Groups > comp.lang.c > #71989

Re: Reading beyond the end of file

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Reading beyond the end of file
Date	2015-10-05 13:15 -0400
Organization	Self
Message-ID	<5612B04A.3030201@verizon.net> (permalink)
References	(8 earlier) <kfnoage5ax4.fsf@x-alumni2.alumni.caltech.edu> <PElQx.11877$3T2.3520@fx29.iad> <musu7o$v4j$1@speranza.aioe.org> <lnio6l6gch.fsf@kst-u.example.com> <muthba$83c$1@speranza.aioe.org>

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On 10/05/2015 05:53 AM, glen herrmannsfeldt wrote:
> Keith Thompson <kst-u@mib.org> wrote:
> 
> (snip, I wrote)
> 
>>> In the case of getchar() with char as unsigned, no valid char
>>> value is -1.  (I am not sure what systems with sizeof(int) equal
>>> to sizeof(char) do.)
> 
>>> If valid char values are 0 to 255, -1 is out of band.
> 
>>> If char values are, for example, -128 to 127 then -1 is in band.
>  
>> getchar() doesn't operate on plain char values.  It returns the next
>> input character *as an unsigned char* converted an int.
> 
> OK, so unsigned even on systems with signed char.
> 
> And what about the sizeof(char)==sizeof(int) case?

The relevant issue isn't the size, but the upper limits. If UCHAR_MAX >
INT_MAX (which requires CHAR_BIT >=16, and makes it likely that
sizeof(int)==1 unless int has at least one entire padding byte), it's
possible for unsigned char values to be too big to be represented as an int.
In that case, paying attention only to the conversion rules (6.3.1.3p3),
it might seem that all you can say is "the result is
implementation-defined or an implementation-defined signal is raised)".
However, 7.21.2p3 says "Data read in from a binary stream shall compare
equal to the data that were earlier written out to that stream, under
the same implementation." Since all C I/O has behavior that traces back
to the behavior defined for fgetc() and fputc(), and since the behavior
of fgetc() and fputc() is defined in terms of conversions between
unsigned char and int, then if UCHAR_MAX > INT_MAX, it's necessarily the
case that a signal cannot be raised, and that the implementation-defined
result of a conversion from unsigned char to int must invert the
standard-defined conversion from int to unsigned char. In particular, if
you use fputc() to write EOF to a binary stream, the result when fgetc()
successfully reads that byte must necessarily be EOF. Therefore, it is
necessary to use ferror() and feof() to detect an error condition or
end-of-file; checking whether fgetc() returns EOF is not sufficient.

UCHAR_MAX > INT_MAX is extremely unlikely, but as far as I can tell it
wouldn't render the implementation non-conforming. It would render it
impossible to ungetc() a byte that contains (unsigned char)EOF.

Thread

Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-11 10:06 -0700
  Re: Reading beyond the end of file Geoff <geoff@invalid.invalid> - 2015-09-11 12:41 -0700
    Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-26 14:24 -0700
      Re: Reading beyond the end of file Geoff <geoff@invalid.invalid> - 2015-09-26 21:49 -0700
        Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-01 13:57 -0700
          Re: Reading beyond the end of file Geoff <geoff@invalid.invalid> - 2015-10-01 17:22 -0700
            Re: Reading beyond the end of file raltbos@xs4all.nl (Richard Bos) - 2015-10-02 13:05 +0000
            Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 17:56 -0700
  Re: Reading beyond the end of file Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-09-11 13:55 -0600
    Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-26 14:14 -0700
      Re: Reading beyond the end of file Siri Cruz <chine.bleu@yahoo.com> - 2015-09-26 14:43 -0700
        Re: Reading beyond the end of file Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-09-26 17:57 -0400
        Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-01 14:01 -0700
      Re: Reading beyond the end of file Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-09-26 22:18 -0600
        Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-01 12:58 -0700
          Re: Reading beyond the end of file Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-10-01 15:08 -0600
            Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 18:03 -0700
              Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 04:02 +0000
      Re: Reading beyond the end of file raltbos@xs4all.nl (Richard Bos) - 2015-10-02 13:11 +0000
        Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-02 09:31 -0700
          Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-03 16:07 -0400
        Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 19:06 -0700
  Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-09-11 14:36 -0700
    Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-26 14:05 -0700
      Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-09-26 18:53 -0700
        Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-01 13:49 -0700
          Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-01 20:23 -0400
            Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-02 06:14 +0000
            Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 19:03 -0700
              Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-04 22:55 -0400
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 04:27 +0000
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-04 22:21 -0700
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 09:53 +0000
                Re: Reading beyond the end of file James Kuyper <jameskuyper@verizon.net> - 2015-10-05 13:15 -0400
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-05 10:25 -0700
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 21:30 +0000
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 21:27 +0000
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-05 10:17 -0700
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-05 11:59 -0700
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-05 13:44 -0700
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 21:58 +0000
                Ones' complement subtraction supercat@casperkitty.com - 2015-10-05 15:28 -0700
                Re: Ones' complement subtraction glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 02:14 +0000
                Re: Ones' complement subtraction supercat@casperkitty.com - 2015-10-06 08:48 -0700
                Re: Ones' complement subtraction glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 20:55 +0000
                Re: Ones' complement subtraction supercat@casperkitty.com - 2015-10-06 14:41 -0700
                Re: Ones' complement subtraction glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-07 00:14 +0000
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-05 15:24 -0700
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 21:39 +0000
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-05 15:31 -0700
                Re: Reading beyond the end of file glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 03:04 +0000
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-06 08:55 -0700
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-06 11:44 -0700
                Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-05 23:16 -0400
                Re: Reading beyond the end of file James Kuyper <jameskuyper@verizon.net> - 2015-10-06 10:29 -0400
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-06 08:56 -0700
                Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-06 23:13 -0400
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-06 22:01 -0700
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-07 08:43 -0700
                Re: Reading beyond the end of file supercat@casperkitty.com - 2015-10-07 10:02 -0700
                Re: Reading beyond the end of file Keith Thompson <kst-u@mib.org> - 2015-10-07 13:08 -0700
                Re: Reading beyond the end of file Richard Damon <Richard@Damon-Family.org> - 2015-10-07 22:03 -0400
                Re: Reading beyond the end of file Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-05 01:47 -0700

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