Re: Regarding assignment to struct

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Regarding assignment to struct
Date	2025-05-05 01:26 -0700
Organization	None to speak of
Message-ID	<87seljh3a3.fsf@nosuchdomain.example.com> (permalink)
References	<vv338b$16oam$1@dont-email.me> <vv4j9p$33vhj$1@dont-email.me> <86plgo7ahu.fsf@linuxsc.com> <vv9hu7$3nomg$1@dont-email.me>

Show all headers | View raw

Andrey Tarasevich <noone@noone.net> writes:
> On Sun 5/4/2025 6:48 AM, Tim Rentsch wrote:
>>> One dark corner this feature has, is that in C (as opposed to C++) the
>>> result of an assignment operator is an rvalue, which can easily lead
>>> to some interesting consequences related to structs with arrays
>>> inside.
>> I'm curious to know what interesting consequences you mean here.  Do
>> you mean something other than cases that have undefined behavior?
>
> I'm referring to the matter of the address identity of the resultant
> rvalue object. At first, "address identity of rvalue" might sound
> strange, but the standard says that there's indeed an object tied to
> such rvalue, and once we start applying array-to-pointer conversion
> (and use `[]` operator), lvalues and addresses quickly come into the
> picture.
>
> The standard says in 6.2.4/8:
>
> "A non-lvalue expression with structure or union type, where the
> structure or union contains a member with array type [...]
> refers to an object with automatic storage duration and temporary
> lifetime. Its lifetime begins when the expression is evaluated and its
> initial value is the value of the expression. Its lifetime ends when
> the evaluation of the containing full expression ends. [...] Such an
> object need not have a unique address."
> https://port70.net/~nsz/c/c11/n1570.html#6.2.4p8
>
> I wondering what the last sentence is intended to mean ("... need not
> have a unique address"). At the first sight, the intent seems to be
> obvious: it simply says that such temporary objects might repeatedly
> appear (and disappear) at the same location in storage, which is a
> natural thing to expect.

You snipped this: "Any attempt to modify an object with temporary
lifetime results in undefined behavior.".  Which means, I think,
that an implementation that shared storage for "such an object"
with something else probably isn't going to cause problems for any
code with defined behavior.

Though I can imagine the possibility of code that modifies `a` and
reads via `pc` within the same full expression.  Perhaps a compiler
would refrain from optimizing the storage if `a` and `*pc` overlap.
I'm too lazy at the moment to write a demo.

But unless I've somehow missed it, the "Such an object need not
have a unique address." wording doesn't appear on that web page or
in my copy of n1570.pdf.  C17 does add these two sentences:

    An object with temporary lifetime behaves as if it were declared
    with the type of its value for the purposes of effective type. Such
    an object need not have a unique address.

Normally any two objects with overlapping lifetime must have distinct
addresses.  This addition, I think, gives compilers permission to have
temporary lifetime objects overlap with other existing objects, but not
to have a modification to one object affect the value of the other
(unless the modification invokes UB, of course).

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

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar

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 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

csiph-web