Re: when is typecasting (unsigned char) to (char) dangerous?

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: when is typecasting (unsigned char) to (char) dangerous?
Date	2011-11-16 16:02 -0500
Organization	A noiseless patient Spider
Message-ID	<4EC424E0.8060506@verizon.net> (permalink)
References	(2 earlier) <ja0pub$bb4$1@speranza.aioe.org> <20111116094733.821@kylheku.com> <lnd3crzx48.fsf@nuthaus.mib.org> <4EC41C0B.2040304@verizon.net> <d784672e-5bb7-44d0-b8af-a141aead4cda@a5g2000vbb.googlegroups.com>

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On 11/16/2011 03:41 PM, Harald van Dĳk wrote:
> On Nov 16, 9:24 pm, James Kuyper <jameskuy...@verizon.net> wrote:
...
>> I know of no reason why signed char (and therefore, char) cannot have
>> trap representations. However, every statement in 6.2.6.1p5 which says
>> that the behavior is undefined when a trap representation is involved,
>> explicitly excludes all character types, not just unsigned char. I'm not
>> quite sure what to make of that fact, but I'm sure that explicitly
>> excluding all character types was intentional; I'm not so sure whether
>> it was intentional to allow signed char to have trap representations.
> 
> 6.2.6.1p5 refers to the trap representations for the type of the
> object. In other words, if an object p of type void * holds a trap
> representation, 6.2.6.1p5 makes it explicit that reading that object
> as void * is not valid.

So, in your opinion, what is the significance of the exclusion of
character types from those statements? What do those statements mean,
with those exclusions, that differs from what they would mean if those
exclusions were dropped? Please accompany your explanation with specific
examples of code that would have defined behavior under the existing
rules, but not with that modification, or vice-versa.

Thread

when is typecasting (unsigned char*) to (char*) dangerous? tim <nospam@nospam.com> - 2011-11-15 19:59 +0000
  Re: when is typecasting (unsigned char*) to (char*) dangerous? Kaz Kylheku <kaz@kylheku.com> - 2011-11-15 20:51 +0000
    Re: when is typecasting (unsigned char*) to (char*) dangerous? tim <nospam@nospam.com> - 2011-11-16 16:55 +0000
      Re: when is typecasting (unsigned char*) to (char*) dangerous? Kaz Kylheku <kaz@kylheku.com> - 2011-11-16 17:54 +0000
        Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 11:27 -0800
          Re: when is typecasting (unsigned char*) to (char*) dangerous? Kaz Kylheku <kaz@kylheku.com> - 2011-11-16 19:49 +0000
            Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 11:52 -0800
              Re: when is typecasting (unsigned char*) to (char*) dangerous? Kaz Kylheku <kaz@kylheku.com> - 2011-11-16 20:06 +0000
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 14:55 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 04:42 -0800
              Re: when is typecasting (unsigned char*) to (char*) dangerous? James Kuyper <jameskuyper@verizon.net> - 2011-11-16 15:15 -0500
          Re: when is typecasting (unsigned char*) to (char*) dangerous? James Kuyper <jameskuyper@verizon.net> - 2011-11-16 15:24 -0500
            Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-16 12:41 -0800
              Re: when is typecasting (unsigned char*) to (char*) dangerous? James Kuyper <jameskuyper@verizon.net> - 2011-11-16 16:02 -0500
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-16 13:23 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 15:32 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-16 17:02 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? pete <pfiland@mindspring.com> - 2011-11-16 21:38 -0500
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 19:22 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-16 23:08 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 23:46 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-17 09:15 -0800
                Re: when is typecasting (unsigned char*) to (char*) dangerous? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 04:54 -0800
          Re: when is typecasting (unsigned char*) to (char*) dangerous? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-16 21:44 +0100
      Re: when is typecasting (unsigned char*) to (char*) dangerous? Kenneth Brody <kenbrody@spamcop.net> - 2011-11-16 13:17 -0500
  Re: when is typecasting (unsigned char*) to (char*) dangerous? Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-11-16 00:58 -0800
  Re: when is typecasting (unsigned char*) to (char*) dangerous? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-11-16 01:36 -0800
    Re: when is typecasting (unsigned char*) to (char*) dangerous? Markus Wichmann <nullplan@gmx.net> - 2011-11-16 13:28 +0100
    Re: when is typecasting (unsigned char*) to (char*) dangerous? nroberts <roberts.noah@gmail.com> - 2011-11-16 08:14 -0800
      Re: when is typecasting (unsigned char*) to (char*) dangerous? BGB <cr88192@hotmail.com> - 2011-11-16 12:03 -0700
  Re: when is typecasting (unsigned char*) to (char*) dangerous? blp@cs.stanford.edu (Ben Pfaff) - 2011-11-16 08:58 -0800
    Re: when is typecasting (unsigned char*) to (char*) dangerous? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-16 19:04 +0100
      Re: when is typecasting (unsigned char*) to (char*) dangerous? Ben Pfaff <blp@cs.stanford.edu> - 2011-11-16 10:05 -0800
      Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-16 11:19 -0800
        Re: when is typecasting (unsigned char*) to (char*) dangerous? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-16 20:58 +0100
          Re: when is typecasting (unsigned char*) to (char*) dangerous? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-18 15:37 +0200
            Re: when is typecasting (unsigned char*) to (char*) dangerous? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-18 19:41 +0100
              Re: when is typecasting (unsigned char*) to (char*) dangerous? Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-18 19:43 +0100
  Re: when is typecasting (unsigned char*) to (char*) dangerous? Lauri Alanko <la@iki.fi> - 2011-11-17 19:09 +0000
    Re: when is typecasting (unsigned char*) to (char*) dangerous? Harald van Dĳk <truedfx@gmail.com> - 2011-11-17 11:44 -0800
    Re: when is typecasting (unsigned char*) to (char*) dangerous? Keith Thompson <kst-u@mib.org> - 2011-11-17 13:16 -0800
  Re: when is casting (unsigned char*) to (signed char*) dangerous? Shao Miller <sha0.miller@gmail.com> - 2012-01-22 02:22 -0500
    Re: when is casting (unsigned char*) to (signed char*) dangerous? Harald van Dijk <haraldvdijk@gmail.com> - 2012-01-22 12:14 -0800
    Re: when is casting (unsigned char*) to (signed char*) dangerous? Keith Thompson <kst-u@mib.org> - 2012-01-23 00:37 -0800
      Re: when is casting (unsigned char*) to (signed char*) dangerous? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-24 15:10 +0100
      Re: when is casting (unsigned char*) to (signed char*) dangerous? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 08:55 -0800
        Re: when is casting (unsigned char*) to (signed char*) dangerous? Keith Thompson <kst-u@mib.org> - 2012-01-26 11:32 -0800
          Re: when is casting (unsigned char*) to (signed char*) dangerous? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 21:57 -0800
            Re: when is casting (unsigned char*) to (signed char*) dangerous? Shao Miller <sha0.miller@gmail.com> - 2012-02-01 01:12 -0500
    Re: when is casting (unsigned char*) to (signed char*) dangerous? Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 08:48 -0800

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