Re: [Caml-list] Wrapping var_args, or C ... in ocaml?

[Richard Jones <rich@annexia.org>]

On Sun, Feb 14, 2010 at 04:46:20PM +0100, Guillaume Yziquel wrote:
> Hello.
> 
> I'm currently looking at:
> 
> 	http://docs.python.org/c-api/arg.html
> 
> and I would like to know how to wrap up C functions with va_list of with 
> an ellipsis. Is this documented somewhere, or has someone already done 
> something like this?

It really depends on the function and how it will be used.  It
might translate to any of:

(1) A collection of functions implementing different aspects of the C
function.  eg. The open(2) function in Unix is really a varargs
function, and depending on whether you want to open a file for input,
output, create, etc. you'd probably be better off with different
functions in OCaml.  (Unix.openfile does _not_ do this ...)

(2) A simple list, eg. for a C function that takes a NULL-terminated
list of strings.

(3) A variant list of variants, or option labels, eg. for a C function
that takes 'type, value'(s), such TIFFSetField in
libtiff. (http://www.remotesensing.org/libtiff/man/TIFFSetField.3tiff.html)

(4) Something very specialized, eg. the 'printw' function in ncurses
is like printf and so would need quite a tricky implementation in
OCaml.  (Probably best to use ksprintf to convert to a string in OCaml
and then pass printw ("%s", str) in C).

In libguestfs where we autogenerate bindings we avoided varargs
altogether, because it's hard to map such a concept to all the
different languages we support.

Rich.

-- 
Richard Jones
Red Hat