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Re: Random thought -- EOF and errno

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> Keith Thompson <kst-u@mib.org> writes:
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>>> Keith Thompson <kst-u@mib.org> writes:
[...]
>>>> Hypothetically,
>>>> <errno.h> has:
>>>>     #define EOF 42 /* Error on Ocelot Farm */
>>>> and <stdio.h> has:
>>>>     #define EOF (-1)
>>>>
>>>> Then the preprocessed expansion of the above source file has:
>>>>     ...
>>>>     #define EOF 42
>>>>     ...
>>>>     #define EOF (-1)
>>>>     ...
>>>> which violates C99 6.10.3p2.
>>>
>>> Right, that's what I thought.  But merely defining EOF as positive in
>>> errno.h is not what makes the implementation fail on this source -- it
>>> is defining EOF in stdio.h without un-defining it first.  Since EOF in
>>> errno.h is an extension, there's no way to know if it's really important
>>> or not, so you can't assume that it can't be redefined.
>>
>> <stdio.h> isn't required to undefine EOF before redefining it.  The
>> identifier EOF is reserved, so if a user program does this:
>>
>>     #define EOF 42
>>     #include <stdio.h>
>>
>> then that program's behavior is at best undefined.
>
> Agreed, but I think we are talking at cross purposes.  I thought you
> were saying that this daft implementation *must* fail to compile the
> program you quoted.  I was just saying that it might not.  I don't think
> there is any disagreement about the facts, but I may well have
> misconstrued what you wrote.

The only program an implementation *must* fail to compile is one with a
#error directive.

The above 2-line program will trigger a diagnostic if <stdio.h> doesn't
undefine EOF before defining it.  There's no rule that says it should
undefine it or that it shouldn't.  (It's probably better QoI-wise if it
doesn't, so that bad code like the above *will* trigger a diagnostic.)

[...]

>> I think you're assuming that this program:
>>
>> #include <errno.h>
>> #include <stdio.h>
>> int main(void){}
>>
>> is strictly conforming.  Of course it *should* be, but I'm arguing that
>> the reservation of the identifier EOF by both <stdio.h> and <errno.h>
>> creates an unintentional loophole, implying that it isn't.  It depends
>> on the implementation-defined choice of whether <errno.h> defines EOF.
>
> How, then, is it strictly conforming?  I though that SCP must not depend
> on anything implementation-defined.

It's not; that was my point.  I inferred (perhaps wrongly) that you were
assuming that it is.  On the other hand, I now think it is; see my
response to Eric Sosman (as soon as I write it).

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Random thought -- EOF and errno Kenneth Brody <kenbrody@spamcop.net> - 2011-08-12 12:04 -0400
  Re: Random thought -- EOF and errno James Kuyper <jameskuyper@verizon.net> - 2011-08-12 12:27 -0400
    Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 00:12 +0100
      Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-12 16:30 -0700
        Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 01:27 +0100
          Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-12 18:28 -0700
            Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 14:38 +0100
              Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-13 11:50 -0700
                Re: Random thought -- EOF and errno Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-13 17:45 -0400
                Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-13 18:04 -0700
                Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 22:53 +0100
                Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-13 17:57 -0700
                Re: Random thought -- EOF and errno Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-13 21:06 -0400
                Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-13 18:39 -0700
                Re: Random thought -- EOF and errno Tim Rentsch <txr@alumni.caltech.edu> - 2011-09-04 12:59 -0700
      Re: Random thought -- EOF and errno James Kuyper <jameskuyper@verizon.net> - 2011-08-12 20:51 -0400
        Re: Random thought -- EOF and errno Nobody <nobody@nowhere.com> - 2011-08-13 05:55 +0100
          Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 14:49 +0100
        Re: Random thought -- EOF and errno Kenneth Brody <kenbrody@spamcop.net> - 2011-08-15 15:02 -0400
          Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-15 12:18 -0700
          Re: Random thought -- EOF and errno James Kuyper <jameskuyper@verizon.net> - 2011-08-15 21:54 -0400
            Re: Random thought -- EOF and errno Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-16 04:21 +0100
        Re: Random thought -- EOF and errno William Ahern <william@wilbur.25thandClement.com> - 2011-08-15 12:56 -0700
  Re: Random thought -- EOF and errno Seebs <usenet-nospam@seebs.net> - 2011-08-12 18:37 +0000
  Re: Random thought -- EOF and errno Nobody <nobody@nowhere.com> - 2011-08-13 06:00 +0100
    Re: Random thought -- EOF and errno Kenneth Brody <kenbrody@spamcop.net> - 2011-08-15 15:05 -0400
      Re: Random thought -- EOF and errno Keith Thompson <kst-u@mib.org> - 2011-08-15 12:14 -0700
      Re: Random thought -- EOF and errno gordonb.gl0ml@burditt.org (Gordon Burditt) - 2011-08-16 04:31 -0500

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