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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Initializing a malloc'ed struct whose fields are const with run-time values |
| Date | 2013-01-23 18:26 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <kdprii$h72$1@dont-email.me> (permalink) |
| References | (5 earlier) <am8fjtFbgsgU4@mid.individual.net> <kfn38xssc0i.fsf@x-alumni2.alumni.caltech.edu> <0.587c540b10d4d98feb70.20130123135415GMT.87zk000zug.fsf@bsb.me.uk> <kdpn11$79e$1@dont-email.me> <amb6hrFbhmbU2@mid.individual.net> |
On 1/23/2013 17:29, Ian Collins wrote:
> Shao Miller wrote:
>> On 1/23/2013 08:54, Ben Bacarisse wrote:
>>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>>
>>>> Ian Collins <ian-news@hotmail.com> writes:
>>>>
>>>>>> [snip]
>>>>>>
>>>>> I just tried this code:
>>>>>
>>>>> #include <stdio.h>
>>>>>
>>>>> struct toto { const int i; const float f; const void *p; };
>>>>>
>>>>> struct toto foo(int i, float f, void *p)
>>>>> {
>>>>> return (struct toto){i, f, p};
>>>>> }
>>>>>
>>>>> int main(void)
>>>>> {
>>>>> float f = foo( 1,2,NULL).f;
>>>>> }
>>>>>
>>>>> and gcc (-Wall -std=c99 -pedantic) compiles it, but Sun c99 (which
>>>>> tends to be both strict and conforming) rejects it with one error:
>>>>>
>>>>> "x.c", line 7: left operand must be modifiable lvalue: op "="
>>>>>
>>>>> where line 7 is the function return.
>>>>
>>>> I believe gcc has it right here. Even the error message from
>>>> the Sun compiler suggests they were thinking of this like a
>>>> kind of assignment, which it certainly isn't in this case.
>>>
>>> As you say elsewhere, the assignment-like nature of the return only
>>> kicks in when the types differ, but even when it does, I'd take "is
>>> converted as if by assignment" to mean that only the conversion and type
>>> constraints of assignment are to be considered. (The constraints being
>>> important because they would, for example, prevent a function with type
>>> 'T *' returning a 'const T * value without a diagnostic).
>>>
>>> More curious to me is that gcc (in conforming mode) permits this:
>>>
>>> struct toto { const int i; const float f; const void *p; };
>>>
>>> struct toto foo(int i, float f, void *p)
>>> {
>>> return (struct toto){i, f, p};
>>> }
>>>
>>> int bar(struct toto p) { return p.i; }
>>>
>>> int main(void)
>>> {
>>> return bar(foo(1, 2, 0));
>>> }
>>>
>>> but 6.5.2.2 p2 says:
>>>
>>> "Each argument shall have a type such that its value may be assigned
>>> to an object with the unqualified version of the type of its
>>> corresponding parameter."
>>>
>>> The phrase "unqualified version of the type" can't surely be intended to
>>> mean the recursive removal of all type qualifiers. Elsewhere it simply
>>> means that the "top level" qualifiers are removed. If that's the right
>>> interpretation, the call to bar should be a constraint violation.
>>>
>>
>> Why? The result of the call to 'foo' yields a 'struct toto', which is
>> not qualified (and also isn't an lvalue).
>
> I think Ben's point is the *parameter* value may be *assigned*, not the
> result of the expression.
>
> I see Sun c99, which rejects the return in foo, also accepts bar.
>
As in, a constraint violation against 6.5.16p2? If so, why would that
constraint apply? There are still constraints in 6.5.16.1p1... As
asked elsethread, why pick out "modifiable" and not "lvalue"? Do we
need to worry about the constraints and semantics of 6.5.16.2? Isn't
backing up to 6.5.16 backing up too far?
--
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.
Cheerily," -- Richard Harter
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Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-22 16:36 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 11:23 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:26 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:41 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:25 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:17 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:09 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 14:16 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values "BartC" <bc@freeuk.com> - 2013-01-23 13:25 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 15:21 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:56 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 16:40 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 18:26 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 20:39 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:14 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ike Naar <ike@sverige.freeshell.org> - 2013-01-22 17:59 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 14:09 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 21:59 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 21:19 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:47 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:05 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:15 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 09:43 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:19 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:35 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 10:45 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 17:07 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:29 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:54 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:08 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 11:29 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 18:26 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 14:03 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 23:58 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 23:36 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 13:55 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 02:44 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 21:39 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-24 00:54 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 17:56 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-24 13:58 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:37 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:22 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:06 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 19:55 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 00:48 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 23:06 +1300
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:02 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:12 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:37 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 22:29 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 04:28 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 00:07 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:04 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 22:10 +0100
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:13 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:25 -0500
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:21 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:32 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:50 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Nick Bowler <nbowler@draconx.ca> - 2013-01-22 21:43 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:45 +0000
Re: Initializing a malloc'ed struct whose fields are const with run-time values Keith Thompson <kst-u@mib.org> - 2013-01-24 08:23 -0800
Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 20:48 -0800
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