How can I retrieve data from mongoc_collection_find_with_opts

Question

Following are snippets of my code in C, redacted for ease of reading:

bCursor = mongoc_collection_find_with_opts(...);
mongoc_cursor_next(bCursor, &bDoc);

where I get the document in bDoc as json:

{
  "_id" : { "$numberInt" : "4184300" },
  "msgTime" : { "$numberInt" : "969481160" },
  "msgLTm" : { "$numberInt" : "969484760" }
}

How do I the fields _id, or msgTime or msgLTm?

I have tried

if (bson_iter_init(&bIter, (bson_t *)&cDoc) && bson_iter_find_descendant(&bIter, "_id", &bVal))
      ....

but to no avail.. as there are no descendants perhaps.

Any feedback will most appreciated. Cheers