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

[Malcolm Matalka <mmatalka@gmail.com>]

Jeremy Yallop <yallop@gmail.com> writes:

> Dear Malcolm,
>
> On 28/02/2016, Malcolm Matalka <mmatalka@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
>

Great, with some small modifications this worked like a charm.


>> 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.