Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #162573

Re: Working code

Show all headers | View raw

On 9/1/2021 10:22 PM, Ike Naar wrote:
> 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.

Ah... I see. Indeed. Good point.

-- 
Best regards,
Andrey Tarasevich

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web