Quoting YOSHIFUJI Hideaki / $B5HF#1QL@(B [off-list ref]: 
 

+#define IN6_IS_ADDR_V4MAPPED(a)				\ 
+	((((a)->s6_addr32[0]) == 0) &&			\ 
+	 (((a)->s6_addr32[1]) == 0) &&			\ 
+	 (((a)->s6_addr32[2]) == __constant_htonl(0x0000ffff))) 

 
Please use plain htonl, __constant_htonl is only needed in static 
initializations, in all other cases with constants as a parameter it 
generates the same code as htonl, so lets prefer using the shorter, 
more readable format. 
 
- Arnaldo