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Re: Working code

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Richard Damon <Richard@Damon-Family.org> writes:

> On 9/2/21 6:54 AM, Ben Bacarisse wrote:
>> Ike Naar <ike@rie.sdf.org> writes:
>> 
>>> On 2021-09-02, Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
>>>> On 9/1/2021 4:58 PM, Tim Rentsch wrote:
>>>>>> The following is simplified version of "working code":  it
>>>>>> is more than 20 years old (probably closer to 30 years old)
>>>>>> and apparently did what its author intended:
>>>>>>
>>>>>> int (*handlers[2])();
>>>>>>
>>>>>> #define DO_IT(n)   ((p = handlers[n]) ? (*p)() : 0)
>>>>>>
>>>>>> static void foo() {
>>>>>>      register int (*p)();
>>>>>>      register int bar = DO_IT(0) | DO_IT(1);
>>>>>>      /* ... */
>>>>>>      /* Use of bar */
>>>>>> }
>>>>>>
>>>>>> /* Code that fills 'handlers' and calls foo() */
>>>>>>
>>>>>> I wonder what you think about this code?
>>>>>
>>>>> I think it has undefined behavior, and needlessly so:
>>>>>
>>>>>      #define DO_IT(n)   (handlers[n] ? handlers[n]() : 0)
>>>>>
>>>>>      ...
>>>>
>>>> That intermediate variable `p` is indeed completely unnecessary. But 
>>>> where did you find undefined behavior in the original version?
>>>
>>> In the expression 'DO_IT(0) | DO_IT(1);' p is being modified more than once.
>>> The behaviour of the expression depends on how its subexpressions are
>>> sequenced.
>> 
>> There is a sequence point after each assignment.  Am I missing
>> something?
>
> No, there is a dependency that that assignment can't happen until after
> the right hand side is evalutated, but no full sequence point.

What's a full sequence point?

> There is no sequencing between the assignments of DO_IT(0) and DO_IT(1)
> and very importantly between the (*p)()s, both assignements can happen
> then both (*p)() happen, using whichever value of p was assigned last.

There is one between each assignment and the corresponding use, but you
are saying that that is not enough.  Have I got that about right?  Is
this derived from the new C11 "sequenced before" wording that replaced
the old wording C99 or was it UB in C99 as well?

-- 
Ben.

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Working code antispam@math.uni.wroc.pl - 2021-09-01 22:23 +0000
  Re: Working code Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-09-01 16:58 -0700
    Re: Working code Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-09-01 20:57 -0700
      Re: Working code Ike Naar <ike@rie.sdf.org> - 2021-09-02 05:22 +0000
        Re: Working code Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-09-01 22:33 -0700
        Re: Working code Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-09-02 11:54 +0100
          Re: Working code Richard Damon <Richard@Damon-Family.org> - 2021-09-02 07:08 -0400
            Re: Working code Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-09-02 13:37 +0100
              Re: Working code James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-02 12:03 -0400
              Re: Working code Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-09-02 09:57 -0700
  Re: Working code Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-09-01 21:02 -0700
    Re: Working code John McCue <jmccue@obsd2.mhome.org> - 2021-09-02 13:31 +0000

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