Bit Field Formations

[Crescioli, Phil]

Hello all,
I'm using gcc version 3.2.2 (RedHat Linux 3.2.2-5) and am having some
questions with regard to how gcc forms bit-fields.

If I create a structure that includes bit-field definitions, how are the
bit fields assigned within a word. In the example below, is variable x1
assigned to bits 31-28 or bits 3-0? My assumption is that the fields are
assigned in order so that the example produces either:

X1:31-28, x2:27-26, x3:25-23, x4:22-18, x5:17-13, x6:12, x7:11, x8:10-6,
x9:5-0

Or

X1:3-0, x2:5-4, x3:8-6, x4:13-9, x5:18-14, x6:19, x7:20, x8:25-21,
x9:31-26

Which is correct?

typdef struct
{
  double        a;
  double        b;
  short int     c;
  short int     d;
  unsigned int  e;
  UINT32        x1 : 4; 
  UINT32        x2 : 2;    
  UINT32        x3 : 3;             
  UINT32        x4 : 5;             
  UINT32        x5 : 5;             
  UINT32        x6 : 1;             
  UINT32        x7 : 1;             
  UINT32        x8 : 5;             
  UINT32        x9 : 6;                      
  unsigned int  f;
  float         g;
} TEST_STRUCT

Thanks,
Phil Crescioli
General Dynamics - Advanced Information Systems
Phil.Crescioli@gd-ais.com