Ctypes: nested anonymous unions

Learning
#1

Assuming I am writing Ctypes bindings, and have a .h file containing the following structure definition:

struct a {
  int foo;
  struct {
    int bar;
  } baz;
}

How do I write a description of this on the OCaml side, using Ctypes? Is it supported? If not, is there a workaround?

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#2

I got an answer from @yallop elsewhere, which is, for posterity:

  • When using the dynamic interface (which dynlinks the C library), the struct names do not matter. If I understand correctly, this means that assigning a dummy name to the anonymous union should work.
  • Using stub generation, it is not really possible at the moment. The best approach is to fix the C library by adding a tag to the anonymous union. For example, see https://github.com/grpc/grpc/pull/12059, and also https://github.com/grpc/grpc/issues/12075 which explains a bit more why anonymous structures do not work well with ctypes and stubgen.

Edit: (also from @yallop)
One workaround is to add an accessor function in a header file, and add a binding to that function:

/* in the C header file */
inline int *bar(struct a *p) { return &p->b.bar; }

(* in the ctypes bindings *)
let bar = foreign "bar" (ptr (structure "a") @-> returning (ptr int))
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