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Re: Never use strncpy!

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c++
Subject Re: Never use strncpy!
Date 2022-09-21 15:56 +0100
Organization A noiseless patient Spider
Message-ID <87mtas22ty.fsf@bsb.me.uk> (permalink)
References <tgenin$qc3$1@gioia.aioe.org>

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Juha Nieminen <nospam@thanks.invalid> writes:

> Well, *almost* never, at least.
>
> I always thought that std::strncpy() works exactly like std::strcpy(),
> except that it stops early if the specified count is reached. Turns out
> that I was gravely mistaken:
>
> "If, after copying the terminating null character from src, count is not
> reached, additional null characters are written to dest until the total
> of count characters have been written."
...
> "If count is reached before the entire string src was copied, the
> resulting character array is not null-terminated."

Yes, a famous oddity that stems from one very specific use in
fixed-width Unix data structures (the most common use for a file name in
a directory entry that had to be zero filled but need not be zero
terminated).

> I can't find any standard library (C or C++) function that does that,
> so you'll just have to write your own. (Luckily it's trivial to do.)

A common trick was to write

   *dest = 0;
   strncat(dest, source, N);

or even (in an expression context)

  strncat((*dest = 0, dest), source, N)

> Perhaps return to the caller some value telling if the string was
> truncated.

If your C library includes Annex K there is the rather gruesome
strncpy_t function.

And there is a very widely available, but non-standard, function called
strlcat.

-- 
Ben.

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Thread

Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-21 10:04 +0000
  Re: Never use strncpy! "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-09-21 15:06 +0200
  Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-21 15:24 +0200
    Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-21 15:30 +0200
      Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-21 19:20 +0200
        Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-21 19:33 +0200
          Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-21 23:14 +0200
            Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-22 04:40 +0200
              Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-22 08:56 +0200
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-22 12:02 +0200
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-23 13:23 +0200
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-23 13:49 +0200
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-23 15:25 +0200
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-23 16:03 +0200
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-23 14:34 +0000
                Re: Never use strncpy! Michael S <already5chosen@yahoo.com> - 2022-09-23 08:19 -0700
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-23 17:57 +0000
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-23 18:36 +0200
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-24 16:22 +0200
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-24 16:25 +0200
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-24 17:09 +0200
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-24 17:14 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-09-26 17:34 -0700
                Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-26 07:53 +0000
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-26 10:21 +0200
                Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-26 08:34 +0000
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-26 13:39 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-09-26 17:47 -0700
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-27 14:17 +0000
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-27 18:07 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-09-27 12:42 -0700
                Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-28 09:32 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-09-28 13:34 -0700
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-28 21:11 +0000
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-01 06:26 +0200
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-10-01 17:01 +0000
                Re: Never use strncpy! Michael S <already5chosen@yahoo.com> - 2022-10-02 15:32 -0700
                Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-10-03 14:01 +0000
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-03 13:49 -0700
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-02 12:46 -0700
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-03 04:14 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-02 19:43 -0700
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-02 19:48 -0700
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-03 04:53 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-02 19:58 -0700
                Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-03 08:27 +0200
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-03 13:49 -0700
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-04 10:53 -0700
                Re: Never use strncpy! "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-02 12:44 -0700
            Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-22 05:59 +0000
              Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-22 09:32 +0200
    Re: Never use strncpy! Muttley@dastardlyhq.com - 2022-09-21 15:42 +0000
      Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-21 18:10 +0200
        Re: Never use strncpy! Muttley@dastardlyhq.com - 2022-09-21 16:19 +0000
          Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-21 18:27 +0200
            Re: Never use strncpy! Muttley@dastardlyhq.com - 2022-09-23 14:47 +0000
      Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-21 16:12 +0000
    Re: Never use strncpy! Philipp Klaus Krause <pkk@spth.de> - 2022-09-23 19:52 +0200
      Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-24 16:25 +0200
  Re: Never use strncpy! scott@slp53.sl.home (Scott Lurndal) - 2022-09-21 14:00 +0000
    Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-22 06:03 +0000
      Re: Never use strncpy! Philipp Klaus Krause <pkk@spth.de> - 2022-09-23 19:50 +0200
        Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-26 07:54 +0000
  Re: Never use strncpy! Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-09-21 15:56 +0100
  Re: Never use strncpy! Muttley@dastardlyhq.com - 2022-09-21 15:36 +0000
  Re: Never use strncpy! Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-09-21 08:59 -0700
  Re: Never use strncpy! Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-09-21 13:23 -0700
    Re: Never use strncpy! Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-09-21 21:49 +0100
      Re: Never use strncpy! Richard Damon <Richard@Damon-Family.org> - 2022-09-21 19:18 -0400
        Re: Never use strncpy! Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-09-22 00:39 +0100
          Re: Never use strncpy! Richard Damon <Richard@Damon-Family.org> - 2022-09-21 20:45 -0400
            Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-21 18:00 -0700
          Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-21 17:55 -0700
        Re: Never use strncpy! Juha Nieminen <nospam@thanks.invalid> - 2022-09-22 06:09 +0000
          Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-22 11:44 -0700
          Re: Never use strncpy! Richard Damon <Richard@Damon-Family.org> - 2022-09-22 23:33 -0400
      Re: Never use strncpy! Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-09-22 19:00 -0700
        Re: Never use strncpy! Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-09-22 19:04 -0700
          Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-22 22:49 -0700
    Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-24 11:38 +0200
      Re: Never use strncpy! Muttley@dastardlyhq.com - 2022-09-24 09:43 +0000
        Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-24 11:59 +0200
      Re: Never use strncpy! Richard Damon <Richard@Damon-Family.org> - 2022-09-24 06:27 -0400
        Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-24 11:53 -0700
          Re: Never use strncpy! Bonita Montero <Bonita.Montero@gmail.com> - 2022-09-24 20:58 +0200
  Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-21 16:01 -0700
    Re: Never use strncpy! David Brown <david.brown@hesbynett.no> - 2022-09-22 09:37 +0200
    Re: Never use strncpy! Manfred <noname@add.invalid> - 2022-10-01 01:24 +0200
      Re: Never use strncpy! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-09-30 16:43 -0700
  Re: Never use strncpy! Lynn McGuire <lynnmcguire5@gmail.com> - 2022-09-22 21:03 -0500

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