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

				#include 								static struct sss{				  double f;				  char a[10];				} sss;								#define _offsetof(st,f) ((char *)&((st *) 16)->f - (char *) 16)								int main (void) {				  printf ("++++Array of char in struct starting with double:\n");				  printf ("size=%d,align=%d\n",				          sizeof (sss), __alignof__ (sss));				  printf ("offset-double=%d,offset-arrayof-char=%d,\nalign-double=%d,align-arrayof-char=%d\n",				          _offsetof (struct sss, f), _offsetof (struct sss, a),				          __alignof__ (sss.f), __alignof__ (sss.a));				  printf ("offset-char-a[5]=%d,align-char-a[5]=%d\n",				          _offsetof (struct sss, a[5]),				          __alignof__ (sss.a[5]));				  return 0;				}