Groups > comp.lang.c > #72021

Re: Reading beyond the end of file

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: Reading beyond the end of file
Date	2015-10-06 03:04 +0000
Organization	Aioe.org NNTP Server
Message-ID	<muvdol$jtn$1@speranza.aioe.org> (permalink)
References	(7 earlier) <lnio6l6gch.fsf@kst-u.example.com> <muthba$83c$1@speranza.aioe.org> <lntwq53zw4.fsf@kst-u.example.com> <muuqmc$gjo$1@speranza.aioe.org> <lnlhbh3q1x.fsf@kst-u.example.com>

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Keith Thompson <kst-u@mib.org> wrote:
>> Keith Thompson <kst-u@mib.org> wrote:

(snip, I wrote)
>>>> And what about the sizeof(char)==sizeof(int) case?
  
(snip, I also wrote)
>> I was thinking about UTF-32, but it also occurs for UTF-16 on
>> systems with 16 bit int.
 
> Plenty of systems support UTF-16 without making CHAR_BIT > 8.

OK, but see below.
 
>>> Another problem is that the input character is treated as an unsigned
>>> char and converted to signed int.  If unsigned char fits in the range of
>>> signed int, that's not a problem.  But if not, and if you read a
>>> character with a value exceeding INT_MAX, then the conversion to int
>>> yields an implementation-defined result or raises an
>>> implementation-defined signal.  Probably the conversion will do the
>>> typical 2's-complement wraparound, but if it doesn't, hilarity ensues.

(and I also wrote)

>> Seems to me that this already happens on systems where char is signed.

>> The (char) is converted to (unsigned char) for fgetc(), then after
>> reading it back to (char). 
 
>> Is (char) conversion defined different from (int) conversion?
 
> There's no such conversion.  N1570 7.21.7.1p1:
 
>    If the end-of-file indicator for the input stream pointed to by
>    stream is not set and a next character is present, the fgetc
>    function obtains that character as an unsigned char converted
>    to an int and advances the associated file position indicator
>    for the stream (if defined).
 
> It doesn't read the character as a (plain) char and then convert it to
> unsigned char.  It reads it directly as an unsigned char.  Before the
> read, it doesn't have a type; it's just bits in whatever the input
> stream is reading from.

Then it will be converted to signed (char) from (int), even
if the value is larger than CHAR_MAX.

I presume the value is the same as if the characters were read
directly into signed (char) without the intermediate (int).
 
>>> In practice, I don't think there are any hosted implementations with
>>> UCHAR_MAX > INT_MAX, and freestanding implementations aren't required to
>>> provide getchar() and friends.

>> I suspect UTF-16 will get more popular, but UTF-32 will likely 
>> stay rare for a while.
 
> Neither of those requires CHAR_BIT > 8.  See char16_t and char32_t in
> <uchar.h> (new in C11).

I was assuming the case where UTF-16 is popular enough that one
would just want to use (char). Java has been doing it for years
now, though it might take some years more for C.

-- glen

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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