[Openmcl-devel] Simple C FFI example for OS X?

[Gary Byers]

On Mon, 2 Feb 2004, Peter Seibel wrote:

> Does anyone have a really trivial example of how to write some C code
> and call it from openmcl on OS X? I looked through the examples and it
> seemed that most of the OS X examples were Cocoa-based while the plain
> C examples were for LinuxPPC? I'm mostly interested in the mechanics
> of how to compile my C code so I can load it in Lisp and then call my
> functions. Like I said trivial stuff, but I'd rather cut-n-paste than
> figure it out from first principles if I can.
>
> FWIW, I'm working on hooking up to the QuickTime Music Architecture
> functions to play some music. My current theory is to write a thin
> layer in C that provides just the API I need in a way that will be
> convenient to call from Lisp and then do the rest up in Lisp.
> (Basically I need to be able to generate chunks of memory with
> QuickTime music events encoded in them and then stuff them into a
> TunePlayer and tell it to play them. No rocket science.)
>
> -Peter


Peter Seibel wrote:
> Does anyone have a really trivial example of how to write some C code
> and call it from openmcl on OS X? I looked through the examples and it
> seemed that most of the OS X examples were Cocoa-based while the plain
> C examples were for LinuxPPC? I'm mostly interested in the mechanics
> of how to compile my C code so I can load it in Lisp and then call my
> functions. Like I said trivial stuff, but I'd rather cut-n-paste than
> figure it out from first principles if I can.
>
> FWIW, I'm working on hooking up to the QuickTime Music Architecture
> functions to play some music. My current theory is to write a thin
> layer in C that provides just the API I need in a way that will be
> convenient to call from Lisp and then do the rest up in Lisp.
> (Basically I need to be able to generate chunks of memory with
> QuickTime music events encoded in them and then stuff them into a
> TunePlayer and tell it to play them. No rocket science.)
>
> -Peter
>

You're right that there should be examples of/pointers to something
that shows how to create a shared library (and, for OSX, there should
be a discussion of the different flavors of shared library available.)

Until a better example exists, here's something that might help to get
you started:

----- add2ints.c ------
/*
  Add 2 'int's together, return the result.
*/

int
add2ints(int x, int y)
{
  return x+y;
}
----- EOF ------

A better example would include a makefile.  Since this is so simple,
let's just ask the C compiler to create a "bundle"-type shared lib.
Since we aren't saying otherwise, all "extern" symbols will be exported
(with a leading underscore on OSX.)

gcc -o add2ints.so -bundle add2ints.c

;;; Load the shared library in lisp; use an absolute pathname in this
;;; case (since it's not installed in a canonical location):

? (ccl:open-shared-library "/Users/gb/add2ints.so")
#<SHLIB /Users/gb/add2ints.so #x546DACE>

;;; Verify that the external entry point "_add2ints" is known:
? (ccl:external "_add2ints")
#<EXTERNAL-ENTRY-POINT "_add2ints" (#x000CEFD0) /Users/gb/add2ints.so #x546D5E6>

;;; Pause a moment to wonder why, after a few years, CCL::EXTERNAL-CALL
;;; is still not exported.  Curse or shrug, then start adding:
? (ccl::external-call "_add2ints" :int 2 :int 2 :int)
4

----
Somewhere or other, there is (or is supposed to be) something that
lets you describe the syntax of foreign functions so that you'd be
able to use the #_ reader macro and not have to provide foreign
argument/result types:

(#_add2ints 2 2)


For things that're more substantial than this example, the
interface translator/interface database do that for you.  For things
that aren't much more substantial than this example, they're probably
overkill.

Somewhere on the Fink website (<http://fink.sourceforge.net>), there
is or was a very good explanation of the different flavors of
shared library available under OSX.

Hope that's trivial enough to get you started.

Gary Byers
gb at clozure.com