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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 16:25 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <kdn050$nbj$1@dont-email.me> (permalink) |
| References | <kdmbl7$4lc$1@dont-email.me> <0.7432cc83d523a65ab134.20130122211350GMT.87ehhc53ap.fsf@bsb.me.uk> |
On 1/22/2013 16:13, Ben Bacarisse wrote:
> Noob <root@127.0.0.1> writes:
>
>> Is it possible, in standard C89 and C99, to initialize a
>> struct whose fields are const with values only known at
>> run-time?
>>
>> For example, consider:
>>
>> struct toto { const int i; const float f; const void *p; };
>>
>> Is it possible to portably implement:
>>
>> struct toto *foo(int i, float f, void *p);
>>
>> which allocates space for a "struct toto", initializes it
>> with i, f, p and returns the address of this struct?
>>
>> The best I could come up with is:
>>
>> #include <stdlib.h>
>> #include <string.h>
>> struct toto *foo(int i, float f, void *p)
>> {
>> struct toto s = { i, f, p };
>> struct toto *res = malloc(sizeof *res);
>> if (res != NULL) memcpy(res, &s, sizeof s);
>> return res;
>> }
>>
>> which has several defects:
>>
>> 1) Apparently, C89 does not allow one to use elements "not computable
>> at load time" in an initialization list. However, it is allowed both
>> in C99 (right?) and in gnu89.
>
> Yes, it's fine in C99 and later. I'm not sure about gnu89 but I would
> not be surprised.
>
Oops. To Noob: Sorry about providing the wrong answer for this one,
earlier. I hope the rest of that post helped.
> BTW, in C99 you don't even need a declaration:
>
> if (res != NULL) memcpy(res, &(struct toto){i, f, p}, sizeof *res);
>
>> 2) I'm not sure it is well-defined to memcpy stuff into some const
>> fields. Does it tickle the cranky UB gods?
>
> No, I think it's fine. You are not modifying anything that is const.
> malloced storage has no declared type and the memcpy probably makes the
> effective type a byte array.
>
To Ben: I think the effective type would be 'struct toto', as 'memcpy'
is included in 6.5p6.
--
- 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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