Calling C Functions

ppx_cstubs abuses external declarations that are not used in regular code. Instead of OCaml types, values of type Ctypes.typ are used inside the declarations:

/* function prototypes */
int puts(const char *s);
char *getenv(const char *name);

To access these functions from OCaml, you simply repeat the prototypes in OCaml syntax with appropriate Ctypes.typ values:

external puts: string -> int = "puts"
external getenv: string -> string_opt = "getenv"

let () = (* regular code *)
  match getenv "HOME" with
  | None -> ()
  | Some s ->
    let i = puts s in

Attributes for External Declarations

External declarations can be annotated with three different attributes:

external foo : void -> bar = "cfoo" [@@ release_runtime_lock] [@@ return_errno] [@@ noalloc]
  • release_runtime_lock: If [@@ release_runtime_lock] is specified, the OCaml runtime lock will be released during the call to the C function, allowing other threads to run. You can’t pass arguments that point to the OCaml heap (like Ctypes.ocaml_string) to such functions.

  • return_errno: If [@@ return_errno] is given, the function returns a pair as result. The first value is the regular result, the second value is the errno code of type Signed.sint.

  • noalloc: If the C function doesn’t interact with the OCaml runtime, e.g. by calling a function you have passed to C, you can add [@@ noalloc] to the declaration. The generated code will be slightly faster. Note: noalloc is here intended as an attribute for your C function, not for the generated stub code and the C function together. You can add it, even if you (believe to) know that the generated stub code has to allocate memory in the OCaml heap. The generated code will still differ.


Inside external declarations (and similar locations like struct definitions) the following parameterized (pseudo)types can be used:

So instead of …

(* function prototype in c:
   void *bsearch(const void *key, const void *base,
                 size_t nmemb, size_t size,
                 int (*compar)(const void *, const void *));
let%c compar = funptr (ptr void @-> ptr void @-> returning int)
let%c ptr_void = ptr void;
external bsearch:
  key: ptr_void
  -> base: ptr_void
  -> nmemb: size_t
  -> size: size_t
  -> compar
  -> ptr_void = "bsearch"

… one can also write:

external bsearch:
  key:void ptr
  -> base: void ptr
  -> nmemb: size_t
  -> size: size_t
  -> (void ptr -> void ptr -> int) funptr
  -> void ptr = "bsearch"

Inline Code

As a slight extension of the scheme above, you can also label your parameters, annotate external (%c) and write a few lines of C code:

external%c puts_flush : str:string -> bool = {|
  int r = puts($str);  /* to escape a regular dollar sign: $$ */
  if ( r < 0 ){
    return false;
  r = fflush(stdout);
  return (r == 0); /* `return` is mandatory, unless your function is void */
|} [@@ release_runtime_lock]

let _ : int = puts_flush ~str:"Hello World"

This way several switches between the OCaml runtime and C are avoided, which has various advantages:

  • Intermediate results can be stored on the C stack. They don’t need to be allocated on the heap and wrapped in a way to appease the OCaml runtime.

  • The C compiler can better optimise your code.

  • Constant parameters don’t need to be exposed to OCaml, just to pass them to the C function.

  • You often have to write (and generate) less code, if you don’t create wrappers for every c function and type, but just wrap snippets of C code.

Implicit Removal of Labels

Labels that end with an underscore will be removed from the generated OCaml function:

external%c puts_flush : str_:string -> bool = {|
  int r = puts($str_);

let _ : int = puts_flush "Hello World" (* no warning about a missing label ever *)

Labels are always removed, when an operator is defined through inline code.