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Re: Which side of bitwise OR is evaluated first?

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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().

> The reason why I wrote it this way is that I wanted to handle all
> errors and close the file pointer in a single block. 

Why?

> But I'm not sure
> if `ferror' is guaranteed to be called before `fclose' there, and the
> standard says once `fclose' is called on `fp', what `ferror(fp)' will
> do is unspecified or something like that.
> 
> So, should I change this or is it good (except being a bit
> unreadable)?
> 

IMHO, you should change it /because/ it is unreadable.  The fact that
you need to ask if the ordering is correct is a sure sign that you have
written it in a "smart-arse" manner that requires a lot of thought to
see if it does the right thing.  There may be exceptional circumstances
dictating your needs here, but I would write this with generous use of
named local variables holding the function returns (or flags indicating
their success or failure), clear conditionals, and braces so that the
flow of the code is absolutely obvious at a glance, and you are sure
that the /apparent/ flow is the same as a the /actual/ flow.

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Thread

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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