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Re: Regarding assignment to struct

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: Regarding assignment to struct
Date 2025-05-05 07:14 -0700
Organization A noiseless patient Spider
Message-ID <86wmav401y.fsf@linuxsc.com> (permalink)
References (2 earlier) <86plgo7ahu.fsf@linuxsc.com> <vv9hu7$3nomg$1@dont-email.me> <20250505111213.00004b55@yahoo.com> <vv9stv$2i8j$1@dont-email.me> <20250505120145.000014f8@yahoo.com>

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Michael S <already5chosen@yahoo.com> writes:

> On Mon, 5 May 2025 01:29:47 -0700
> Andrey Tarasevich <noone@noone.net> wrote:
>
>> On Mon 5/5/2025 1:12 AM, Michael S wrote:
>>
>>> According to my understanding, you are wrong.
>>> Taking pointer of non-lvalue is UB, so anything compiler does is
>>> conforming.
>>
>> Er... What?  What specifically do you mean by "taking pointers"?
>>
>> The whole functionality of `[]` operator in C is based on pointers.
>> This expression
>>
>>    (a = b).a[5]
>> [...]
>> is already doing your "taking pointers of non-lvalue" (if I
>> understood you correctly) as part of array-to-pointer conversion.
>> And no, it is not UB.
>>
>> This is not UB either
>>
>>    struct S foo(void) { return (struct S) { 1, 2, 3 }; }
>>    ...
>>    int *p;
>>    p = &foo().a[2], printf("%d\n", *p);
>
> That is not UB:
> int a5 = (a = b).a[5];
>
> That is UB:
> int* pa5 = &(a = b).a[5];
>
>> So, what you are basing your "UB" claim on is not clear to me.
>
> If you read the post of Keith Thompson and it is still not clears to
> you then I can not help.

Under C11 semantics, both

   int a5 = (a = b).a[5];
   
and

   int* pa5 = &(a = b).a[5];

have well-defined behavior.  The undefined behavior of the
upthread example comes later, only after the statement assigning
to the pointer (or here, initializing) completes.  It isn't
taking the address with & that has undefined behavior;  it is
using the stored pointer value in a subsequent statement, /after
the full expression containing the & operator has completed/,
that results in undefined behavior.

