From: Ben Giddings <bg-rubytalk@...>
Date: 2005-02-02T11:14:58+09:00
Subject: Ruby/DL and functions that modify a pointer

I just started using Ruby/DL and am amazed at what it can do.  I've 
almost got a really impressive demo to show off to my cow-orkers, but I 
have one cow left to ork... er... I have one thing left to figure out.

Given some C code like this:

typdef struct {
   int status;
   int results[16];
} foo_t;

int get_results(handle_t the_handle, foo_t *results_ptr)
{
   results_ptr->status = 0;
   results_ptr->results[0] = 0xFF;

   return 0;
}

How do I interface to that code in Ruby/DL?

I already have functions that take handles, I just treat it as a pointer 
and everything works great:

require 'dl/import'
module LIBMYAPI
   extend DL::Importable
   dlload "./libmyapi.so.1.0.1"
   typealias("size_t", "unsigned int")
   typealias("id_t", "int")
   extern "void *open_handle(char *)"
   extern "char *get_version(void *)"
   extern "int close_handle(void *)"
end

if __FILE__ == $0
   hostname = 'foo'

   handle = LIBMYAPI.open_handle("m4-c0050c")
   result = LIBMYAPI.get_version(handle)
   puts result
   LIBM4API.close_handle(handle)
end

I just don't know how to tell Ruby/DL that get_results takes a pointer, 
then changes it.

While I'm at it, what's the proper way to deal with a structure 
containing an array, if you want to access things by name?

Something like this works for the 'timeval' struct, but I don't know how 
to use it with structs containing arrays.

ptr = DL.malloc(DL.sizeof('LL'))
ptr.struct!('LL', :tv_sec, :tv_usec)
ptr[:tv_sec] = 10
ptr[:tv_usec] = 100
sec = ptr[:tv_sec]
usec = ptr[:tv_usec]

Thanks!

Ben