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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:
>>> James Kuyper <jameskuyper@verizon.net> writes:
>>>> On 08/12/2011 12:04 PM, Kenneth Brody wrote:
>>>>> A stray thought wandered into my head this morning...
>>>>> 
>>>>> Given that 7.5p4 reserves macros that begin with E+digit or
>>>>> E+capital-letter for error codes, what allows the Standard to also
>>>>> define EOF in a manner unrelated to <errno.h>?  (Aside from "duh,
>>>>> it's the Standard, and it can do whatever it likes", that is.)
>>>>
>>>> Those macros are reserved for use by the implementation; EOF is
>>>> #defined by the implementation; there's no conflict. In principle,
>>>> EOF could even be a valid errno value.
>>>
>>> I'm not sure it could be (or at least not in a useful way).  7.5p3 says
>>> that errno "is set to a positive error number by several library
>>> functions" but EOF must have a negative value (7.19.1p3).  It certainly
>>> seems as if setting errno to EOF would not be confirming to the spirit
>>> of the standard.
>>
>> If an implementation defined EOF with a positive value in <errno.h>, I
>> presume it would violate the standard, but what clause would it violate?
>>
>> Such an implementation would fail to compile this:
>>
>> #include <errno.h>
>> #include <stdio.h>
>> int main(void){}
>>
>> which clearly would be A Bad Thing, but does the standard actually
>> require it to be accepted?
>
> What's your line of reasoning here?  I don't see why such an
> implementation could not compile the above but I also don't see how you
> could be in doubt that the above should be accepted.
>
> (I have guesses about both of these but it seems wiser just to ask for
> more detail.)

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.

It's "obvious" that a program that has #include directives for any
combination of standard headers and is otherwise legal is still legal
(handwave meaning of "legal").  The question is, what clause of the
standard does this implementation violate by rejecting the program?
(I have no doubt that it violates the *intent* of the standard.)

-- 
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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