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Groups > comp.lang.c > #102948
| From | Kelsey Bjarnason <kbjarnason@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: lvalues from the ternary op |
| Date | 2017-02-01 22:47 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <i91bmd-no6.ln1@spanky.localhost.net> (permalink) |
| References | (17 earlier) <2f20253f-9ce6-4dcd-82e7-2fcdc2eae5b0@googlegroups.com> <ln1svxh6i1.fsf@kst-u.example.com> <aa4dec3a-ebfc-495a-9160-e260037473f5@googlegroups.com> <kfnlgtrdvq3.fsf@x-alumni2.alumni.caltech.edu> <698d5687-fb5e-488d-841e-99991d26c314@googlegroups.com> |
[snips]
[PMFJI]
On Tue, 31 Jan 2017 10:17:02 -0800, supercat wrote:
> 6. The Standard authorizes compilers to do anything whatsoever if the
> storage of an object with one Effective Type is accessed using an
> lvalue of an incompatible type, and "int" is not a type compatible
> with "float".
This makes me wonder...
union {
type_1 a;
type_2 b;
} u;
Assume type_1 has no trap representations, while type_2 does; or each
does but they're different, and further assume they overlap such that
sizeof(u) is more or less the size of the largest, rather than the
combined size of each...
... let's ponder a pseudo-assembly code view of using this:
load 0x0fffffff into int_register_a
store int_register_a into [memory_address]
In our imaginary system, int_register_a can handle a bit pattern
associated with 0x0fffffff, while a floating point register cannot, it
would cause a trap.
If we subsequently did this:
load [memory_address] into int_register_a
we would not expect a trap, as [memory_address] has a legitimate
representation of a value for the register. If, instead, we did this:
load [memory_address] into float_register_f
we would expect a trap.
I submit that the correct semantics is that any valid value of o.a can be
stored into a, regardless of the details of o.b; and likewise it can be
retrieved from u.a without invoking u.b's behaviour.
Which suggests that what counts is not "accessing the storage *of* an
object", but, rather, accessing the storage *as* an object. As long as
object u.b is not accessed (without an intervening store of a legitimate
value into u.b), then there should be no unexpected behaviour, the
accessing of u.a should behave exactly as if u.a were a member of a
struct, rather than a union.
Or did I miss something?
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Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-04 19:46 +0100
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 11:08 -0800
Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-04 21:10 +0000
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 13:48 -0800
Re: lvalues from the ternary op Ian Collins <ian-news@hotmail.com> - 2017-01-05 11:07 +1300
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 14:20 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 14:41 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 15:21 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 15:52 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 16:37 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 18:31 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-13 17:26 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-13 17:49 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 22:53 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-13 17:08 -0800
Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-14 02:15 +0000
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 22:51 -0800
Re: lvalues from the ternary op luser droog <luser.droog@gmail.com> - 2017-01-16 23:09 -0800
Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-18 14:51 +0100
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 09:59 -0800
Re: lvalues from the ternary op Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-01-18 13:12 -0500
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 12:11 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 12:45 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 14:07 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 14:40 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 16:10 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 18:08 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-19 06:44 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-19 08:40 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-19 12:32 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 06:01 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-20 08:15 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 10:51 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-20 11:32 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 13:41 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-23 07:04 -0800
Re: lvalues from the ternary op Ike Naar <ike@iceland.freeshell.org> - 2017-01-23 17:59 +0000
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-23 10:57 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:45 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-31 09:13 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-31 10:17 -0800
Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-01 01:51 +0000
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-01 14:24 -0800
Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-02 03:07 +0000
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 04:43 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-04 08:01 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 09:06 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-09 11:56 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-12 10:29 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-12 13:43 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-19 21:44 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-20 08:15 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-24 07:58 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 10:00 -0800
Re: lvalues from the ternary op scott@slp53.sl.home (Scott Lurndal) - 2017-02-24 18:58 +0000
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 13:03 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 14:11 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 12:42 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 13:49 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 13:58 -0800
Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 14:56 -0800
Re: lvalues from the ternary op supercat <flatfinger@casperkitty.com> - 2017-02-25 14:00 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-25 09:23 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-25 09:43 -0800
Re: lvalues from the ternary op Kelsey Bjarnason <kbjarnason@gmail.com> - 2017-02-01 22:47 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-02 10:12 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 07:51 -0800
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-04 08:18 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 08:58 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 04:00 -0800
Re: lvalues from the ternary op Kelsey Bjarnason <kbjarnason@gmail.com> - 2017-02-04 09:35 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 09:35 -0800
Re: lvalues from the ternary op Robert Wessel <robertwessel2@yahoo.com> - 2017-01-31 13:36 -0600
Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-31 12:23 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-01 13:48 -0800
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:53 -0800
Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-19 14:10 +0100
Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-30 12:05 -0800
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