Call a foreign-language function

Show how a foreign language function can be called from the language.

As an example, consider calling functions defined in the C language. Create a string containing "Hello World!" of the string type typical to the language. Pass the string content to C's strdup. The content can be copied if necessary. Get the result from strdup and print it using language means. Do not forget to free the result of strdup (allocated in the heap).

Notes:

It is not mandated if the C run-time library is to be loaded statically or dynamically. You are free to use either way.
C++ and C solutions can take some other language to communicate with.
It is not mandatory to use strdup, especially if the foreign function interface being demonstrated makes that uninformative.

Ada

Ada provides standard interfaces to C, C++, Fortran and Cobol. Other language interfaces can be provided as well, but are not mandatory. Usually it is possible to communicate to any language that supports calling conventions standard to the OS (cdecl, stdcall etc). <lang Ada> with Ada.Text_IO; use Ada.Text_IO; with Interfaces.C; use Interfaces.C; with Interfaces.C.Strings; use Interfaces.C.Strings;

procedure Test_C_Interface is

  function strdup (s1 : Char_Array) return Chars_Ptr;
  pragma Import (C, strdup, "_strdup");

  S1 : constant String := "Hello World!";
  S2 : Chars_Ptr;

begin

  S2 := strdup (To_C (S1));
  Put_Line (Value (S2));
  Free (S2);

end Test_C_Interface; </lang>

Common Lisp

Library: CFFI

<lang lisp>CL-USER> (let* ((string "Hello World!")

               (c-string (cffi:foreign-funcall "strdup" :string string :pointer)))
          (unwind-protect (write-line (cffi:foreign-string-to-lisp c-string))
            (cffi:foreign-funcall "free" :pointer c-string :void))
          (values))

Hello World!

No value</lang>

OCaml

Outline of what is linked against

For the hypothetical C library that contains functions described by a header file with this in: <lang c>void myfunc_a(); float myfunc_b(int, float); char *myfunc_c(int *);</lang>

The header file is named "mylib.h", and linked against the library with -lmylib and compiled with -I/usr/include/mylib.

Required files

Here are provided all the files, including a Makefile.

file "mylib.ml":

<lang ocaml>external myfunc_a: unit -> unit = "caml_myfunc_a" external myfunc_b: int -> float -> float = "caml_myfunc_b" external myfunc_c: int array -> string = "caml_myfunc_c"</lang>

file "wrap_mylib.c":

<lang C>#include <caml/mlvalues.h>

include <caml/alloc.h>
include <mylib.h>

CAMLprim value caml_myfunc_a(value unit) {

   myfunc_a();
   return Val_unit;

}

CAMLprim value caml_myfunc_b(value a; value b) {

   float c = myfunc_b(Int_val(a), Double_val(b));
   return caml_copy_double(c);

}

CAMLprim value caml_myfunc_c(value ml_array) {

   int i, len;
   int *arr;
   char *s;
   len = Wosize_val(ml_array);
   arr = malloc(len * sizeof(int));
   for (i=0; i < len; i++) {
       arr[i] = Int_val(Field(ml_array, i));
   }
   char *s = myfunc_c(int *);
   free(arr);
   return caml_copy_string(s);

}</lang>

the Makefile:

<lang Makefile>wrap_mylib.o: wrap_mylib.c

       ocamlc -c -ccopt -I/usr/include/mylib $<

dllmylib_stubs.so: wrap_mylib.o

       ocamlmklib -o mylib_stubs $< -lmylib

mylib.mli: mylib.ml

       ocamlc -i $< > $@

mylib.cmi: mylib.mli

       ocamlc -c $<

mylib.cmo: mylib.ml mylib.cmi

       ocamlc -c $<

mylib.cma: mylib.cmo dllmylib_stubs.so

       ocamlc -a -o $@ $< -dllib -lmylib_stubs -cclib -lmylib

mylib.cmx: mylib.ml mylib.cmi

       ocamlopt -c $<

mylib.cmxa: mylib.cmx dllmylib_stubs.so

       ocamlopt -a -o $@ $< -cclib -lmylib_stubs -cclib -lmylib

clean:

       rm -f *.[oa] *.so *.cm[ixoa] *.cmxa</lang>

the file mylib.cma is used for the interpreted and bytecode modes, and mylib.cmxa is for the native mode.

Tcl

Wrapping up the ilogb function from C's math library with

Library: critcl

so that it becomes one of Tcl's normal functions (assuming Tcl 8.5):

<lang tcl>package require critcl critcl::code {

   #include <math.h>

} critcl::cproc tcl::mathfunc::ilogb {double value} int {

   return ilogb(value);

} package provide ilogb 1.0</lang>