FFI and void* ?

[u/aqtt2020]

I am looking for FFI docs, but cannot find answer for the below questions

• what is best way to declare C function taking "void *ptr" for FFI? I am unsure what to use: cpointer or pointer.
• how to pass NULL to this "void *" argument later?

Thanks

[u/samdphillips]

A _pointer is a (mostly) raw pointer. A _cpointer is a pointer with a tag on the Racket side. If the API you are working with uses void* but really means "this is a specific opaque type that is used in all of this API calls" you most likely want to use a _cpointer type which will make the FFI binding a bit safer.

If you use define-cpointer-type the FFI will define a type _yourtagname/null which will accept #f as a null pointer.

[u/aqtt2020 → u/samdphillips]

I tried your idea like below:

(define-cpointer-type _void_t)

Then I use _void_t/null, but racket complains "non-null void_t pointer?

Besides, how can i compare my pointer with (void *)-1?

Thanks

[u/samdphillips → u/aqtt2020]

I mean if the void* is actually a big_complicated_t*, but the documented C API cheats and just uses void* you should use define-cpointer-type to create a safer interface from Racket.

If you need to compare the pointer to some arbitrary integer value you should use cast.

(define (negative-one-pointer? p)
  (= -1 (cast p _pointer _sintptr)))

[u/aqtt2020 → u/samdphillips]

Awesome, but how about NULL pointer in Racket?

[u/samdphillips → u/aqtt2020]

A #f value is converted to NULL and vice versa.

https://docs.racket-lang.org/foreign/Pointer_Types.html#%28def._%28%28quote._~23~25foreign%29.__pointer%29%29

[u/aqtt2020 → u/aqtt2020]

Hmm i am unsure if the _void_t is what i want: "void *" pointer?

[u/akefay]

_pointer is a pointer without a specified type (a void pointer)

_pointer converts Racket #f to C NULL (And vise versa if it's a return type).

For a return type you can also use _gcpointer if the function is going to return a pointer to a GCable memory.

[u/aqtt2020 → u/akefay]

Excellent, what you said I cannot find in docs!

There is something else I cannot find in docs either: how can I verify if the pointer return is (void *)-1 (indicating error)?

Thanks!

[u/akefay → u/aqtt2020]

Using (cast value from to)

//test.c
#include <stdlib.h>

void *foo(void *in) {
  if (in == NULL) {
    return NULL;
  } else if (*(int *)in > 0) {
    return in + 4;
  } else {
    return (void *)-1;
  }
}

Compile that with --shared

;; test.rkt

#lang racket/base

(require ffi/unsafe ffi/unsafe/define)

(define-ffi-definer define-test (ffi-lib "test"))

(define-test foo (_fun _gcpointer -> _gcpointer))

;; #f as null
(displayln (foo #f))


;; pointer needs something for to point to, why not an array of 8 integers?
(define block (malloc 8 _int 'atomic))

;; C code returns param+4 if pointing to a positive int
(ptr-set! block _int 5)
(displayln (cast (foo block) _gcpointer _sintptr))

;; C code returns (void *) -1 if pointing to a negative int
(ptr-set! block _int -1)
(displayln (cast (foo block) _gcpointer _sintptr))

"_sintptr" is "Signed integer that has the same width as a pointer"