Re: How many wide characters may mbstowcs store?

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: How many wide characters may mbstowcs store?
Date	2020-05-11 13:03 +0100
Organization	A noiseless patient Spider
Message-ID	<87tv0mvfun.fsf@bsb.me.uk> (permalink)
References	<r9bd16$nbt$1@solani.org>

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Philipp Klaus Krause <pkk@spth.de> writes:

> In the current C draft (N2479) - wording hasn't changed since at least
> C17 - it seems unclear how many wide characters mbstowcs may store:
>
> "size_t mbstowcs(wchar_t * restrict pwcs, const char *restrict s, size_t n);
>
> The mbstowcs function converts a sequence of multibyte characters that
> begins in the initial shift state from the array pointed to by s into a
> sequence of corresponding wide characters and stores not more than n
> wide characters into the array pointed to by pwcs. No multibyte
> characters that follow a null character (which is converted into a null
> wide character) will be examined or converted. Each multibyte character
> is converted as if by a call to the mbtowc function, except that the
> conversion state of the mbtowc function is not affected.
>
> No more than n elements will be modified in the array pointed to by
> pwcs. If copying takes place between objects that overlap, the behavior
> is undefined."
>
> One could read that so that only converted characters are stored, so
> nothing may be stored beyond a terminating 0. And indeed SDCC and my
> Debian GNU/Linux system behave that way.
>
> On the other hand, one could read that the only limit on the number of
> characters stored is n. And indeed someone reported to me that on their
> Ubuntu 18.04 LTS with gcc 7.5.0 both mbstowcs and wcstombs apparently
> write beyond a terminating 0 (but do not exceed the limit n).

I'm not getting the ambiguity.

  "No multibyte characters that follow a null character ... will be
  examined or converted."

I think you are saying that the wording permits arbitrary wide
characters (not resulting from examining and converting the input
string) to be written after the converted null?  That seems like a
stretch to me, but would only be a problem in that paragraph 4 says

 "... the mbstowcs function returns the number of array elements
  modified, not including a terminating null wide character, if any."

At least that paragraph makes the intent clear -- modifying the array
beyond a converted null would make the returned value useless.

-- 
Ben.

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How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 13:30 +0200
  Re: How many wide characters may mbstowcs store? Manfred <noname@add.invalid> - 2020-05-11 13:55 +0200
    Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 14:01 +0200
      Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-05-11 09:08 -0400
        Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 15:19 +0200
        Re: How many wide characters may mbstowcs store? Manfred <noname@add.invalid> - 2020-05-11 18:32 +0200
        Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 18:59 +0200
          Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 17:42 +0000
            Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 20:30 +0200
            Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-05-12 00:29 -0400
              Re: How many wide characters may mbstowcs store? Manfred <noname@add.invalid> - 2020-05-12 14:41 +0200
              Re: How many wide characters may mbstowcs store? Bonita Montero <Bonita.Montero@gmail.com> - 2020-05-12 16:19 +0200
  Re: How many wide characters may mbstowcs store? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-05-11 13:03 +0100
    Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 14:07 +0200
  Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 15:20 +0200
    Re: How many wide characters may mbstowcs store? Bonita Montero <Bonita.Montero@gmail.com> - 2020-05-11 16:31 +0200
    Re: How many wide characters may mbstowcs store? Barry Schwarz <schwarzb@delq.com> - 2020-05-11 10:06 -0700
  Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 15:58 +0200
    Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-05-11 10:24 -0400
    Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 16:52 +0200
      Re: How many wide characters may mbstowcs store? Manfred <noname@add.invalid> - 2020-05-11 18:55 +0200
    Re: How many wide characters may mbstowcs store? richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-05-11 15:51 +0000
      Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 19:01 +0200
        Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 17:33 +0000
          Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 20:57 +0200
            Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 19:17 +0000
              Re: How many wide characters may mbstowcs store? richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-05-11 19:41 +0000
                Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 20:01 +0000
            Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 19:19 +0000
    Re: How many wide characters may mbstowcs store? Florian Weimer <fw@deneb.enyo.de> - 2020-05-11 20:24 +0200
      Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 20:59 +0200
        Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 19:17 +0000
          Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 21:24 +0200
        Re: How many wide characters may mbstowcs store? Florian Weimer <fw@deneb.enyo.de> - 2020-05-11 22:30 +0200
  Re: How many wide characters may mbstowcs store? Bonita Montero <Bonita.Montero@gmail.com> - 2020-05-11 16:44 +0200
    Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 16:54 +0200
      Re: How many wide characters may mbstowcs store? Bonita Montero <Bonita.Montero@gmail.com> - 2020-05-11 16:57 +0200
        Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 17:07 +0200
          Re: How many wide characters may mbstowcs store? Bonita Montero <Bonita.Montero@gmail.com> - 2020-05-11 17:08 +0200
          Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-05-11 11:25 -0400
      Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-11 09:06 -0700
        Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 19:05 +0200
          Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 19:19 +0200
            Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-23 07:51 -0700
              Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-23 20:27 +0200
                Re: How many wide characters may mbstowcs store? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-05-23 14:25 -0700
                Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-26 07:09 -0700
                Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-26 07:14 -0700
                Re: How many wide characters may mbstowcs store? Spiros Bousbouras <spibou@gmail.com> - 2020-05-26 16:00 +0000
                Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-29 21:23 -0700
                Re: How many wide characters may mbstowcs store? Spiros Bousbouras <spibou@gmail.com> - 2020-05-30 20:08 +0000
                Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-06-03 08:46 -0700
                Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-06-03 10:18 -0700
                Re: How many wide characters may mbstowcs store? raltbos@xs4all.nl (Richard Bos) - 2020-06-04 20:34 +0000
        Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 17:39 +0000
          Re: How many wide characters may mbstowcs store? Autist <autist69@gmail.com> - 2020-05-11 19:42 +0200
          Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-11 20:28 +0200
            Re: How many wide characters may mbstowcs store? scott@slp53.sl.home (Scott Lurndal) - 2020-05-11 18:37 +0000
              Re: How many wide characters may mbstowcs store? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-05-11 11:50 -0700
                Re: How many wide characters may mbstowcs store? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-05-12 20:02 +0100
                Re: How many wide characters may mbstowcs store? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-05-12 13:12 -0700
          Re: How many wide characters may mbstowcs store? richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-05-11 19:56 +0000
          Re: How many wide characters may mbstowcs store? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-05-24 16:49 -0700
      Re: How many wide characters may mbstowcs store? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-05-11 14:19 -0700
        Re: How many wide characters may mbstowcs store? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-05-11 14:22 -0700
          Re: How many wide characters may mbstowcs store? Philipp Klaus Krause <pkk@spth.de> - 2020-05-12 09:17 +0200
        Re: How many wide characters may mbstowcs store? raltbos@xs4all.nl (Richard Bos) - 2020-05-24 16:12 +0000
          Re: How many wide characters may mbstowcs store? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-05-24 15:10 -0700
          Re: How many wide characters may mbstowcs store? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-05-24 22:58 -0400
  Re: How many wide characters may mbstowcs store? richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-05-11 20:07 +0000

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