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Groups > comp.lang.c > #124120
| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: NULL as the empty string |
| Date | 2017-12-10 19:20 -0800 |
| Organization | None to speak of |
| Message-ID | <lnbmj6q7an.fsf@kst-u.example.com> (permalink) |
| References | <ov2an8$7sq$1@dont-email.me> <f92tlaFp91dU8@mid.individual.net> <lny3mbr6rv.fsf@kst-u.example.com> <p0kk96$h7s$1@dont-email.me> |
jacobnavia <jacob@jacob.remcomp.fr> writes:
> Le 09/12/2017 à 21:22, Keith Thompson a écrit :
>> But if the standard were changed to treat a null pointer as if it
>> were a pointer to an empty string, then non-standard functions
>> would likely return a null pointer to denote an empty string.
>> For example, strdup() (which is POSIX, not ISO C) might return
>> a null pointer given a null pointer argument -- or even given a
>> non-null pointer to an empty string.
>
> Truth table:
>
> strdup(str) --> NULL or a malloced string.
> strdup(NULL) --> NULL.
>
> Everything would stay the same.
(That's not a "truth table". The entries in a truth table are true or
false.)
Would strdup(NULL) returning a malloced empty string be non-conforming?
> Unless you WANT to crash within strdup and not when the NULL pointer
> result is dereferenced. In that case (as in the others)
>
> #define strdup TrappingStrdup
>
> char *TrappingStrdup(char *str)
> {
> if (*str == 0)
> ;
> return strdup(str);
> }
Or I can just use strdup() as it's currently implemented (though *str
has undefined behavior and is not guaranteed to trap). For that matter,
since evaluting *str has undefined behavior, a compiler could assume
that str is non-null thereafter.
But that raises an interesting question. In C as it's currently
defined, if s is a pointer to an empty string:
const char *s = "";
then *s and s[0] both yield '\0', and more generally, if s is a pointer
to a string then s[strlen(s)] == '\0'. With your proposed change, that
guarantee would be broken. s could be (treated as) a pointer to an
empty string, but if s==NULL then *s and s[0] would have undefined
behavior (and would likely trap).
If I want to traverse a string, I can write:
for (ptr = s; *ptr != '\0' ptr ++) {
do_something_with(*ptr);
}
and it will work if ptr points to a string. Under your proposal,
ptr==NULL would make it a valid pointer to a string, but the above
code would break.
Your change would not affect existing programs that (correctly) assume a
null pointer doesn't point to a string, but such code could be broken if
it's called by new code that relies on your new definitions.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-21 23:52 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-11-21 15:16 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 00:38 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-11-21 16:02 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 01:13 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-11-21 16:52 -0800
Re: NULL as the empty string Robert Wessel <robertwessel2@yahoo.com> - 2017-11-21 18:09 -0600
Re: NULL as the empty string Siri Cruise <chine.bleu@yahoo.com> - 2017-11-21 16:34 -0800
Re: NULL as the empty string David Brown <david.brown@hesbynett.no> - 2017-11-22 12:12 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-21 15:57 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 01:06 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-22 15:42 -0800
Re: NULL as the empty string Melzzzzz <Melzzzzz@zzzzz.com> - 2017-11-22 23:49 +0000
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-22 15:56 -0800
Re: NULL as the empty string Melzzzzz <Melzzzzz@zzzzz.com> - 2017-11-23 00:06 +0000
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-23 17:31 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-24 09:42 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-24 13:47 -0800
Re: NULL as the empty string Jorgen Grahn <grahn+nntp@snipabacken.se> - 2017-11-22 06:46 +0000
Re: NULL as the empty string John Bode <jfbode1029@gmail.com> - 2017-12-08 10:27 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 11:11 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-08 21:39 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 13:03 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-08 22:50 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 15:19 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-09 00:35 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 16:05 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-09 01:22 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 17:39 -0800
Re: NULL as the empty string John Bode <jfbode1029@gmail.com> - 2017-12-11 12:22 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-09 01:29 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-08 17:47 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-09 07:05 +0100
Re: NULL as the empty string David Brown <david.brown@hesbynett.no> - 2017-12-09 18:37 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-09 11:53 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-12 10:49 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-12 13:39 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-12 16:05 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-13 03:43 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-13 08:45 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-13 09:12 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-13 13:27 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-13 14:02 -0800
Re: NULL as the empty string asetofsymbols@gmail.com - 2017-12-13 14:58 -0800
Re: NULL as the empty string asetofsymbols@gmail.com - 2017-12-13 15:11 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-14 03:49 -0800
Re: NULL as the empty string mark.bluemel@gmail.com - 2017-12-14 04:05 -0800
Re: NULL as the empty string David Brown <david.brown@hesbynett.no> - 2017-12-14 13:09 +0100
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-14 05:02 -0800
Re: NULL as the empty string David Brown <david.brown@hesbynett.no> - 2017-12-14 14:54 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-14 07:38 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-14 09:50 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-14 09:20 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-14 09:53 -0800
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-14 12:57 -0800
Re: NULL as the empty string herrmannsfeldt@gmail.com - 2017-12-14 17:22 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-14 17:26 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-08 21:23 +0100
Re: NULL as the empty string Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-12-08 13:41 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-08 22:54 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-21 15:17 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 00:26 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-11-21 16:03 -0800
Re: NULL as the empty string "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-22 00:27 +0100
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 00:42 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-11-21 16:05 -0800
Re: NULL as the empty string herrmannsfeldt@gmail.com - 2017-12-06 22:33 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-07 12:04 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-07 23:20 +0100
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-07 15:04 -0800
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-11-21 15:28 -0800
Re: NULL as the empty string Thiago Adams <thiago.adams@gmail.com> - 2017-11-21 16:04 -0800
Re: NULL as the empty string Siri Cruise <chine.bleu@yahoo.com> - 2017-11-21 16:25 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-22 01:34 +0100
Re: NULL as the empty string bartc <bc@freeuk.com> - 2017-11-22 00:36 +0000
Re: NULL as the empty string Öö Tiib <ootiib@hot.ee> - 2017-11-21 23:07 -0800
NULL as the empty string asetofsymbols@gmail.com - 2017-11-23 22:23 -0800
Re: NULL as the empty string Geoff <geoff@invalid.invalid> - 2017-12-09 09:05 -0800
Re: NULL as the empty string Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-12-09 12:40 -0500
Re: NULL as the empty string gordonb.yj0bc@burditt.org (Gordon Burditt) - 2017-12-09 13:50 -0600
Re: NULL as the empty string Ian Collins <ian-news@hotmail.com> - 2017-12-10 08:59 +1300
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-09 12:22 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-11 01:42 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-10 19:20 -0800
Re: NULL as the empty string jacobnavia <jacob@jacob.remcomp.fr> - 2017-12-11 18:56 +0100
Re: NULL as the empty string Keith Thompson <kst-u@mib.org> - 2017-12-11 11:19 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-15 09:29 -0800
Re: NULL as the empty string Thiago Adams <thiago.adams@gmail.com> - 2018-01-05 08:28 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2018-01-05 09:37 -0800
Re: NULL as the empty string Thiago Adams <thiago.adams@gmail.com> - 2018-01-05 17:08 -0800
Re: NULL as the empty string supercat@casperkitty.com - 2017-12-15 09:18 -0800
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