This section illustrates the callback facilities described in section 18.7. We are going to package some Caml functions in such a way that they can be linked with C code and called from C just like any C functions. The Caml functions are defined in the following mod.ml Caml source:
(* File mod.ml -- some ``useful'' Caml functions *) let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2) let format_result n = Printf.sprintf "Result is: %d\n" n (* Export those two functions to C *) let _ = Callback.register "fib" fib let _ = Callback.register "format_result" format_result
Here is the C stub code for calling these functions from C:
/* File modwrap.c -- wrappers around the Caml functions */ #include <stdio.h> #include <string.h> #include <caml/mlvalues.h> #include <caml/callback.h> int fib(int n) { static value * fib_closure = NULL; if (fib_closure == NULL) fib_closure = caml_named_value("fib"); return Int_val(caml_callback(*fib_closure, Val_int(n))); } char * format_result(int n) { static value * format_result_closure = NULL; if (format_result_closure == NULL) format_result_closure = caml_named_value("format_result"); return strdup(String_val(caml_callback(*format_result_closure, Val_int(n)))); /* We copy the C string returned by String_val to the C heap so that it remains valid after garbage collection. */ }
We now compile the Caml code to a C object file and put it in a C library along with the stub code in modwrap.c and the Caml runtime system:
ocamlc -custom -output-obj -o modcaml.o mod.ml ocamlc -c modwrap.c cp /usr/local/lib/ocaml/libcamlrun.a mod.a ar r mod.a modcaml.o modwrap.o
(One can also use ocamlopt -output-obj instead of ocamlc -custom -output-obj. In this case, replace libcamlrun.a (the bytecode runtime library) by libasmrun.a (the native-code runtime library).)
Now, we can use the two functions fib and format_result in any C program, just like regular C functions. Just remember to call caml_startup once before.
/* File main.c -- a sample client for the Caml functions */ #include <stdio.h> int main(int argc, char ** argv) { int result; /* Initialize Caml code */ caml_startup(argv); /* Do some computation */ result = fib(10); printf("fib(10) = %s\n", format_result(result)); return 0; }
To build the whole program, just invoke the C compiler as follows:
cc -o prog main.c mod.a -lcurses
(On some machines, you may need to put -ltermcap or -lcurses -ltermcap instead of -lcurses.)