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| From | Andrey Tarasevich <andreytarasevich@hotmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Bit-field union bug |
| Date | 2011-09-01 16:27 -0700 |
| Organization | A noiseless patient Spider |
| Message-ID | <j3p4cs$5a2$1@dont-email.me> (permalink) |
| References | <j3m8np$8gv$1@speranza.aioe.org> |
On 8/31/2011 2:23 PM, Quentin Pope wrote:
> On my compiler, the output of "sizeof(foo)" of the following is 8,
> instead of 4.
>
> Which indicates that the compiler is aligning the union at
> the next int boundary, rather than coalescing it with "proto".
>
> I think this is a bug.
>
> struct s_skip_ind {
> unsigned skip_ind:4;
> };
>
> struct s_trans_id {
> unsigned trans_val:3;
> unsigned trans_id:1;
> };
>
> struct foo {
> unsigned proto:4;
> union {
> s_skip_ind skip_ind;
> s_trans_id trans_id;
> };
> };
No. You are making invalid conclusions from translating invalid code.
C language does not allow anonymous union members in the struct
declaration. Your declaration of `struct foo` is invalid C. If your
compiler accepted this declaration, it is a mere compiler-specific
extension, which has nothing to do with standard C language.
In C language you are required to give that union member an explicit
name, as in
struct foo {
unsigned proto:4;
union {
s_skip_ind skip_ind;
s_trans_id trans_id;
} u; /* <- the name is required */
};
Now note that the above member `u` should have its own offset and its
own address within `struct foo`, since at any place in your code you can
something like
struct foo f;
void *p = &f.u;
For this reason, the compiler simply cannot merge this union member with
the previous bit-field. It has too align `u` at the beginning of an
addressable memory unit.
Apparently your compiler was forgiving enough to let you (illegally)
omit the member name, but wasn't brave enough to take the next step and
do the merging.
In any case, if there is any compiler bug here, it is the failure of the
compiler to report the illegal anonymous union member, not the failure
to merge the bit-fields.
--
Best regards,
Andrey Tarasevich
Back to comp.lang.c | Previous | Next — Previous in thread | Find similar | Unroll thread
Bit-field union bug Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-08-31 21:23 +0000
Re: Bit-field union bug Keith Thompson <kst-u@mib.org> - 2011-08-31 14:34 -0700
Re: Bit-field union bug Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-09-03 19:06 +0000
Re: Bit-field union bug Ian Collins <ian-news@hotmail.com> - 2011-09-04 08:19 +1200
Re: Bit-field union bug Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-09-03 21:12 +0000
Re: Bit-field union bug Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-03 22:30 +0100
Re: Bit-field union bug Ian Collins <ian-news@hotmail.com> - 2011-09-04 11:01 +1200
Re: Bit-field union bug Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-04 00:37 +0100
Re: Bit-field union bug Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-09-06 00:36 +0300
Re: Bit-field union bug Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 12:23 -0800
Re: Bit-field union bug Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-24 21:15 +0000
Re: Bit-field union bug Ian Collins <ian-news@hotmail.com> - 2011-09-01 09:36 +1200
Re: Bit-field union bug jacob navia <jacob@spamsink.net> - 2011-08-31 23:38 +0200
Re: Bit-field union bug Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-08-31 15:42 -0600
Re: Bit-field union bug Seebs <usenet-nospam@seebs.net> - 2011-08-31 21:57 +0000
Re: Bit-field union bug James Kuyper <jameskuyper@verizon.net> - 2011-08-31 18:21 -0400
Re: Bit-field union bug Seebs <usenet-nospam@seebs.net> - 2011-09-01 02:45 +0000
Re: Bit-field union bug James Kuyper <jameskuyper@verizon.net> - 2011-09-01 06:37 -0400
Re: Bit-field union bug Kenneth Brody <kenbrody@spamcop.net> - 2011-09-01 14:19 -0400
Re: Bit-field union bug James Kuyper <jameskuyper@verizon.net> - 2011-09-01 14:46 -0400
Re: Bit-field union bug Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-09-01 15:34 +0300
Re: Bit-field union bug Keith Thompson <kst-u@mib.org> - 2011-09-01 07:24 -0700
Re: Bit-field union bug James Kuyper <jameskuyper@verizon.net> - 2011-08-31 17:55 -0400
Re: Bit-field union bug "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-08-31 19:01 -0400
Re: Bit-field union bug Peter Nilsson <airia@acay.com.au> - 2011-08-31 16:03 -0700
Re: Bit-field union bug David Thompson <dave.thompson2@verizon.net> - 2011-09-06 01:13 -0400
Re: Bit-field union bug Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 12:17 -0800
Re: Bit-field union bug ec429 <ec429@cam.ac.uk> - 2012-01-25 00:13 +0000
Re: Bit-field union bug Kaz Kylheku <kaz@kylheku.com> - 2012-01-25 00:36 +0000
Re: Bit-field union bug Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 14:54 -0800
Re: Bit-field union bug Andrey Tarasevich <andreytarasevich@hotmail.com> - 2011-09-01 16:27 -0700
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