Calling OCaml from C - nothing shown on stdout

Calling OCaml from C - nothing shown on stdout

[Andreas Sommer]

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Hi everybody,

I'm trying to call a OCaml function from C code. This is the OCaml file:

    open Printf

    let hello () =
        Printf.fprintf stdout "%s" "test"
       
    let () =
        let oc = open_out "/tmp/testfile" in
        Printf.fprintf oc "test";
        close_out oc;
       
        Printf.fprintf stdout "Caml main function\n";
       
        Callback.register "Hello callback" hello;
       
        Printf.fprintf stdout "%s" "Finished with OCaml initialization"
    ;;


and this is the C file:

    #include <stdio.h>
    #include <caml/memory.h>
    #include <caml/mlvalues.h>
    #include <caml/callback.h>

    void helloWrapper()
    {
        static value *closure_f = NULL;

        if(closure_f == NULL)
        {
            printf("1\n");
            closure_f = caml_named_value("Hello callback");
            printf("2\n");
            printf("Closure pointer: %p\n", closure_f);
        }
        printf("3\n");
        caml_callback(*closure_f, Val_unit);
    }

    int main(int argc, char **argv)
    {
        caml_main(argv);

        printf("calling wrapper\n");
        helloWrapper();

        printf("Done.");
        return 0;
    }


Everything seems to work: The file "/tmp/testfile" is created correctly
(so the OCaml function did get called), and the C program ends with
"Done.", *but* nothing is printed from the OCaml fprintf functions.

I am linking the executable with

    $(OCAMLOPT) -output-obj test.ml -o test_obj.o
    $(CC) -c linkwiththis.c -o linkwiththis.o -I"`$(OCAMLC) -where`"
    $(CC) linkwiththis.o test_obj.o $(NATIVECCLIBS) -lasmrun -o
    camltestprogram -L"`$(OCAMLC) -where`"


What am I doing wrong - how can I make OCaml print to stdout correctly?

Best regards,
    Andreas

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Re: [Caml-list] Calling OCaml from C - nothing shown on stdout

[Erik de Castro Lopo]

Andreas Sommer wrote:

> Hi everybody,
> 
> I'm trying to call a OCaml function from C code.


I don't see a call to caml_startup() from the C code which I believe
is necessary.

I've blogged this exact problem (calling into Ocaml from C) here:

    http://www.mega-nerd.com/erikd/Blog/CodeHacking/Ocaml/calling_ocaml.html

Cheers,
Erik
-- 
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/

RE: [Caml-list] Calling OCaml from C - nothing shown on stdout

[David Allsopp]

Andreas Sommer wrote:
> Hi everybody,
>
> I'm trying to call a OCaml function from C code. This is the OCaml file:
> open Printf
>
> let hello () =
>    Printf.fprintf stdout "%s" "test"
>    
> let () =
>    let oc = open_out "/tmp/testfile" in
>    Printf.fprintf oc "test";
>    close_out oc;
>    
>    Printf.fprintf stdout "Caml main function\n";
>    
>    Callback.register "Hello callback" hello;
>    
>    Printf.fprintf stdout "%s" "Finished with OCaml initialization"
>;;

A couple of observations on your coding style - you open Printf and then proceed to use qualified Printf.fprintf calls. There's no need for the open statement. Printf.printf is a shorthand for "Printf.fprintf stdout" which makes the code a bit clearer if you're skimming through it (as Printf.fprintf at a glance looks like actual file I/O rather than channel I/O).

> and this is the C file:
<snip>

As far as I can tell from the manual, there's no particular difference in native code between using caml_main and caml_startup. Your problem is the buffering of stdout in the OCaml runtime. There are two ways of working around this:

1. Allow the runtime to exit using Pervasives.exit at some point on the ML side (or call caml_sys_exit directly in your C code but I haven't tried that...). For example, if I added [; exit 0] to the end of your hello ML function then I get the lines from the Printf calls on the ML side, but out of order (all of the OCaml ones come at the end after the C ones)

2. Insert a flush stdout statement after each Printf.printf call. Better, create another function using Printf.kprintf which flushes stdout each time:
    let flushed_printf x = Printf.kprintf (fun s -> print_string s; flush stdout) in
    flushed_printf "%s" "Finished with..."


David

Re: [Caml-list] Calling OCaml from C - nothing shown on stdout

[Matthieu Dubuget]

-------- Original Message --------
Subject: Re: [Caml-list] Calling OCaml from C - nothing shown on stdout
From: David Allsopp <dra-news@metastack.com>
To: 'Andreas Sommer' <AndiDog@web.de>, 'caml-list@yquem.inria.fr' 
<caml-list@yquem.inria.fr>
Date: 29/06/2010 13:18
>
> 2. Insert a flush stdout statement after each Printf.printf call. Better, create another function using Printf.kprintf which flushes stdout each time:
>      let flushed_printf x = Printf.kprintf (fun s ->  print_string s; flush stdout) in
>      flushed_printf "%s" "Finished with..."
>    

Or use "%!" in your string format to flush.

Salutations

Matt

Re: [Caml-list] Calling OCaml from C - nothing shown on stdout

[Andreas Sommer]

David Allsopp wrote:
>
>
> As far as I can tell from the manual, there's no particular difference in native code between using caml_main and caml_startup. Your problem is the buffering of stdout in the OCaml runtime. There are two ways of working around this:
>   

The caml_main and caml_startup functions are indeed identical
(caml_startup only calls caml_main). You were right about the stdout
buffering, and I will use your flushed_printf solution. Thanks for your
help!

Andreas