engine: use GCC provided offsetof implementation

engine/client/mod_dbghulls.c

@@ -17,7 +17,7 @@ GNU General Public License for more details.
 #include "mod_local.h"
 #include "xash3d_mathlib.h"
 #include "world.h"
-
+#include "eiface.h" // offsetof
 
 #define MAX_CLIPNODE_DEPTH		256	// should never exceeds
 
@@ -67,7 +67,7 @@ _inline void list_del( hullnode_t *entry )
 
 static winding_t * winding_alloc( uint numpoints )
 {
-	return (winding_t *)malloc( (int)((winding_t *)0)->p[numpoints] );
+	return (winding_t *)malloc( offsetof( winding_t, p[numpoints] ) );
 }
 
 static void free_winding( winding_t *w )
@@ -84,7 +84,7 @@ static winding_t *winding_copy( winding_t *w )
 	winding_t	*neww;
 
 	neww = winding_alloc( w->numpoints );
-	memcpy( neww, w, (int)((winding_t *)0)->p[w->numpoints] );
+	memcpy( neww, w, offsetof( winding_t, p[w->numpoints] ) );
 
 	return neww;
 }
@@ -111,7 +111,7 @@ static void winding_reverse( winding_t *w )
 static winding_t *winding_shrink( winding_t *w )
 {
 	winding_t	*neww = winding_alloc( w->numpoints );
-	memcpy( neww, w, (int)((winding_t *)0)->p[w->numpoints] );
+	memcpy( neww, w, offsetof( winding_t, p[w->numpoints] ));
 	free_winding( w );
 
 	return neww;

engine/common/net_encode.h

@@ -28,8 +28,6 @@ GNU General Public License for more details.
 #define DT_STRING		BIT( 7 )	// A null terminated string, sent as 8 byte chars
 #define DT_SIGNED		BIT( 8 )	// sign modificator
 
-#undef offsetof
-#define offsetof( s, m )	(size_t)&(((s *)0)->m)
 #define NUM_FIELDS( x )	((sizeof( x ) / sizeof( x[0] )) - 1)
 
 // helper macroses

engine/eiface.h

@@ -366,8 +366,12 @@ typedef enum _fieldtypes
 } FIELDTYPE;
 
 #ifndef offsetof
+#ifdef __GNUC__
+#define offsetof(s,m) __builtin_offsetof(s,m)
+#else
 #define offsetof(s,m) (size_t)&(((s *)0)->m)
 #endif
+#endif
 
 #define _FIELD(type,name,fieldtype,count,flags)		{ fieldtype, #name, offsetof(type, name), count, flags }
 #define DEFINE_FIELD(type,name,fieldtype)		_FIELD(type, name, fieldtype, 1, 0)

engine/physint.h

@@ -16,9 +16,11 @@ GNU General Public License for more details.
 #ifndef PHYSINT_H
 #define PHYSINT_H
 
+#include "eiface.h" // offsetof
+
 #define SV_PHYSICS_INTERFACE_VERSION	6
 
-#define STRUCT_FROM_LINK( l, t, m )	((t *)((byte *)l - (int)&(((t *)0)->m)))
+#define STRUCT_FROM_LINK( l, t, m )	((t *)((byte *)l - offsetof(t, m)))
 #define EDICT_FROM_AREA( l )		STRUCT_FROM_LINK( l, edict_t, area )
 
 // values that can be returned with pfnServerState