Re: AVL tree insertion - programming exercise

From	cross@spitfire.i.gajendra.net (Dan Cross)
Newsgroups	comp.lang.c
Subject	Re: AVL tree insertion - programming exercise
Date	2026-10-16 11:27 +0000
Organization	PANIX Public Access Internet and UNIX, NYC
Message-ID	<11at1nj$rtn$1@reader2.panix.com> (permalink)
References	<86ece2kppv.fsf@linuxsc.com> <11arlj9$2q39c$1@kst.eternal-september.org> <11as4h9$7ht$1@reader2.panix.com> <11as9gl$2vtou$1@kst.eternal-september.org>

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In article <11as9gl$2vtou$1@kst.eternal-september.org>,
Keith Thompson  <Keith.S.Thompson+u@gmail.com> wrote:
>cross@spitfire.i.gajendra.net (Dan Cross) writes:
>[Snip stuff I agree with]
>
>> Right.  The only reason I wanted to write it that way was to
>> drive that point home; in the "strong-alignment" reading, for
>> small allocations, the alignment of the address the pointer
>> returned from `malloc` refers to is totally decoupled from the
>> requested size: `sizeof(char[3])` is three, which reflects the
>> fundamental alignment requirement of `char[3]` being 1; because
>> `malloc` doesn't know whether the resulting allocation will be
>> used to store something with a fundamental alignment criteria of
>> two or not (assuming such a type exists in the implementation),
>> it must return a pointer to an address that is at least aligned
>> to an even boundary.  In any event, whether `malloc` must return
>> something with stronger alignment, or merely can at its own
>> convenience, is the interesting high order bit here.
>
>Just to avoid any misunderstanding, you refer to "the fundamental
>alignment requirement of `char[3]` being 1".

What I'm trying to say (that I think you said better), is that
the alignment requirement of `char` is fundamental, and that the
aggregate type (the array, `char[3]`) has the same alignment
requirement.  In this case, the that is `1`.

>The phrase "fundamental alignment requirement" might be misleading.
>It could imply to someone unfamiliar with the terminology that
>char[3] could have a fundamental alignment requirement and a
>differing non-fundamental alignment requirement.  (I had thought
>the term "fundamental alignment" was new in C23, but it was added
>in C11.)
>
>It's possible that I'm the only one who thought this might be
>confusing.  If so, ignore this.

Hmm, I don't know that anyone else thought that, but I cannot
say that they did not, and it was useful to make the point.

>To over-explain: Each object type has an alignment, which is an integer
>value.
>
>    A *fundamental alignment* is a valid alignment less than or equal to
>    alignof(max_align_t).
>
>    An *extended alignment* is represented by an alignment greater than
>    alignof(max_align_t).
>
>As far as the language is concerned `char[3]` has an alignment of
>1, and that alignment happens to be a fundamental alignment.  (A
>compiler might allocate standalone `char[3]` objects at a stricter
>alignment, but the standard doesn't address that; compilers can
>over-align anything they like.)

Yes, and succinctly put.

A couple of minor quibbles: it bears mentioning that valid
alignments are positive powers of two.  Also, a point you had
aluded to earlier: for e.g., `malloc(sizeof(char[3]))`, by the
time the size argument gets to `malloc` the type information
(including the alignment requirement) has been lost; malloc just
sees `3`.  For an implementation where there exists types with
an alignment requirement of two and `sizeof(max_align_t)>=2`,
such an alignment requirement is also fundamental.  `malloc`
cannot know that its return value won't be used to hold an
object of such type, so it must return a pointer to an address
that is aligned on an even boundary.

>Which might be part of the confusion about the malloc() wording in C23.
>
>    The pointer returned if the allocation succeeds is suitably aligned
>    so that it may be assigned to a pointer to any type of object with a
>    fundamental alignment requirement and size less than or equal to the
>    size requested.
>
>I read the phrase "with a fundamental alignment requirement" as a
>predicate.  A type has a fundamental alignment requirement or it
>doesn't.
>
>The alternative reading is "with a fundamental alignment requirement
>(and size) less than or equal to the size requested", which says that
>the alignment requirement (a) is fundamental *and* (b) is less than or
>equal to the size requested.  I think that's a rather strained
>interpretation -- and I can't think of any cases where the distinction
>between the two readings of the C23 wording could be significant, given
>that an object type can't have an alignment bigger than its size.

Yes, and I do see what you mean: it's a distinction without a
difference due to other constraints imposed on types generally.

Specifically (and speaking somewhat loosely), if a type has an
alignment requirement that is fundamental, then it is <= the
type's size; no such type may have alignment>size.  Therefore,
whether or not the "size" condition is to applied to the
"fundamental alignment requirement" phrase in the C23 sentence,
the result is the same.

This is in contrast to the C17 phrasing, which was read by many
good judges as requiring alignment to `>=alignof(max_align_t)`
regardless of size.

>[Snip more stuff I agree with]

I think we're all agreed about the following: a conforming
implementation may choose for a successful call to `malloc` et
al to always return an "over-aligned" pointer to an address
aligned to a boundary that is >= `alignof(max_align_t)`, but
is not required to do so.

If you want to steal the low bits of an address for something,
be careful: there be dragons.

	- Dan C.

