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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 | 2013-01-22 17:07 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <kdn2ji$878$1@dont-email.me> (permalink) |
| References | (1 earlier) <slrn3vfskftksd.ddo.ike@sverige.freeshell.org> <am8bvaFbgsgU2@mid.individual.net> <gcc3t9-6br.ln1@wilbur.25thandClement.com> <49d3t9-7rb.ln1@wilbur.25thandClement.com> <am8fjtFbgsgU4@mid.individual.net> |
On 1/22/2013 16:45, Ian Collins wrote:
> William Ahern wrote:
>> William Ahern <william@wilbur.25thandclement.com> wrote:
>>> Ian Collins <ian-news@hotmail.com> wrote:
>>>> Ike Naar wrote:
>> <snip>
>>>>> And how about returning by value?
>>>>>
>>>>> struct toto foo(int i, float f, void *p)
>>>>> {
>>>>> struct toto res = {i, f, p};
>>>>> return res;
>>>>> }
>>
>>>> Is that legal? struct toto has const members, so can it be returned by
>>>> value from a function?
>>
>>> What about foo().f? Does the simple return constitute modification of a
>>> const-qualified lvalue?
>>
>>> I'm still not convinced that "modifying" a const-qualified member
>>> through
>>> assignment of a non-const-qualified aggregate is undefined behaviour.
>>> But I
>>> don't know enough about the rules of lvalues to prove it.
>>
>> Ok, now I am:
>>
>> C99 (N1256) 6.3.2.1
>>
>> A modifiable lvalue is an lvalue that does not have array type, does
>> not have an incomplete type, does not have a const-qualified type,
>> and if it is a structure or union, does not have any member
>> (including, recursively, any member or element of all contained
>> aggregates or unions) with a const-qualified type.
>>
>> But foo().f should still be fine, I think.
>
> So res above isn't a modifiable lvalue.
>
> 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.
>
That seems a bit odd. 'return' has little to do with an lvalue or object:
"The properties associated with qualified types are meaningful only
for expressions that are lvalues.132)"
So "as if by assignment" should be satisfied by:
"the left operand has an atomic, qualified, or unqualified version of
a structure or union type compatible with the type of the right;"
No?
--
- 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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