Calling C++ from Caml

Calling C++ from Caml

[Ohad Rodeh]

Hello,
  I'm trying to use C++ code from within Caml. Currently,
the compiler cannot find the requested functions in the compiled
C++ code. 

  A very simple example fails. I have two files:
dh.ml : ML code
xx.cpp : C++ code

The C++ code contains the function:
	value dhml_Try(value dummy) {
	   return Val_unit;
	}
The ML code calls dhml_Try using:
	external dhml_Try : unit -> unit 
	  = "dhml_Try"

	let _ = 
	   dhml_Try();
	   ()	

The compilation seqeunce is as follows:
	c++ -c -w -I$(CAMLLIB) xx.cpp -o xx.o
	ocamlc -c dh.ml
	ocamlc -cc c++ -custom -o dh  xx.o dh.cmo	
/tmp/ccEPQ4W1.o(.data+0x248): undefined reference to `dhml_Try'

  If I use the gcc compiler with C code, instead of C++, this 
sequence works fine. How can I fix this problem? 

    Thanks,
	Ohad.

Re: Calling C++ from Caml

[John Prevost]

>>>>> "or" == Ohad Rodeh <orodeh@cs.huji.ac.il> writes:

    or> Hello, I'm trying to use C++ code from within Caml. Currently,
    or> the compiler cannot find the requested functions in the
    or> compiled C++ code.

        {...}

    or>   If I use the gcc compiler with C code, instead of C++, this
    or> sequence works fine. How can I fix this problem?

C++ compilers need to do "name-mangling" in order to support
overloading functions.  This adds information about the types of the
arguments to the function's name, and makes it hard for O'Caml to
find.

I suspect that using the following prototype would work for you:

extern "C" {
  value dhml_Try(value dummy);
}

the `extern "C"' part requests that the following be done with C
calling conventions.  I'm not up on my C++, so this may be
insufficient when you're defining the function in the current
file--you may need to use the extern declaration around the function's
definition.

John.