OpaqueValue, a new native type introduced in Issue 2162 (void *
in DII Chapter) for add_arg, was never documented in any of the language
mappings. Language Mapping that claim that the DII interfaces map
according to regular rules need to provide mapping rules for OpaqueValue
and document it. The mapping would simply be OpaqueValue --> (void *) or
equivalent.

The C mapping says in section 1.9:

OMG IDL structures map directly onto C structs. Note that all OMG
IDL types that map to C structs may potentially include padding.

This is not as clear as it should be. In particular, a requirement should
be added to preserve the order of structure members as they appear in the
IDL.