[ocaml-ctypes] [Caml-list] ctypes - Advice for binding big structs?

[Jeremy Yallop]

Dear Malcolm,

On 28/02/2016, Malcolm Matalka <mmatalka at gmail.com> wrote:
> I have a large/complex struct I am trying to create bindings for
> operations on it in Ocaml.  I have an API that tells me how many bytes
> the struct is so I can allocate it just fine and pass it around to C
> functions I've bound with ctypes.  But some data in it is accessed via
> members.  I started implementing a structure in ctypes for it, but it's
> getting large and awkward.  Are there any best practices around doing
> this?

The best approach is to use the Cstubs_structs module, which allows
you to declare just the parts of the structs that you need to access
in your program, and which generates code that uses the C struct
declarations to work out sizes, alignments, field offsets, etc.  The
basic API is the familiar set of functions "structure", "field" and
"seal" from the Ctypes module, but the build process is a little more
involved.  However, in return for the more complex build, all the
issues that you're concerned about are addressed.

The Cstubs_structs API is not yet very well documented, but there's a
brief guide with examples in the pull request that introduced it:

   https://github.com/ocamllabs/ocaml-ctypes/pull/62

> Some concerns I have:
>
> - It seems fragile - a different version of the library might have
>   different members in the struct so keeping my ocaml code in-synch
>   seems error prone.

The Cstubs_structs module addresses this by using generated C code to
determine the offsets of fields each time you build your library.

> - It's annoying because the struct has a lot of members I don't care
>   about in my case.  I only want access to a few members that have
>   important details.

Since Cstubs_structs retrieves layout rather than computing it you
only need to declare the members that you care about.

> - The struct is large with lots of types that I don't necessarily want
>   to create so creating the struct becomes somewhat awkward.  If I know
>   the size of the types I might be able to pretend it's an array of N
>   chars or something instead of trying to implement the type just to
>   fill out this struct, but I don't know if that is valid.

Again, since Cstubs_structs retrieves struct layout and alignment
information from C, you can use Ctypes.make to create struct values,
even if you haven't declared all the fields.

Kind regards,

Jeremy.