Best Practice for Representing `uint8_t`-Backed Enums with `ctypes`?

Hi,

I’m working with OCaml’s ctypes to bind to a C library using cstubs, and I’ve run into a common pattern in C code: enums that are not declared as enum types, but are instead stored as raw uint8_t values in structs.

For example:

struct example {
    uint32_t foo;
    uint8_t mode;
};

The mode field is meant to represent a specific set of values (like an enum), but it’s specified as a uint8_t. For example, the enum could look like this:

enum spng_crc_action {
    A = 0,
    B = 1,
    C = 2
};

In OCaml, I’d like to map this to a variant type.

Is the idiomatic way to handle this to use a Ctypes.view over uint8_t, mapping between the integer values and OCaml variants?

Something like (ignore the missing Unsigned.Uint8 usage):

type mode = A | B | C

let to_uint8 = function
  | A -> 0
  | B -> 1
  | C -> 2

let of_uint8 = function
  | 0 -> A
  | 1 -> B
  | 2 -> C
  | _ -> invalid_arg "Unknown mode value"

let mode_view = view ~read:of_uint8 ~write:to_uint8 uint8_t

Then used in a struct field as:

let mode = Ctypes.field example "mode" mode_view

Is this the right approach? Or is there a better way to model these kinds of enums that are stored as fixed-size integers? Apologies if I got something wrong, I’m not very experienced with C.

Side note: I also tried using enum for this, but it didn’t seem to work correctly — possibly because the backing type is uint8_t. Happy to hear if that can be made to work too.

Thanks in advance!

Is there something in particular that you’re concerned about?

Not really, just wanted to be sure if this is the only option. I guess one bad thing with this approach is that it might stop working if the C enum values are changed. I tried defining it with enum first, but that didn’t work, sadly.

That’s true, but it seems difficult to avoid, if there’s no connection in the C declarations between the possible values of the enum and the mode field.

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I see, thank you though!