this is a gcc file, you can download it and learn its usetility, for further detail please look at t

				#include 								struct sss{				  int i1:21;				  int i2:19;				  int i3:4;				};								static union u{				  struct sss sss;				  unsigned char a[sizeof (struct sss)];				} u;								int main (void) {				  int i;				  for (i = 0; i < sizeof (struct sss); i++)				    u.a[i] = 0;				  u.sss.i1 = 2097151.0;				  for (i = 0; i < sizeof (struct sss); i++)				    printf ("%x ", u.a[i]);				  printf ("\n");				  u.sss.i2 = 524287.0;				  for (i = 0; i < sizeof (struct sss); i++)				    printf ("%x ", u.a[i]);				  printf ("\n");				  u.sss.i3 = 15.0;				  for (i = 0; i < sizeof (struct sss); i++)				    printf ("%x ", u.a[i]);				  printf ("\n");				  return 0;				}