Add FIELD_SIZEOF macro to types.h
diff --git a/src/types.h b/src/types.h
index 0a7e2f7..7e09de6 100644
--- a/src/types.h
+++ b/src/types.h
@@ -27,6 +27,7 @@
 
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
+#define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
 
 #define NULL ((void *)0)