Problem with packed decimal

I'm passing a struct that contains fields of various datatypes, including
packed decimal, from a RPG program to a C++ srvpgm. The issue is that the
decimal fields are received on the C++ side as zoned decimal, not as packed
decimal. I haven't found anything in the Programmer's guide explaining this
behaviour.

Here's the RPG prototype:
d functionA pr 10i 0 extproc(*CWIDEN: 'functionA__+
d FP30FRDDTAD_ONLHDRA_ONLHDRA_both_t')
d headerrec likeds(headerrec) const

Here's the C++ prototype:
int functionA (FRDDTAD_ONLHDRA_ONLHDRA_both_t *prec);

The struct on the C++ side corresponds to a database file record and has
been created with GENCSRC, i.e the decimal members are of the _DecimalT type
and should work fine with packed decimal data.

For testing purposes I created another C++ srvpgm that was passed 2 parms, a
packed decimal and a char array by reference from RPG. That worked. So
there's something with packed decimal inside a datastructure that I don't
know about. Any ideas on what that is?

Many thanks,
Erik