# Questions about ctypes funptr and const pointer

**URL:** <https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534>\
**Category:** Learning\
**Tags:** ctypes\
**Created:** [October 13, 2019, 2:24pm UTC](https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534 "2019-10-13T14:24:49Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![darrenldl](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/darrenldl/32/291_2.png) [@darrenldl](https://discuss.ocaml.org/u/darrenldl)\
**Post date:** [October 13, 2019, 2:24pm UTC](https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534/1 "2019-10-13T14:24:49Z")

</div>

Hello everyone,

I’m currently writing an [OCaml binding](https://github.com/darrenldl/ocaml-lxc) to LXC (Linux Containers), and have some questions w.r.t. usage of ctypes.

#### 1. Using function pointer as function?

My understanding after going through the ctypes document is one can convert an OCaml function to a function pointer to C side, carry function pointer around etc, but cannot use a C function pointer in OCaml’s side directly as if it’s a normal OCaml function.

Is this correct?

**EDIT** :  
Context is LXC stores function pointers in the main struct (`lxc_container`), and one would invoke the function via lines like `c->do_stuff(c, arg1, arg2, ...)` to access the functionality.  
Right now I’m relying on [glue code](https://github.com/darrenldl/ocaml-lxc/blob/master/code_gen/lxc_glue.c) generated using cinaps to access and invoke function pointer on C’s side, which seems to work fine. But I’m curious if there’s a better way.

#### 2. Equivalent of `const *` in ctypes?

It seems that I cannot specify a `const` pointer via ctypes, which would make sense, but I want to double check I’m not being silly in any case.

Thanks very much for your time!  
Darren

---

<div class="post-metadata">

**Author:** ![yallop](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yallop/32/517_2.png) [@yallop](https://discuss.ocaml.org/u/yallop)\
**Post date:** [October 13, 2019, 11:39pm UTC](https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534/2 "2019-10-13T23:39:58Z")

</div>

> My understanding after going through the ctypes document is one can convert an OCaml function to a function pointer to C side, carry function pointer around etc, but cannot use a C function pointer in OCaml’s side directly as if it’s a normal OCaml function.
> 
> Is this correct?

In fact, the conversion works both ways; you can use a C function pointer as an OCaml function on the OCaml side. Here’s a little working example:

```C
/* funlib.c */
struct adder { int (*add) (int, int); };
int add(int x, int y) { return x + y; }
extern struct adder make_adder(void) { return (struct adder){ .add = add }; }

```

```bash
$ gcc -shared -o libfun.so -std=c99 -fPIC -pedantic -W -Wall funlib.c 

```

```OCaml
(* fun.ml *)
#use "topfind";;
#require "ctypes.foreign";;

open Ctypes
open Foreign

let adder = structure "adder"
let add = field adder "add" (funptr (int @-> int @-> returning int))
let () = seal adder

let libfun = Dl.(dlopen ~filename:"./libfun.so" ~flags:[RTLD_NOW])
let make_adder = foreign "make_adder" (void @-> returning adder)
               ~from:libfun

let () =
  let s = make_adder () in
  Printf.printf "add(3,4) = %d\n" (getf s add 3 4)

```

```bash
$ ocaml fun.ml 
add(3,4) = 7

```

> It seems that I cannot specify a `const` pointer via ctypes, which would make sense, but I want to double check I’m not being silly in any case.

That’s correct: there’s currently no support for `const`. (Adding `#define const` before you `#include` a file in generated C code will probably give you something that works, even if it’s not strictly legitimate.)

---

<div class="post-metadata">

**Author:** ![darrenldl](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/darrenldl/32/291_2.png) [@darrenldl](https://discuss.ocaml.org/u/darrenldl)\
**Post date:** [October 13, 2019, 11:44pm UTC](https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534/3 "2019-10-13T23:44:11Z")

</div>

Thanks for the detailed reply!

One silly question just to check my sanity - the above would work the same for stub generation case I presume?

**EDIT** : specifically the `static_funptr` case.

**EDIT2** : Ended up using `coerce`, e.g.

```auto
let is_defined' (c : Types.lxc_container structure ptr) =
  let c_field = getf (!@c) Type_stubs.is_defined in
  let f = coerce (field_type Type_stubs.is_defined)
      (Foreign.funptr (ptr Types.lxc_container @-> returning bool)) c_field
  in
  f c

```

---

<div class="post-metadata">

**Author:** ![yallop](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yallop/32/517_2.png) [@yallop](https://discuss.ocaml.org/u/yallop)\
**Post date:** [August 16, 2024, 10:17am UTC](https://discuss.ocaml.org/t/questions-about-ctypes-funptr-and-const-pointer/4534/4 "2024-08-16T10:17:48Z")

</div>

> [@darrenldl](#):
>
> It seems that I cannot specify a `const` pointer via ctypes, which would make sense, but I want to double check I’m not being silly in any case.

The latest release of ctypes (0.23.0), now available from OPAM, includes new support for `const`.