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Re: Bit-field union bug

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On 08/31/2011 05:23 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;
>    };

Ian has already pointed out to you the syntax errors in that declaration.

> };

"An implementation may allocate any addressable storage unit large
enough to hold a bit-field. If enough space remains, a bit-field that
immediately follows another bit-field in a structure shall be packed
into adjacent bits of the same unit." (6.7.2.1p10)

However, the thing that follows proto is not another bit field, but a
union. The union contains two structs, both structs contain bit fields,
but those bit fields are not members of the same struct that proto is.
Therefore, 6.7.2.1p10 does not apply.

Here's an alternative approach that should cause the fields to be merged
into the same storage unit. However, there's no guarantees about what
size the "storage unit" is. It could have a size of 8.

	struct s_skip_ind {
	   unsigned proto:4;
	   unsigned skip_ind:4;
	};

	struct s_trans_id {
	   unsigned proto:4;
	   unsigned trans_val:3;
	   unsigned trans_id:1;
	};
	
	union foo {
	   struct s_skip_ind skip_ind;
	   struct s_trans_id trans_id;
	};


Note that, with this definition, both 'proto's are required to occupy
the same bits (6.5.2.3p5).

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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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