The utility global_types_gen in the cg_xml package shows you the layout of all the structures, including the offset from the beginning of the structure ...
Here is some example output
_Fnarray // Size in MADUs == 12 struct _Fnarray * _Next // Offset == 0 int _Index // Offset == 4 event_callback _Pfn // Offset == 8 _Iosarray // Size in MADUs == 24 struct _Iosarray * _Next // Offset == 0 int _Index // Offset == 4 long _Lo // Offset == 8 void * _Vp // Offset == 16 _Locimp // Size in MADUs == 52 struct facet __b_Q3_3std6locale5facet // Offset == 0 struct facet * * _Facetvec // Offset == 8 size_t _Facetcount // Offset == 12 category _Catmask // Offset == 16 signed char _Xparent // Offset == 20 string _Name // Offset == 24
Hope this helps ...
-George