Re: "undefined behavior"?

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: "undefined behavior"?
Date	2024-06-13 17:09 -0700
Organization	None to speak of
Message-ID	<878qz85qr7.fsf@nosuchdomain.example.com> (permalink)
References	(4 earlier) <666a2a30$0$952$882e4bbb@reader.netnews.com> <87tthx65qu.fsf@nosuchdomain.example.com> <v4dtlt$23m6i$1@dont-email.me> <87plsk5xbz.fsf@nosuchdomain.example.com> <v4g02k$2gfm9$2@dont-email.me>

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Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
> On 13/06/2024 22:47, Keith Thompson wrote:
>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>>> On 13/06/2024 01:33, Keith Thompson wrote:
>>>> printf is a variadic function, so the types of the arguments after
>>>> the format string are not specified in its declaration.  The printf
>>>> function has to *assume* that arguments have the types specified
>>>> by the format string.  This:
>>>>       printf("%d\n", foo);
>>>> (probably) has undefined behavior if foo is of type size_t.
>>>>
>>> And isn't that a nightmare?
>> Not at all.  Compilers commonly diagnose mismatches when the format
>> string is a string literal, as it most commonly is.  The format
>> specifier for size_t is "%zu", since C99.
>> 
>>>> There is no implicit conversion to the expected type.  Note that
>>>> the format string doesn't have to be a string literal, so it's
>>>> not always even possible for the compiler to check the types.
>>>> Variadic functions give you a lot of flexibility at the cost of
>>>> making some type errors difficult to detect.
>>>> (I wrote "probably" because size_t *might* be a typedef for unsigned
>>>> int, and there are special rules about arguments of corresponding
>>>> signed and unsigned types.)
>>>
>>> We just can't have size_t variables swilling around in prgrams for
>>> these reasons.
>> We can and do.
>> 
> And this is how things break.
>
> Now, running a third party editor under your control so that user can
> edit an text and return control and the edited text back to you when
> he exits the editor. Yes, I understand that this is a difficult thing
> to do, the software engineeering isn't consistent, and theway you have
> to do it may change from one version of C to another.
> But printing out a variable which holds the length of a string? And
> something so basic breaks from one version of C to the next? We should 
> ahave no tolerance for that at all.

What broke?  And how would *you* print the result of strlen()?

strlen() has returned a result of type size_t since C89/C90.

C99 (that's 25 years ago) added the "%zu" format specifier.  Today,
you're unlikely to find an implementation that doesn't support
    printf("%zu\n", strlen(s));
But even if you need to deal with pre-C99 implementations for some
reason, this:
    printf("%lu\n", (unsigned long)(strlen(s));
works reliably in C90, and works in C99 and later as long as size_t is
no wider than unsigned long -- and even then it breaks (printing an
incorrect value) only if the actual value returned by strlen(s) exceeds
ULONG_MAX, which is at least 4294967295.  If you're using 4-gigabyte
strings, you probably want to avoid calling strlen() on them anyway.

This:
    printf("%d\n", strlen(s));
has *never* been valid (it has undefined behavior unless the
implementation you're using happens to make size_t a typedef for
unsigned int and the value doesn't exceed INT_MAX, which might be
as small as 32767).

We're simply not going to throw away the last quarter century of
progress in C and go back to C90.  You can if you like, but don't
expect anyone else to follow you.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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"undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 16:47 -0400
  Re: "undefined behavior"? Barry Schwarz <schwarzb@delq.com> - 2024-06-12 14:30 -0700
    Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 17:53 -0400
      Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:30 -0700
        Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 19:07 -0400
          Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 17:33 -0700
            Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-13 05:47 +0100
              Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-13 15:39 +0000
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-13 18:08 +0100
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-13 19:01 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-13 19:54 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-13 12:34 -0700
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 00:32 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 00:55 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 02:48 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 12:44 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 15:30 +0100
                Re: "undefined behavior"? Richard Harnden <richard.nospam@gmail.invalid> - 2024-06-14 16:32 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 19:06 +0100
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-14 19:31 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 20:13 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 22:29 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 23:35 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-15 00:14 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 20:57 +0200
                Re: "undefined behavior"? Richard Harnden <richard.nospam@gmail.invalid> - 2024-06-15 20:27 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-15 23:13 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-16 12:53 +0200
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-16 14:44 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-14 11:49 -0700
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 22:32 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-15 00:56 -0700
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 15:58 -0700
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-14 02:18 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:08 +0200
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 12:34 -0700
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:13 +0200
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 13:43 -0700
              Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 14:47 -0700
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 00:41 +0100
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 17:09 -0700
  Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-12 23:38 +0200
    Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:18 -0700
      Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 14:42 +0200
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 16:39 -0700
      Re: "undefined behavior"? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-18 17:23 -0700
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-18 17:42 -0700
          Re: "undefined behavior"? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-22 09:28 -0700
    Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 18:29 -0400
      Re: "undefined behavior"? Ike Naar <ike@sdf.org> - 2024-06-13 07:25 +0000
        Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 11:13 -0400
          Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-13 15:40 +0000
          Re: "undefined behavior"? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2024-06-13 15:49 +0000
            Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 13:05 -0400
      Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:15 +0200
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 16:47 -0700
          Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:13 +0200
  Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-12 23:38 +0200
    Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:22 -0700
      Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 18:34 -0400
        Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:21 +0200
          Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 10:38 -0400
            Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:18 +0200
              Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-14 17:36 +0000
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:15 +0200
              Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-14 19:05 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 18:39 -0700
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-14 23:49 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 20:56 -0700
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-15 00:45 -0400
                Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-15 07:03 +0200
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-15 07:39 -0400
                Re: "undefined behavior"? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-06-15 01:05 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 22:20 -0700
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-15 09:37 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:22 +0200
      Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-13 02:19 +0200
        Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:28 +0200
  Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 14:57 -0700
  Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-13 10:43 +0100
  Re: "undefined behavior"? Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-13 11:45 +0200

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