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Re: scanf used for to get a line

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supercat@casperkitty.com writes:
> On Friday, November 4, 2016 at 6:44:22 PM UTC-5, Keith Thompson wrote:
>> supercat writes:
>> > The Standard describes behavior in many scenarios, but the only way in which
>> > it could be construed as defining behavior when a file ends with a partial
>> > line would be if such a partial line was considered an error.  I don't see
>> > anything in the Standard, however, which would indicate that hitting
>> > end-of-file when the preceding character isn't a newline should be considered
>> > an error condition.
>> 
>> N1570 7.21.2p2 says:
>> 
>>     A text stream is an ordered sequence of characters composed into
>>     *lines*, each line consisting of zero or more characters plus a
>>     terminating new-line character. Whether the last line requires
>>     a terminating new-line character is implementation-defined.
>> 
>> If the implementation does require a terminating new-line and a given
>> stream doesn't have one, then my interpretation is that the behavior of
>> reading that stream is undefined.
>
> Does anything in the Standard specify the behavior in the event that the
> implementation does not require a newline before EOF, and indeed none
> exists?

The behavior of what?  A stream has no behavior by itself.  The behavior
of any function that operates on a FILE* is defined by the description
of that function.  If there's some construct whose behavior you think is
not defined in that case, please specify what it is.

>> That's unlikely to be a problem in practice.  I speculate that it's
>> intended mainly to cater to systems with complex text file formats,
>> where an ill-formed text file might be unreadable.  In particular, I
>> believe the new-line is *not* required on Unix-like systems, and
>> probably not on Windows either.  Calling fgets() at the end of such a
>> file would just give the last line as a string with no trailing
>> new-line.
>
> Would anything in the Standard guarantee sane behavior, or are you merely
> assuming that because it would be silly for implementations to do anything
> nasty, implementations won't do anything nasty?

See above.

> I've certainly seen programs where failure to include a newline after the
> last line of a file would cause the program to process the line preceding
> the last line twice; I have no idea if they were written in C or some
> other language, but certainly line-input routines which behave badly on
> files that end with partial lines are not unknown in this world.

Yes, some programs have bugs.

>> > I don't know if any counter-examples.  On the other hand, since the
>> > semantics of scanf make reasonable error-trapping behavior at the
>> > application level awkward, and I don't think it's hard to imagine--
>> > especially in the days before the Standard--that an implementor might
>> > decide to implement design their "scanf" function in such a way that
>> > scanf("%d %d", &var1, &var2); would be a reasonable way to read two
>> > numbers from the console.
>> 
>> Apart from the numeric overflow problem, that *is* a reasonable way to
>> read two numbers from the console (assuming you don't require them to be
>> on the same line).  Was that intended to have something to do with the
>> buzzer scanario?
>
> What happens if someone types x, or 12x, or 12 34x, etc.?  A smart routine
> for an interactive system could validate input as it was typed and reject
> invalid characters immediately.  I don't know that any implementations
> defined their scanf routines to do so, but a routine that did so could
> certainly be helpful.

Sure.  scanf doesn't do that.


-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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scanf used for to get a line asetofsymbols@gmail.com - 2016-11-01 01:50 -0700
  scanf used for to get a line asetofsymbols@gmail.com - 2016-11-01 01:55 -0700
  Re: scanf used for to get a line mark.bluemel@gmail.com - 2016-11-01 03:55 -0700
    Re: scanf used for to get a line "Charles Richmond" <numerist@aquaporin4.com> - 2016-11-03 11:23 -0500
      Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-03 09:38 -0700
        Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-03 10:08 -0700
          Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-03 11:29 -0700
            Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-03 14:37 -0700
              Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-03 16:34 -0700
                Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-04 10:21 -0700
                Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-04 11:46 -0700
                Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-04 15:04 -0700
                Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-04 16:44 -0700
                Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-04 17:13 -0700
                Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-04 19:12 -0700
                Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-05 14:10 -0700
                Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-05 14:22 -0700
          Re: scanf used for to get a line Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-15 10:57 -0800
            Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-15 12:05 -0800
              Re: scanf used for to get a line jameskuyper@verizon.net - 2016-11-15 14:01 -0800
              Re: scanf used for to get a line Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-16 10:54 -0800
              Re: scanf used for to get a line Richard Damon <Richard@Damon-Family.org> - 2016-11-21 18:01 -0500
                Re: scanf used for to get a line supercat@casperkitty.com - 2016-11-21 15:44 -0800
      Re: scanf used for to get a line asetofsymbols@gmail.com - 2016-11-03 13:35 -0700
        Re: scanf used for to get a line Keith Thompson <kst-u@mib.org> - 2016-11-03 13:50 -0700
      Re: scanf used for to get a line ImperiusDamian <damianblack.wolf@gmail.com> - 2016-11-05 16:10 -0400
  Re: scanf used for to get a line Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-01 11:39 +0000
    Re: scanf used for to get a line asetofsymbols@gmail.com - 2016-11-01 05:47 -0700
      Re: scanf used for to get a line Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-01 15:05 +0000
        Re: scanf used for to get a line asetofsymbols@gmail.com - 2016-11-02 01:42 -0700

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