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Re: memcpy not memcpying?

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From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: memcpy not memcpying?
Date Mon, 04 Nov 2019 20:58:51 -0800
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Keith Thompson <kst-u@mib.org> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> [...]
>
>> I think I might use _Generic here because checking the size is not
>> always going to be enough:
>>
>>      _Static_assert(
>>           _Generic(i, int: 1, default: 0),
>>           "Check this because symbolcount_t is not compatible with int."
>>      );
>>
>> This works (in both clang and gcc) with -std=c11 -pedantic, but I am not
>> sure that _Generic /must/ be allowed here.  It would be a shame if
>> generic selections were not considered to be standard constant
>> expressions.  Have I missed something in the standard?
>
> I think it's covered in 6.6, which has this contraint in p2:
>
>     Constant expressions shall not contain assignment, increment,
>     decrement, function-call, or comma operators, except when they
>     are contained within a subexpression that is not evaluated.
>
> Which implies, I think, that a generic selection is a constant
> expression if the result expression is a constant expression,
> even if the other subexpressions are not.

What a strange inference.  The quoted sentence can tell us only when
an expression is not a constant expression, never when it is a
constant expression.

Besides which, a _Static_assert needs not just a constant expression
but an integer constant expression.

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Thread

memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 08:42 -0400
  Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 08:49 -0400
    Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:24 -0400
      Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 09:46 -0400
        Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:49 -0400
        Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-01 10:19 -0700
          Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-01 21:14 +0100
          Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 17:16 -0400
            Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-01 22:42 +0000
              Re: memcpy not memcpying? Richard Damon <Richard@Damon-Family.org> - 2019-11-01 21:10 -0400
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 11:36 +0000
            Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 02:15 -0700
            Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 13:04 +0100
              Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 13:43 +0000
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 17:24 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 17:00 +0000
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 13:06 -0700
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 21:05 +0000
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 22:40 +0000
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 15:59 -0700
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 23:47 +0000
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 18:22 -0700
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 13:27 +0000
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 17:46 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 17:20 +0000
                Re: memcpy not memcpying? Manfred <noname@add.invalid> - 2019-11-03 19:44 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 19:36 +0000
                Re: memcpy not memcpying? Manfred <noname@add.invalid> - 2019-11-03 22:57 +0100
                Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-03 22:39 +0000
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 23:06 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 23:28 +0000
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-04 11:00 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 11:14 +0000
                Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 11:22 +0000
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-04 13:10 +0100
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-04 10:03 -0800
                Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 19:36 +0000
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-05 10:04 -0800
                Re: memcpy not memcpying? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-04 20:58 -0800
                Re: memcpy not memcpying? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-04 22:01 -0800
                Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-04 12:56 +0100
                Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-04 09:50 -0500
                Re: memcpy not memcpying? scott@slp53.sl.home (Scott Lurndal) - 2019-11-04 20:26 +0000
                Re: memcpy not memcpying? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-04 04:34 -0800
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 13:05 +0000
                Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 14:09 +0000
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 15:11 +0000
                Re: memcpy not memcpying? Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-04 15:15 +0000
                Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 15:43 +0000
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 20:20 +0000
                Re: memcpy not memcpying? Ian Collins <ian-news@hotmail.com> - 2019-11-03 14:37 +1300
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 11:01 +0000
                Re: memcpy not memcpying? Öö Tiib <ootiib@hot.ee> - 2019-11-03 04:03 -0800
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 12:52 +0000
                Re: memcpy not memcpying? Öö Tiib <ootiib@hot.ee> - 2019-11-03 23:34 -0800
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 09:36 +0000
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 16:20 +0000
                Re: memcpy not memcpying? Ian Collins <ian-news@hotmail.com> - 2019-11-04 07:46 +1300
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-03 12:26 -0800
                Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 17:31 +0100
                Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 17:05 +0000
              Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 09:14 -0800
                Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-03 12:32 -0800
                Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 14:59 -0800
                Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 15:19 -0800
                Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 17:00 -0800
          Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-02 11:16 -0400
            Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 17:26 +0100
    Re: memcpy not memcpying? Barry Schwarz <schwarzb@delq.com> - 2019-11-01 06:39 -0700
      Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 09:56 -0400
        Re: memcpy not memcpying? Barry Schwarz <schwarzb@delq.com> - 2019-11-01 12:39 -0700
    Re: memcpy not memcpying? guinness.tony@gmail.com - 2019-11-01 06:49 -0700
    Re: memcpy not memcpying? jak <nospam@please.ty> - 2019-11-01 20:40 +0100
  Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:26 +0100
    Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-01 10:22 -0700
    Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-01 19:44 +0000
      Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 21:31 +0100
  Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:39 -0400
    Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:42 +0100
      Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:48 -0400
  Re: memcpy not memcpying? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-01 06:47 -0700
    Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:58 +0100
      Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 10:22 -0400
        Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 18:38 +0100
          Re: memcpy not memcpying? scott@slp53.sl.home (Scott Lurndal) - 2019-11-01 17:54 +0000
            Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 20:24 +0100
          Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 15:24 -0400
        Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-01 23:19 +0000

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