20020910-1.c

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

unsigned int  x0  = 0;

typedef struct {
  unsigned int  field1 : 20;
  unsigned int  field2 : 12;
} XX;

static XX yy;

static void foo (void)
{
  yy.field1 = (unsigned int ) (&x0);
}