From: Aaron Patterson <aaron@...>
Date: 2010-01-26T03:53:26+09:00
Subject: Re: ruby bounties--list of bounties

On Tue, Jan 26, 2010 at 03:12:17AM +0900, Charles Oliver Nutter wrote:
> On Mon, Jan 25, 2010 at 6:25 PM, Mike Dalessio <mike.dalessio@gmail.com> wrote:
> > Charlie, you're making a great case against using FFI.
> 
> FFI is much better than writing any C code at all, due to the
> security, stability, and portability problems of writing your own C
> bindings.

References please.

Last I checked, it was just as easy to segv from an FFI library as a C
library.  Plus with FFI you don't get any benefits of compile time
checks.  You can't, for example, check for #define constants.

With FFI you must:

1. Duplicate header files (see below for more problems)
2. Understand struct layouts and the sizeof() for each member
3. Do runtime checking of library features
4. Worry about weak ref maps when using void pointers (see the id2ref
   problem in nokogiri)
5. Pay a runtime conversion price from ruby data types to FFI types
6. Educate users on LD_LIBRARY_PATH
7. Worry about 32bit and 64bit issues (like Tony mentioned)

The duplication of header files becomes an even larger problem if the
library you're wrapping changes it's struct layout.  Where a simple
recompile would have solved the problem, now (without warning) you're getting
surprising values in your FFI program.  Plus typical debugging tools
like gdb get you nowhere.

Example:

Library "foo" ships with a struct like this:

    struct awesome {
      float hello;
      char * world;
    };

Then later changes to:

    struct awesome {
      char * world;
      float hello;
    };

You wrapped the first one, upgrade the library, then boom.  It doesn't work.
With a compiled program, you wouldn't care.

Unfortunately, none of the problems I've just listed off are
theoretical.  I have personally run in to every one of them and can
provide you with real world examples.  FFI is awesome for certain,
confined, small, stable use cases.  I use FFI, and I enjoy it.  But
saying that it's "the only logical choice" seems wrong.

I am curious what your experience has been, and why you haven't run in to the
same problems?  How do other people overcome these issues?

-- 
Aaron Patterson
http://tenderlovemaking.com/