+Virtual structs are prefaced with a header, which consists of
+the following, in order:
+1. 128-bit GUID of the most derived vstruct type
+2. 160-bit SHA-1 of the vstruct type info
+3. 32-bit objlist-offset of an introspection object
+4. 32-bit length of vstruct, including header, and all referenced
+data chunks, which must be contiguously appended to the vstruct body
+5. 32-bit reserved field
+
+The requirement for all referenced data chunks to be contiguous with
+the vstruct means that scatter-gather and zero-copy cannot be done;
+however, it prevents the loss of derived-type data when only a base
+type is known at unmarshalling time (the derived-type data could
+later be accessed through introspection, or be passed to another
+process that does know the derived type). For this reason, vstructs
+should generally not be used to pass large chunks of data.
+
+The introspection object reference may be NULL; in this case, the
+SHA-1 field is ignored.
+