? (Was: Is this defined behavior?) Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-06 15:50 +0000
              Re: Is this defined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-02-04 23:21 -0800
          Re: Is this defined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2024-02-04 15:27 +0000
            Re: Is this defined behavior? David Brown <david.brown@hesbynett.no> - 2024-02-04 18:23 +0100

csiph-web