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Re: Sequencing guarantees for macro versions of standard functions

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>
>>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>>
>>>> Andrey Tarasevich <andreytarasevich@hotmail.com> writes:
>>>
>>> [...]
>>>
>>>>> Consider the following example
>>>>>
>>>>>   double x = 1.5;
>>>>>   x = modf(x, &x);
>>>>>
>>>>> This is fine in case `modf` is actually a function.  But does the
>>>>> standard intend to guarantee that this is specified and defined even
>>>>> if `modf` is implemented as a macro?
>>>>
>>>> No.
>>>>
>>>>> I would expect so, and it is quite possible that the following passage
>>>>>
>>>>> "[...] Likewise, those function-like macros described in the following
>>>>> subclauses may be invoked in an expression anywhere a function with a
>>>>> compatible return type could be called.  [...]"
>>>>>
>>>>> is intended to guarantee exactly that.
>>>>
>>>> The quoted passage guarantees what it says:  such function-like
>>>> macros may be /invoked/ anywhere their corresponding functions
>>>> could.  It does /not/ guarantee that such invocations will work
>>>> if there are sequencing issues.
>>>>
>>>>> But still, what exactly are they trying to point out in footnote 186?
>>>>
>>>> If not having the same sequencing guarantees might cause a
>>>> problem (ie, if a "call" might invoke a macro rather than calling
>>>> a function), avoid using a function call expression that might
>>>> invoke a macro, as for example in this particular case
>>>>
>>>>     x = (modf)( x, &x );
>>>>
>>>> so that we're sure an actual function call takes place.  Hence we
>>>> know there will be no sequencing problems.  Make sense?
>>>
>>> Or
>>>
>>>     x = modf(x, &y);
>>>     x = y;
>>>
>>> (I'm already a bit uncomfortable with x = (modf)(x, &x), though
>>> I know it's well defined.)
>>
>> I understand having a queasy reaction to 'x = (modf)(x, &x)', and
>> to some degree I have such a reaction myself.  I deliberately
>> chose not to comment about that, since what was being asked about
>> is only about sequencing issues in macros vs functions.
>>
>> However, the suggested rewrite has different semantics than the
>> previous single assignment with function call.  If we want to
>> keep the same semantics but still be able to use the macro
>> form (but without any sequencing issues), a different writing
>> is needed, as for example
>>
>>     x = modf( x, (double[]){ 0 } );
>>
>> Writing the call this way should make it obvious that the value
>> stored in/through the second argument is ignored and discarded.
>
> Right, I should have paid closer attention to the semantics of modf()
> (which I don't think I've ever used "in anger").

Why do you make the "in anger" comment?  I couldn't find any
connection to earlier remarks in this thread.

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Thread

Sequencing guarantees for macro versions of standard functions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-20 15:59 -0800
  Re: Sequencing guarantees for macro versions of standard functions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-20 16:35 -0800
    Re: Sequencing guarantees for macro versions of standard functions Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-23 14:38 -0800
      Re: Sequencing guarantees for macro versions of standard functions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-23 19:36 -0800
        Re: Sequencing guarantees for macro versions of standard functions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-23 19:56 -0800
          Re: Sequencing guarantees for macro versions of standard functions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-23 19:58 -0800
            Re: Sequencing guarantees for macro versions of standard functions Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-23 23:50 -0800
            Re: Sequencing guarantees for macro versions of standard functions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-24 07:27 -0800
        Re: Sequencing guarantees for macro versions of standard functions Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-23 23:49 -0800
          Re: Sequencing guarantees for macro versions of standard functions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-24 07:31 -0800
            Re: Sequencing guarantees for macro versions of standard functions Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-24 13:18 -0800
              Re: Sequencing guarantees for macro versions of standard functions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-29 03:36 -0800

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