ppx-ctypes-helper  

Purpose

The purpose of this preprocessor extension is to heavily simplify the usage of the ctypes library, especially when dealing with structures and enums.

Installation

Using esy

esy add ppx-ctypes-helper

With dune

Add the following line to your dune file.

(preprocess ( pps ppx-ctypes-helper.lib ))

With pesy

Add the following key to your package.json or esy.json file.

"preprocess": "pps ppx-ctypes-helper.lib"

Usage

%%struct

The %%struct extension sets up a full structure comprised of a ctype definition with fields, utility operators, types and a view that will automatically serialize the structure into/from an ocaml record.

The available typenames are the same as those supported by ctypes. See here for the list.

Syntax

(typename)("*" - 0 or more)("?" - 0 or 1 occurence)([fieldName] - 0 or 1 occurence with an optional field name between the brackets)

Spaces are allowed between parts.

Conventions

• To be considered as a structure, a typename must start with an uppercase letter.
• To be considered as an enum, a typename must contain only uppercase letters and underscores.

Example

Reasonml syntax

%struct
(
    "StructureName",
    {
        integer: "int",
        text: "string",
        optional_string: "string_opt",
        // By default, uses the "size" field to determine the length  of the array.
        string_array: "string[]",
        size: "int32_t",
        other_structure: "OtherStructure",
        other_structure_pointer: "OtherStructure *",
        other_structure_pointer_optional: "OtherStructure *?",
        // Uses the "count" field to determine the length.
        other_structure_array: "OtherStructure[count]",
        count: "int",
        other_structure_pointer_optional_array: "OtherStructure *?[]",
        enumeration: "ENUM_NAME"
    }
)

/* Then, simply use it like a ctype view. */
foreign("function_name", StructureName.view @-> (returning @@ void))

Ocaml syntax

[%%struct ("StructureName", {
    integer = "int";
    text = "string";
    optional_string = "string_opt";
    string_array = "string[]";
    size = "int32_t";
    other_structure = "OtherStructure";
    other_structure_pointer = "OtherStructure *?";
    other_structure_pointer_optional = "OtherStructure *?";
    other_structure_array = "OtherStructure[count]";
    count = "int";
    other_structure_pointer_optional_array = "OtherStructure *?[]";
    enumeration = "ENUM_NAME";
})]

Generated code

module StructureName {
  /* Abstract type to specialize the structure. */
  type t;

  /* Ctypes structure definition. */
  let structure: Ctypes.typ(Ctypes.structure(t));

  /* Operators to access and set structure fields. */

  /* To set some fields: structure >. field >= value >. field2 >= value2 */
  /* To get a field value: structure >? field */

  let ( >. ): ('a, 'b) => ('a, 'b);
  let ( >= ):
    ((Ctypes.structured('a, 'b), Ctypes.field('c, Ctypes.structured('a, 'b))),
    'c) => Ctypes.structured('a, 'b);
  let ( >? ):
    (Ctypes.structured('a, 'b),
    Ctypes.field('c, Ctypes.structured('a, 'b))) => 'c;

  /* Field definitions. */

  let integer: Ctypes.field(int, Ctypes.structure(t));
  let text: Ctypes.field(string, Ctypes.structure(t));
  let optional_string: Ctypes.field(option(string), Ctypes.structure(t));
  let string_array:
    Ctypes.field(option(Ctypes_static.ptr(string)), Ctypes.structure(t));
  let size: Ctypes.field(int32, Ctypes.structure(t));
  let other_structure:
    Ctypes.field(OtherStructure.t_view, Ctypes.structure(t));
  let other_structure_pointer:
    Ctypes.field(Ctypes_static.ptr(OtherStructure.t_view),
                  Ctypes.structure(t));
  let other_structure_pointer_optional:
    Ctypes.field(option(Ctypes_static.ptr(OtherStructure.t_view)),
                  Ctypes.structure(t));
  let other_structure_array:
    Ctypes.field(option(Ctypes_static.ptr(OtherStructure.t_view)),
                  Ctypes.structure(t));
  let count: Ctypes.field(int, Ctypes.structure(t));
  let other_structure_pointer_optional_array:
    Ctypes.field(option(Ctypes_static.ptr(option(Ctypes_static.ptr(OtherStructure.t_view)))),
                  Ctypes.structure(t));
  let enumeration: Ctypes.field(ENUM_NAME.t, Ctypes.structure(t));

  /* Type definition for the serialized record. */

  type t_view = {
    enumeration: ENUM_NAME.t,
    other_structure_pointer_optional_array:
      list(option(OtherStructure.t_view)),
    count: int,
    other_structure_array: list(OtherStructure.t_view),
    other_structure_pointer_optional: option(OtherStructure.t_view),
    other_structure_pointer: OtherStructure.t_view,
    other_structure: OtherStructure.t_view,
    size: int32,
    string_array: list(string),
    optional_string: option(string),
    text: string,
    integer: int,
  };

  /* A pair of functions + a view that are used to automatically convert from/to the record with the ctypes structure. */

  let read: Ctypes.structured(t, [ `Struct ]) => t_view;
  let write: t_view => Ctypes.structured(t, [ `Struct ]);
  let view: Ctypes.typ(t_view);
}

%%enum

In the same fashion, the %%enum extension generates converters between an enum and its native value.

Syntax

Decorate a variant type with the extension.

This will generate a module of the same name as the type, but fully uppercased and containing a ctypes view. Optionally annotate variants with the @as decorator to associate a specific value.

By default, enums are associated with integers starting from 0. Use the @as annotation to change the inferred type.

Available types:

• string
• int
• float
• char

Example

Ocaml syntax

[%%enum type int_enum = Ten [@as 10] | One | Two | Twenty [@as 20]]
[%%enum type string_enum = Hello [@as "Hello"] | World]

Reasonml syntax

%enum
type int_enum =
    | [@as 10] Ten
    | One
    | Two
    | [@as 20] Twenty;

%enum
type string_enum =
    | [@as "Hello"] Hello
    | World;

Generated code

module INT_ENUM =
  struct
    type t =
      | Ten [@as 10]
      | One
      | Two
      | Twenty [@as 20]
    let view =
      Ctypes.view
        ~read:((function | 10 -> Ten | 1 -> One | 2 -> Two | 20 -> Twenty)
        [@warning "-8"])
        ~write:(function | Ten -> 10 | One -> 1 | Two -> 2 | Twenty -> 20)
        Ctypes.int
  end
module STRING_ENUM =
  struct
    type t =
      | Hello [@as "Hello"]
      | World
    let view =
      Ctypes.view ~read:((function | "Hello" -> Hello | "World" -> World)
        [@warning "-8"])
        ~write:(function | Hello -> "Hello" | World -> "World") Ctypes.string
  end