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Re: Which side of bitwise OR is evaluated first?

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David Brown <david.brown@hesbynett.no> writes:
> (Please try to get a proper newsreader and newsserver, rather than
> google groups - gg is particularly inconvenient when you have code
> snippets, making it hard to fix gg's broken formatting.)
>
> On 14/04/2021 07:43, Oğuz wrote:
>> I wrote this:
>> 
>>     fp = fopen(optarg, "r");
>>     if (!fp || (readfrom(fp), ferror(fp)|fclose(fp)))
>>         perror(optarg)
>> 
>> `readfrom' is a function of type `void' that calls `getline' until
>> it returns -1.
>> 
>
> C guarantees that the left side of || and && is evaluated (including
> side-effects) first, and the right side is then evaluated only if
> necessary (depending on the first side).  So the check of !fp first is
> safe.  Similarly, the comma operator evaluates the left side first, then
> the right side.
>
> Other operators, such as | and &, have no such rules.  The evaluation of
> the left and right sides are unsequenced, meaning not only that their
> order of evaluation is not fixed, but the can be interleaved and it can
> vary between runs of the code.  Thus the "ferror(fp)" and "fclose(fp)"
> can be done in any order - and the code can in theory do half of
> fclose() first, then ferror(), then the rest of fclose().

Function calls cannot be interleaved like that.  However, standard
library calls can be implemented as macros.  And in any case calling
ferror(fp) after close(fp) has undefined behavior.

N1570 6.5.2.2p10:

    There is a sequence point after the evaluations of the function
    designator and the actual arguments but before the actual
    call. Every evaluation in the calling function (including other
    function calls) that is not otherwise specifically sequenced
    before or after the execution of the body of the called function
    is indeterminately sequenced with respect to the execution of
    the called function.

and a footnote:

    In other words, function executions do not ‘‘interleave’’ with each other.

[...]

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */

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Which side of bitwise OR is evaluated first? Oğuz <oguzismailuysal@gmail.com> - 2021-04-13 22:43 -0700
  Re: Which side of bitwise OR is evaluated first? Barry Schwarz <schwarzb@delq.com> - 2021-04-14 00:04 -0700
  Re: Which side of bitwise OR is evaluated first? David Brown <david.brown@hesbynett.no> - 2021-04-14 09:42 +0200
    Re: Which side of bitwise OR is evaluated first? Oğuz <oguzismailuysal@gmail.com> - 2021-04-14 00:51 -0700
      Re: Which side of bitwise OR is evaluated first? David Brown <david.brown@hesbynett.no> - 2021-04-14 11:25 +0200
    Re: Which side of bitwise OR is evaluated first? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-04-14 03:27 -0700
      Re: Which side of bitwise OR is evaluated first? David Brown <david.brown@hesbynett.no> - 2021-04-14 13:28 +0200
        Re: Which side of bitwise OR is evaluated first? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-04-14 10:31 -0700
          Re: Which side of bitwise OR is evaluated first? David Brown <david.brown@hesbynett.no> - 2021-04-14 19:40 +0200
    Re: Which side of bitwise OR is evaluated first? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-04-14 10:26 -0700
  Re: Which side of bitwise OR is evaluated first? Richard Damon <Richard@Damon-Family.org> - 2021-04-14 07:22 -0400
    Re: Which side of bitwise OR is evaluated first? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-04-14 07:54 -0700
    Re: Which side of bitwise OR is evaluated first? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-04-14 14:23 -0400
  Re: Which side of bitwise OR is evaluated first? scott@slp53.sl.home (Scott Lurndal) - 2021-04-14 14:33 +0000
    Re: Which side of bitwise OR is evaluated first? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-04-14 13:15 -0400
  Re: Which side of bitwise OR is evaluated first? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-04-14 08:06 -0700
    Re: Which side of bitwise OR is evaluated first? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2021-04-14 10:34 -0600
      Re: Which side of bitwise OR is evaluated first? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-11 00:24 -0700

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