Re: Dymamic arrays: memory management and naming

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Dymamic arrays: memory management and naming
Date	2023-09-25 19:38 -0700
Organization	None to speak of
Message-ID	<87msx9u09c.fsf@nosuchdomain.example.com> (permalink)
References	(4 earlier) <20230924003915.51bb21339ab1c551eba59162@gmail.moc> <KZf6tlV2s29qC6sCU@bongo-ra.co> <20230924081911.947@kylheku.com> <20230925155845.139@kylheku.com> <86sf71itgt.fsf@linuxsc.com>

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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Kaz Kylheku <864-117-4973@kylheku.com> writes:
>> On 2023-09-24, Kaz Kylheku <864-117-4973@kylheku.com> wrote:
>>> h only makes sense if you're passing a signed argument and want it
>>> printed as unsigned.  Say we have 16 bit short and 32 bit int.
>>> Then "%u"/(short) -1 will print as 429496725, whereas "%hu"/(short) -1
>>> will obtain the desired 65535.
>>
>> There is something I had neglected to mention:  the use of h for portability
>>
>> If we have "%u" paired with an unsigned short, argument we have
>> a mismatch on a large number of platforms, where unsigned short promotes
>> to int.  The %u specifier wants unsigned int.
>
> A %u conversion specification is always okay for an unsigned
> short argument, regardless of whether the argument promotes to
> int or to unsigned int.  Reading an integer-typed variadic
> argument with the corresponding type of the other signedness
> is allowed when the argument value is representable in both
> types, which it always will be in this case even if unsigned
> short promotes to int.

Agreed, but unfortunately the standard doesn't quite say that in
normative text -- until C23, as I discovered while writing this.

In C17 and earlier, 7.21.6.1p9 (The fprintf function) says:
    If any argument is not the correct type for the corresponding
    conversion specification, the behavior is undefined.

(The word "the" implies that there is only one "correct type".)

6.2.5p9 (Types) says:
    The range of nonnegative values of a signed integer type is a
    subrange of the corresponding unsigned integer type, and the
    representation of the same value in each type is the same.
with a footnote:
    The same representation and alignment requirements are meant to
    imply interchangeability as arguments to functions, return values
    from functions, and members of unions.
but of course footnotes are non-normative.

I can imagine a (contrived) conforming implementation that passes
int arguments in one set of CPU registers and unsigned int arguments
in another set of CPU registers, so that `printf("%u\n", 42);`
would very probably *not* print "42".  Such an implementation
would violate the intent of that footnote, but would not as far
as I can tell violate any normative requirement.  (In real life,
I'm sure no such implementation exists.)

But C23 (N3096 draft) changes the description for fprintf:
    fprintf shall behave as if it uses va_arg with a type argument
    naming the type resulting from applying the default argument
    promotions to the type corresponding to the conversion
    specification and then converting the result of the va_arg
    expansion to the type corresponding to the conversion
    specification.

and the description of the va_arg() macro explicitly allows the type
of the first argument and the type given by the second argument
to be corresponding signed and unsigned types, if the value is
representable in both types.  (This goes back to C99.)

Prior to C23, though the <stdarg.h> macros are the obvious way to
implement the *printf() functions, the standard didn't require it
to behave that way.

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

Thread

Dymamic arrays: memory management and naming Anton Shepelev <anton.txt@gmail.moc> - 2023-09-09 13:23 +0300
  Re: Dymamic arrays: memory management and naming Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-09-09 03:56 -0700
  Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-09 15:06 +0000
  Re: Dymamic arrays: memory management and naming scott@slp53.sl.home (Scott Lurndal) - 2023-09-09 15:44 +0000
  Re: Dymamic arrays: memory management and naming Spiros Bousbouras <spibou@gmail.com> - 2023-09-10 17:57 +0000
    Re: Dymamic arrays: memory management and naming Anton Shepelev <anton.txt@gmail.moc> - 2023-09-10 23:49 +0300
      Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-10 21:25 +0000
  Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-12 16:07 -0700
    Re: Dymamic arrays: memory management and naming Anton Shepelev <anton.txt@gmail.moc> - 2023-09-17 00:44 +0300
  Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-17 16:51 -0700
    Re: Dymamic arrays: memory management and naming Anton Shepelev <anton.txt@gmail.moc> - 2023-09-19 01:10 +0300
      Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-19 07:58 -0700
      Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-19 10:33 -0700
        Re: Dymamic arrays: memory management and naming Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-19 15:08 -0700
          Re: Dymamic arrays: memory management and naming "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-19 15:35 -0700
          Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-19 22:49 +0000
            Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-19 22:30 -0700
              Re: Dymamic arrays: memory management and naming Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-20 02:46 -0700
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-20 07:06 -0700
              Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-20 21:03 +0000
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 14:08 -0700
                Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-25 21:27 +0000
          Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-19 22:17 -0700
        Re: Dymamic arrays: memory management and naming Anton Shepelev <anton.txt@gmail.moc> - 2023-09-24 00:39 +0300
          Re: Dymamic arrays: memory management and naming Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-09-23 21:48 +0000
          Re: Dymamic arrays: memory management and naming James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-09-24 00:15 -0400
          Re: Dymamic arrays: memory management and naming Spiros Bousbouras <spibou@gmail.com> - 2023-09-24 11:48 +0000
            Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-24 16:08 +0000
              Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 13:21 -0700
                Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-25 21:06 +0000
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 18:43 -0700
              Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-25 23:14 +0000
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 19:00 -0700
                Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-26 02:38 +0000
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 23:42 -0700
                Re: Dymamic arrays: memory management and naming Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-25 19:38 -0700
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 23:41 -0700
                Re: Dymamic arrays: memory management and naming Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-26 02:42 -0700
                Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-03 03:29 -0700
            Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-24 16:11 +0000
            Re: Dymamic arrays: memory management and naming James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-09-24 12:28 -0400
              Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-24 17:10 +0000
                Re: Dymamic arrays: memory management and naming James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-09-24 13:56 -0400
            Re: Dymamic arrays: memory management and naming Phil Carmody <pc+usenet@asdf.org> - 2023-09-30 13:50 +0300
              Re: Dymamic arrays: memory management and naming Spiros Bousbouras <spibou@gmail.com> - 2023-10-01 17:43 +0000
          Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 09:21 -0700
            Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-25 18:06 +0000
              Re: Dymamic arrays: memory management and naming Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-25 15:24 -0700
                Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-25 23:49 +0000
                Re: Dymamic arrays: memory management and naming Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-26 00:07 +0000

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