Thread

AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-06 23:08 -0700
  Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-07 15:25 +0200
    Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-07 12:13 -0700
      Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-07 21:28 +0200
        Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-08 05:31 -0700
          Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-09 11:12 +0200
            Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-09 11:12 +0200
      Re: AVL tree insertion - programming exercise BGB <cr88192@gmail.com> - 2026-10-07 20:10 -0500
        Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-08 05:56 -0700
          Re: AVL tree insertion - programming exercise BGB <cr88192@gmail.com> - 2026-10-08 15:09 -0500
            Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-08 22:56 +0200
              Re: AVL tree insertion - programming exercise BGB <cr88192@gmail.com> - 2026-10-08 17:29 -0500
                Re: AVL tree insertion - programming exercise fir <profesor.fir@gmail.com> - 2026-10-09 09:56 +0200
            Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-09 01:39 +0200
              Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-09 00:03 -0700
            Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-08 23:53 -0700
              Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-09 10:34 +0200
                Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-09 12:27 +0200
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-09 12:59 +0200
                Re: AVL tree insertion - programming exercise bart <bc@freeuk.com> - 2026-10-09 11:43 +0100
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-09 13:20 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-09 12:38 +0000
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-09 16:05 +0200
                Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-09 17:54 +0200
                Re: AVL tree insertion - programming exercise Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-10-10 10:38 +0800
                Re: AVL tree insertion - programming exercise ram@zedat.fu-berlin.de (Stefan Ram) - 2026-10-10 03:12 +0000
                Re: AVL tree insertion - programming exercise Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-10-10 04:44 +0000
                Re: AVL tree insertion - programming exercise "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-10 14:12 -0700
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-09 19:48 +0000
                Re: AVL tree insertion - programming exercise scott@slp53.sl.home (Scott Lurndal) - 2026-10-09 23:18 +0000
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-10 15:27 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-12 18:26 +0000
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-12 21:17 +0200
                Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-09 17:47 +0300
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-09 16:48 +0000
                Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-10 19:08 +0300
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-10 18:29 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-10 17:44 +0000
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-10 23:22 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-10 17:43 +0000
                Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-10 20:35 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-10 19:36 +0000
                Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-10 22:08 +0200
                Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-10 23:21 +0300
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-10 20:55 +0000
                Re: AVL tree insertion - programming exercise scott@slp53.sl.home (Scott Lurndal) - 2026-10-10 20:22 +0000
                Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-10 23:32 +0300
                Re: AVL tree insertion - programming exercise scott@slp53.sl.home (Scott Lurndal) - 2026-10-11 15:24 +0000
  Re: AVL tree insertion - programming exercise Bonita Montero <Bonita.Montero@gmail.com> - 2026-10-10 19:40 +0200
  Re: AVL tree insertion - programming exercise Andrey Tarasevich <noone@noone.net> - 2026-10-10 12:39 -0700
    Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-10 23:00 +0300
    Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-10 22:28 +0200
    Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-10 19:02 -0700
  Re: AVL tree insertion - programming exercise "chine.bleu" <chine.bleu@yahoo.com> - 2026-10-13 01:38 -0700
    Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-13 11:42 +0200
      Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-13 10:55 +0000
      Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-13 04:29 -0700
        Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-13 14:01 +0200
          Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-13 05:09 -0700
          Re: AVL tree insertion - programming exercise antispam@fricas.org (Waldek Hebisch) - 2026-10-13 19:01 +0000
        Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-13 15:17 +0000
          Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-13 17:39 +0200
          Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-13 09:01 -0700
            Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-13 21:17 +0000
              Re: AVL tree insertion - programming exercise "chine.bleu" <chine.bleu@yahoo.com> - 2026-10-13 18:01 -0700
              Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-13 21:13 -0700
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-14 09:43 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-14 20:11 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-14 15:25 -0700
                Re: AVL tree insertion - programming exercise David Brown <david.brown@hesbynett.no> - 2026-10-15 09:28 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-14 14:13 +0000
                Re: AVL tree insertion - programming exercise Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-14 18:03 +0200
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-14 18:21 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-14 15:42 -0700
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-15 14:25 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-15 15:54 -0700
                Re: AVL tree insertion - programming exercise Kaz Kylheku <046-301-5902@kylheku.com> - 2026-10-15 23:51 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-15 18:14 -0700
                Re: AVL tree insertion - programming exercise Kaz Kylheku <046-301-5902@kylheku.com> - 2026-10-16 20:26 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-16 15:27 -0700
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-16 03:09 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-15 21:34 -0700
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-16 11:27 +0000
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-16 11:27 -0700
                Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-17 08:21 -0700
                Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-17 10:48 -0700
                Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-17 18:10 +0000
                Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-14 10:39 -0700
          Re: AVL tree insertion - programming exercise scott@slp53.sl.home (Scott Lurndal) - 2026-10-13 17:15 +0000
            Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-13 21:30 +0000
      Re: AVL tree insertion - programming exercise "chine.bleu" <chine.bleu@yahoo.com> - 2026-10-13 08:19 -0700
      Re: AVL tree insertion - programming exercise "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-13 13:16 -0700
        Re: AVL tree insertion - programming exercise Michael S <already5chosen@yahoo.com> - 2026-10-13 23:28 +0300
    Re: AVL tree insertion - programming exercise James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-13 06:20 -0400
      Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-13 14:34 +0000
        Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-13 20:45 -0700
          Re: AVL tree insertion - programming exercise cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-14 14:25 +0000
          Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-14 20:54 -0700
            Re: AVL tree insertion - programming exercise Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-14 22:47 -0700
              Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-15 04:42 -0700
              Re: AVL tree insertion - programming exercise "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-15 14:07 -0700
    Re: AVL tree insertion - programming exercise Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-13 03:52 -0700

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