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

Learning
1

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!

2

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

3

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.

4

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.

1 Like
5

I see, thank you though!