From: Robert Feldt <feldt@...>
Date: 2003-09-24T15:01:26+09:00
Subject: Re: A DL-Ruby Question

Kevin Proctor <K_Proctor@sympaticoSPORK.ca> skrev den Wed, 24 Sep 2003 13:57:58 +0900:

> But, and there's always a but, you may have noticed the ugliness in my sqlite_exec function.  I'm just passing an int (0 specifically) - I really want to put a 'function pointer' there to do neat things with the results of my sql statement.  For the life of me I can not find an example of doing something like this in DL.  I'm sure it's there somewhere but I'm stumped.
>
If you're trying to do a callback to Ruby from C then this example
might help you:

# Example 7. Calling back to a Ruby function from a C function
#
# Sometimes C code takes a function as a parameter and calls that function.
# To be able to use such C code from Ruby we need to be able to call back
# from C to Ruby and transfer parameters and results back and forth.
# Here's a simple example to show how this can be done.
#
require 'example_runner'

cf = CFile.new "ex1.c", <<EOC

#include <stdio.h>

/* CompareStringLengths is a pointer to a function comparing the length
 * of two strings and returning -1, 0, or 1 */
typedef int (*CompareStringLengths)(char *str1, char *str2);

extern void report_string_compare(char *str1, char *str2, 				  CompareStringLengths compare_func) {
  int result = (*compare_func)(str1, str2);

  printf("Results was %d.\\n", result);
}
EOC

cf.compile_to_dll_with_command "gcc -shared -o test.so"

require 'dl/import'
require 'dl/struct'

module Ex07
  extend DL::Importable
  dlload './test.so'

  def str_len_compare(str1, str2)
    puts "Callback succeeded: We are in Ruby from C now!"
    str1.length <=> str2.length
  end
  STRLENCOMPARE = callback "int str_len_compare(char*,char*)"
  extern "void report_string_compare(char*, char*, void*)"
end

Ex07.report_string_compare("ab", "cde", Ex07::STRLENCOMPARE)
Ex07.report_string_compare("abc", "d", Ex07::STRLENCOMPARE)
Ex07.report_string_compare("ab", "cd", Ex07::STRLENCOMPARE)


I did a series of such examples and intended to release them but then
I got stuck on the non-portability of nested structs and
didn't go further in my efforts to use dl for a full extension.

The example_runner is just a little class to compile the C code
to a dll; excluded here for brevity.

Regards,

Robert Feldt