Chicken-Scheme FFI Examples

June 16, 2020 in programming, dev, research, code

I'm currently working on refactoring the FFI implementation for the Rebel Game Engine. It was previously written using the Bind chicken egg but I wanted to have more control over the implementation by using the low level foreign functions.

To help me better understand I made some examples that has the basic FFI implementations that I'll be needing for my project.

foreign-lambda example

Let's say we have a structure Vec3 and a function Vec3Create that we want to access from chicken-scheme.

typedef struct Vec3 {
    float x;
    float y;
    float z;
} Vec3;

Vec3* Vec3Create(float x, float y, float z)
{
    Vec3* v = (Vec3*)malloc(sizeof(Vec3));
    v->x = x;
    v->y = y;
    v->z = z;
    return v;
}

We could use foreign-lambda to bind to the function:

(define vec3_create
  (foreign-lambda
    (c-pointer (struct "Vec3"))   ; Return type, a pointer to a struct object of Vec3
    "Vec3Create"                  ; Name fo the function
    float float float))           ; The three parameters (x,y,z) to pass to the function

This would allow us to call the vec3_create function like so:

(vec3_create 1.1 2.2 3.3)

foreign-lambda* example

Let's bind another C function Vec3Print.

void Vec3Print(Vec3 v)
{
    printf("Vec3 to print: (%f, %f, %f)\n", v.x, v.y, v.z);
}

We could also use foreign-lambda* (Notice the asterisk). This is similar to foreign-lambda but accepts C code as a string.

(define vec3_print
  (foreign-lambda*
    void                          ; The return type
    (((c-pointer (struct "Vec3")) a0)) ; The parameter to pass, a pointer to a Vec3 object
    "Vec3Print(*a0);"))           ; The C code in string from
                                  ; Vec3Print accepts a non pointer, we dereference it

(vec3_print (vec3_create 1.1 2.2 3.3))   ; Creates a vec3 and prints it

Inline C code in foreign-lambda*

Here's another example using foreign-lambda*. This time there is no predefined C function, but instead we define the code inside the lisp function's body.

(define vec3_zero
  (foreign-lambda*
    (c-pointer (struct "Vec3"))
    ()                            ; Empty variables
    "Vec3* v = (Vec3*)Vec3Create(0.0f, 0.0f, 0.0f); 
    C_return(v);"))               ; Instead of "return", we use "C_return".

(vec3_print (vec3_zero))          ; Calls vec3_zero and prints the returned Vec3

Note that due to obscure technical reasons C_return must be used instead of return when returning a value. More info about this here.

Free-ing Vec3 pointers

Since Vec3Create allocates memory for a Vec3 struct using malloc, it's a good idea to free this when we are done using it. To do this we could bind a function to free.

(define free (foreign-lambda void "free" c-pointer))

(let ((v (vec3_zero)))
  (vec3_print v)
  (free v))

Setting up getters and setters

If we want to have access to variables of a struct object. We could do something like this:

(define vec3_x
  (foreign-lambda*
    float
    (((c-pointer (struct "Vec3")) a0))
    "C_return(a0->x);"))

(display (vec3_x (vec3_create 8.8 8.8 8.8)))

Now this is fine but it'll be a pain to specify an accessor for every variable. A better solution is to use the foreigners egg which allows the use of macros that will make our lives easier.

(import (chicken foreign))
(import foreigners)

;; Set up the accessors for Vec3 struct
(define-foreign-record-type (vec3 Vec3)
  (float x vec3_x vec3_x!)   ; vec3_x is a getter, vec3_x! is a setter
  (float y vec3_y vec3_y!)
  (float z vec3_z vec3_z!))

(let ((v (vec3_create 4.4 5.5 6.6)))
  (display (vec3_x v)) ; Display value of x
  (newline)

  (vec3_x! v 7.7)      ; Set x to 7.7
  (display (vec3_x v)) ; Display value of x
  (newline)

  (free v))

Binding enums

The foreigners egg also allows for the binding of enums using define-foreign-enum-type. Say we have an enum declaration Keys.

enum Keys {
  UP = 0,
  RIGHT = 1,
  DOWN = 2,
  LEFT = 3
};

:::scheme
(define-foreign-enum-type (keys int)
  (keys->int int->keys)
  ((up keys/up) UP)
  ((right keys/right) RIGHT)
  ((down keys/down) DOWN)
  ((left keys/left) LEFT))

(display keys/right)
(display keys/down)

These are the basic FFI implementations that I have explored. It should be enough for most uses. The example project with working code can be found on Github.

Also, check out the chicken-bind egg, it already has all of the code above conveniently in one package. If you don't need full control and just want a simple FFI solution then this is what you need.

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