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Re: Repeated types in union

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On 12/12/2011 3:09 PM, Edward Rutherford wrote:
> [...]
> Does that mean the "It's Legal" brigade would say it's always legal to
> read an unsigned char from an union, whatever was previously stored in
> it, on the grounds that an unsigned char cannot contain a trap
> representation?

     The varieties of `char' are something of a special case, because
C has always had the notion that it's possible to inspect and maybe
fiddle with the individual bytes of a multi-byte object.  At your
peril, of course, since you might invalidate the multi-byte thing.
But still: Things like memcpy() are defined in terms of copying the
individual bytes, and the copy of a valid object must itself be
valid.

     The Standard tightens this just a trifle, by allowing the `char'
flavors other than `unsigned' to have trap representations.  Still,
`unsigned char' remains as the "atom" of C memory: Its mapping between
representations and values is one-to-one, which guarantees fidelity
both in value and in representation when copying or comparing, and
also guarantees that there are no trap representations.

     But back to the `union' issue: I'm still not 100% comfortable
with the idea of writing to one member and reading another.  It sort
of looks like it should work, but I've not heard a watertight argument
that it *must* work, even in the face of a ferociously aggressive
optimizer.  I think the "It's legal" faction have found arguments they
deem satisfactory; perhaps they've looked more diligently than I have.

     Down to nuts and bolts: Is this a theoretical question, or do you
have an actual use case in mind?  If the latter, could you describe it?
Maybe someone will be able to say "Well, in *that* case it works" or
"If you did it *this other* way you wouldn't care."

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

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Repeated types in union Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2011-12-10 22:06 +0000
  Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-11 00:11 +0100
  Re: Repeated types in union Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-10 18:14 -0500
    Re: Repeated types in union Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2011-12-12 20:09 +0000
      Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-12 23:45 +0100
      Re: Repeated types in union Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-12 19:48 -0500
        Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-13 15:17 -0800
          Re: Repeated types in union Keith Thompson <kst-u@mib.org> - 2011-12-13 15:29 -0800
          Re: Repeated types in union James Kuyper <jameskuyper@verizon.net> - 2011-12-13 18:30 -0500
          Re: Repeated types in union Noob <root@127.0.0.1> - 2011-12-14 17:40 +0100
          Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:28 -0800
        Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 15:04 -0800
    Re: Repeated types in union ralph <nt_consulting64@yahoo.net> - 2011-12-12 14:50 -0600
    Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 14:53 -0800
  Re: Repeated types in union Barry Schwarz <schwarzb@dqel.com> - 2011-12-10 22:04 -0800
    Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-12 09:49 -0800
      Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-12 20:09 +0100
        Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-13 15:10 -0800
          Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-14 00:24 +0100
          Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:46 -0800
      Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:42 -0800
    Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:36 -0800

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