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Thread

Regarding assignment to struct Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2025-05-02 18:34 +0000
  Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-02 13:17 -0700
  Re: Regarding assignment to struct Barry Schwarz <schwarzb@delq.com> - 2025-05-02 13:35 -0700
    That depends... (Was: Regarding assignment to struct) gazelle@shell.xmission.com (Kenny McCormack) - 2025-05-02 20:44 +0000
      Re: That depends... (Was: Regarding assignment to struct) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2025-05-03 01:13 +0000
        Re: That depends... (Was: Regarding assignment to struct) Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-03 02:28 +0000
        Re: That depends... (Was: Regarding assignment to struct) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-03 06:17 +0200
        Re: That depends... (Was: Regarding assignment to struct) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-03 04:31 +0000
          Re: That depends... (Was: Regarding assignment to struct) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-03 05:11 +0000
            Re: That depends... (Was: Regarding assignment to struct) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-05 12:30 +0200
              Re: That depends... (Was: Regarding assignment to struct) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-05 18:47 +0000
          Re: That depends... (Was: Regarding assignment to struct) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-04 11:05 -0700
        Re: That depends... (Was: Regarding assignment to struct) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-03 00:47 -0400
        Re: That depends... (Was: Regarding assignment to struct) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-04 10:59 -0700
          Re: That depends... (Was: Regarding assignment to struct) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2025-05-04 18:16 +0000
  Re: Regarding assignment to struct antispam@fricas.org (Waldek Hebisch) - 2025-05-02 21:35 +0000
    Re: Regarding assignment to struct Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2025-05-03 01:43 +0000
  Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-03 01:14 -0700
    Re: Regarding assignment to struct Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-03 22:46 +0000
      Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-03 17:37 -0700
        Re: Regarding assignment to struct James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-03 23:38 -0400
          Re: Regarding assignment to struct gazelle@shell.xmission.com (Kenny McCormack) - 2025-05-04 09:25 +0000
          Re: Regarding assignment to struct scott@slp53.sl.home (Scott Lurndal) - 2025-05-04 14:27 +0000
            Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-04 18:45 +0200
          Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-04 13:20 -0700
            Re: Regarding assignment to struct scott@slp53.sl.home (Scott Lurndal) - 2025-05-05 00:41 +0000
              Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-04 18:42 -0700
              Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 21:57 -0700
            Re: Regarding assignment to struct James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-04 21:08 -0400
    Re: Regarding assignment to struct Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-03 22:47 +0000
    Re: Regarding assignment to struct Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-03 22:46 +0000
    Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-04 06:48 -0700
      Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-04 22:22 -0700
        Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-05 11:12 +0300
          Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-05 01:29 -0700
            Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-05 12:01 +0300
              Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 07:14 -0700
              Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-05 08:45 -0700
                Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-05 20:20 +0300
                Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 22:26 -0700
                Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:11 -0700
                Re: Regarding assignment to struct James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-29 12:57 -0400
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 13:27 -0700
                Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 17:04 -0700
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 17:53 -0700
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-06 11:35 +0200
                Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:19 -0700
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-29 21:05 +0200
                Re: Regarding assignment to struct antispam@fricas.org (Waldek Hebisch) - 2025-05-06 17:36 +0000
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-06 20:46 +0200
                Re: Regarding assignment to struct scott@slp53.sl.home (Scott Lurndal) - 2025-05-06 19:22 +0000
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-07 09:37 +0200
                Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:49 -0700
                Re: Regarding assignment to struct Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-29 16:33 +0200
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-29 21:20 +0200
                Re: Regarding assignment to struct scott@slp53.sl.home (Scott Lurndal) - 2025-05-29 21:15 +0000
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-29 14:54 -0700
                Re: Regarding assignment to struct scott@slp53.sl.home (Scott Lurndal) - 2025-05-30 14:29 +0000
                Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-30 10:50 +0200
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-06 13:06 -0700
                Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:21 -0700
                Re: Regarding assignment to struct Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-29 16:43 +0200
                Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:14 -0700
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-29 13:56 -0700
          Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 07:03 -0700
        Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 01:26 -0700
          Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-05 10:14 -0700
            Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-08 12:45 -0700
              Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-08 22:20 +0200
        Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 01:34 -0700
          Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-05 12:03 +0300
            Re: Regarding assignment to struct gazelle@shell.xmission.com (Kenny McCormack) - 2025-05-05 11:30 +0000
            Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 13:32 -0700
              Re: Regarding assignment to struct Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-05 21:10 +0000
                Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 22:57 -0700
            Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 22:40 -0700
          Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 06:34 -0700
            Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 13:43 -0700
              Re: Regarding assignment to struct Nick Bowler <nbowler@draconx.ca> - 2025-05-06 19:06 +0000
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-06 13:21 -0700
                Re: Regarding assignment to struct Nick Bowler <nbowler@draconx.ca> - 2025-05-07 19:09 +0000
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-07 14:23 -0700
                Re: Regarding assignment to struct Nick Bowler <nbowler@draconx.ca> - 2025-05-08 12:58 +0000
                Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-07 21:17 -0700
              Re: Regarding assignment to struct Andrey Tarasevich <noone@noone.net> - 2025-05-29 05:36 -0700
                Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-29 14:36 -0700
        Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 07:56 -0700
          Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-05 20:00 +0200
        Re: Regarding assignment to struct NotAorB <atod101101@gmail.com> - 2025-05-12 16:38 -0400
  Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-03 11:46 +0200
    Re: Regarding assignment to struct Muttley@dastardlyhq.com - 2025-05-05 08:50 +0000
      Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-05 13:34 +0200
      Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-05 13:53 -0700
        Re: Regarding assignment to struct Muttley@DastardlyHQ.org - 2025-05-06 07:16 +0000
        Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-06 11:46 +0200
          Re: Regarding assignment to struct Muttley@DastardlyHQ.org - 2025-05-06 10:18 +0000
        Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-06 16:34 +0300
  Re: Regarding assignment to struct Richard Damon <richard@damon-family.org> - 2025-05-03 21:42 -0400
    Re: Regarding assignment to struct Michael S <already5chosen@yahoo.com> - 2025-05-04 11:01 +0300
      Re: Regarding assignment to struct Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-05-04 08:34 +0000
        Re: Regarding assignment to struct David Brown <david.brown@hesbynett.no> - 2025-05-04 14:06 +0200
      Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-05 21:25 -0700
      Re: Regarding assignment to struct Rosario19 <Ros@invalid.invalid> - 2025-05-12 11:23 +0200
  Re: Regarding assignment to struct Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-04 07:49 -0700
  Re: Regarding assignment to struct Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-04 14:09 -0700

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