From: "Kent S." <happy@...>
Date: 2004-03-12T02:54:44+09:00
Subject: Re: A few questions about Ruby/DL

Josh Huber wrote:

 > I've had quite good luck using Ruby/DL to wrap a shared library, but
 > the lack of documentation in english is possibly preventing me from
 > using it in the best way.
 >
 > I have a few questions:
 >
 > 1. Memory management -- how do I free memory?  When do I need to free
 >    memory?  I can't seem to find any free's in the examples, but
 >    there are references to setting a freeing function in the PtrData
 >    class documentation (ext/dl/doc/dl.txt)


You don't have to free memory yourself. With Ruby/DL you are dealing 
with DL::PtrData objects that basically represent a memory pointer. When 
there are no references to a PtrData object left, GC will collect this 
object and invoke its free routine, which takes care of the allocated 
memory. Take a look at PtrData#free attribute. BTW, if you know that 
your library function returns a pointer to some kind of static buffer 
and you don't want a PtrData object release this buffer, just set its 
free attribute to nil.

 >
 > 2. What's the best way to handle pointer outputs which allocate
 >    memory?  I've done things like the following, but I'm not sure
 >    that it's the best way:
 >
 >    # say the C interface has a prototype like
 >
 >    # int get_data(char **data, int *len);
 >
 >    # on success, memory is allocated for a string and the pointer is
 >    # assigned to the data parameter.  The length of the string is
 >    # written into the len parameter.
 >
 >    data = DL::malloc(DL::sizeof("P"))
 >    len  = DL::malloc(DL::sizeof("L"))
 >    len.struct!("L", "val")
 >
 >    ret  = LIB.get_data(data, len)
 >
 >    if ret == LIB::STATUS_OK
 >      puts "got a string of length #{len[:val]}: #{data.ptr}"
 >    else
 >      puts "failed to get data: #{LIB::strerror(ret)}"
 >    end


I would do it like that:

$ cat test.c
#include <stdio.h>

int get_data(char** data, int* len) {
    int i;
    *len = 30;
    *data = (char*)malloc(sizeof(char)*(*len));
    sprintf(*data, "string of size %d", *len);
    return (*len);
}
$ cc -dynamic -bundle -o test.bundle test.c
$ irb
irb(main):001:0> require 'dl/import'
=> true
irb(main):002:0> module TestDLL
irb(main):003:1>   extend DL::Importable
irb(main):004:1>   dlload 'test.bundle'
irb(main):005:1>   extern "int get_data(char**, int*)"
irb(main):006:1> end
=> #<DL::Symbol:0x0x4c9910 func=0x0x1e6f28 'int get_data(void *, void *);'>
irb(main):007:0> data = DL::PtrData.new(0)
=> #<DL::PtrData:0x0x4905f0 ptr=0x0x0 size=0 free=0x0x0>
irb(main):008:0> data.free = DL::FREE
=> #<DL::Symbol:0x0x456120 func=0x0x5876ec 'void free(void *);'>
irb(main):009:0> len = DL::PtrData.malloc(DL.sizeof('I'))
=> #<DL::PtrData:0x0x441f30 ptr=0x0x441710 size=4 free=0x0x0>
irb(main):010:0> len.struct!('I', 'val')
=> nil
irb(main):011:0> TestDLL.get_data(data.ref, len)
=> 30
irb(main):012:0> len['val']
=> 30
irb(main):013:0> data.to_s
=> "string of size 30"
irb(main):014:0>

/kent