[Peaker]

It depends on whether you delegate all memory allocations to the C file.

In the C code I work on, dynamic memory allocations are considered expensive and problematic (fragmentation, runtime errors) so we prefer to move the responsibility for allocation as far up the call-stack as possible, where it is often possible to minimize or coalesce dynamic allocations.

To do this, the interface files must expose the correct sizeof() so that the caller can allocate. We do this by splitting the header file into foo.h and foo_private.h (foo.h #includes foo_private.h). This separation is not for enforcement, but for API clarity and convention (if it's in a private.h, you have no business assuming anything about it).

This forces us to rebuild when the sizes of private structures change (which can be a significant downside) but allows us to avoid dynamic allocations, their costs and their many potential error paths.

In this approach, foo_private.h may very well #include a hash_table or whatever header which should not* be exposed to the #includer.