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Re: How does strict aliasing work with parallel arrays in a union or const-initialized data

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Tim Rentsch <txr@alumni.caltech.edu> writes:
> supercat@casperkitty.com writes:
[...]
>> Do any versions of the C Standard allow a structure containing an
>> FAM type to appear within a union?  GCC allows that as an
>> extension, but the C99 compiler I use won't allow that unless gcc
>> extensions are enabled, [snip]
>
> What makes you think C99 (or later) does not allow declaring
> of struct-wth-FAM objects, either as ordinary identfiers or
> as struct/union members?  AFAIK they are allowed.  When I tried
> compiling a simple test case with gcc, using -std=c99 -pedantic,
> no diagnostics were given.  So I'm at a loss to understand your
> question here.

The restriction isn't in the same paragraph that discusses flexible
array members, so it's easy to miss.

N1570 6.7.2.1p18 describes flexible array members.   Paragraph 3 of the
same section says:

    A structure or union shall not contain a member with incomplete
    or function type (hence, a structure shall not contain an
    instance of itself, but may contain a pointer to an instance of
    itself), except that the last member of a structure with more
    than one named member may have incomplete array type; such
    a structure (and any union containing, possibly recursively,
    a member that is such a structure) shall not be a member of a
    structure or an element of an array.

Annoyingly, that paragraph doesn't use the phrase "flexible array
member".

So the answer to supercat's question is no: neither C99 nor C11 permits
an FAM as a member of a structure or as an element of an array.  It
does, however, permit a FAM as a member of a union.  (Tim, perhaps your
test case used a FAM as a union member?)

This translation unit:

    struct foo { int len; unsigned char fam[]; };
    struct bar { struct foo nope; };

violates a constraint (but gcc accepts it if extensions are enabled).
This one:

    struct foo { int len; unsigned char fam[]; };
    union  bar { struct foo nope; };

does not violate any constraint in C99 or C11.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-21 16:47 -0700
  Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-24 21:20 -0700
    Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-25 11:44 -0700
      Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 12:15 -0700
        Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-28 14:25 -0700
          Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 16:23 -0700
            Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 17:08 -0700
              Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 18:09 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-28 20:02 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Bartc <bc@freeuk.com> - 2015-10-29 10:55 +0000
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-29 06:50 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-29 08:52 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 10:21 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-29 08:50 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-29 12:25 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 13:02 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-30 13:35 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 19:57 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-31 12:27 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-30 17:02 -0400
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-30 14:16 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 08:13 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-31 08:42 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 10:40 -0700
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-02 01:45 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-02 06:57 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-02 08:12 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-02 08:54 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-04 07:24 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-04 09:22 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-04 19:48 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-05 09:44 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-07 10:17 -0800
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data raltbos@xs4all.nl (Richard Bos) - 2015-11-10 15:37 +0000
                Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-10 09:01 -0800
          Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 16:48 -0700
        Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 16:11 -0700
          Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 16:37 -0700

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