Re: Guile continuations

[Eric Cooper <ecc@cmu.edu>]

On Fri, Aug 21, 2009 at 06:53:29PM -0500, Linas Vepstas wrote:
> I'm trying to find an elegant way of multi-threading between C
> and scheme code, without actually using threads... Basically,
> I want to do some scheme stuff for a while, break off, run some
> C code for a while, then resume the scheme execution where
> I left off.

You can implement coroutines using call/cc, and the coroutines can be
either Scheme or C code.

Note that this only works if the threads explicitly call (yield) -- it
won't do the "right thing" when a thread does some blocking I/O
operation.  For that, you either need to define alternate versions of
those potentially blocking operations, or switch to real
(pre-emptively scheduled) POSIX threads.

Here's a simple proof of concept:

    ;;; A naive queue for thread scheduling.

    (define *queue* '())

    (define (empty-queue?)
      (null? *queue*))

    (define (enqueue x)
      (set! *queue* (append *queue* (list x))))

    (define (dequeue)
      (let ((x (car *queue*)))
	(set! *queue* (cdr *queue*))
	x))

    ;;; This starts a new thread running (proc).

    (define (fork proc)
      (call/cc
       (lambda (k)
	 (enqueue k)
	 (proc))))

    ;;; This yields the processor to another thread, if there is one.

    (define (yield)
      (call/cc
       (lambda (k)
	 (enqueue k)
	 ((dequeue)))))

    ;;; This terminates the current thread, or the entire program
    ;;; if there are no other threads left.

    (define (thread-exit)
      (if (empty-queue?)
	  (exit)
	  ((dequeue))))

    ;;; The body of a Scheme thread:

    (define (printer str)
      (lambda ()
	(let loop ((n 0))
	  (format #t "~A (~A)\n" str n)
	  (yield)
	  (loop (1+ n)))))

The C code:
    #include <libguile.h>

    SCM yield;

    SCM printer() {
	int n = 0;
	for (;;) {
	    printf("C thread (%d)\n", n);
	    scm_call_0(yield);
	    n += 1;
	}
    }

    void init_printer() {
	yield = scm_c_eval_string("yield");
	scm_c_define_gsubr("c-printer", 0, 0, 0, printer);
    }

Compile this as a shared library:
    gcc -shared -o printer.so -fPIC printer.c

Then run guile with the shared library accessible to it:

    $ LTDL_LIBRARY_PATH=. guile
    guile> (load "threads.scm")
    guile> (fork (printer "Scheme!"))
    Scheme! (0)
    guile> (load-extension "printer" "init_printer")
    guile> (fork c-printer)
    C thread (0)
    Scheme! (1)
    guile> (thread-exit)
    C thread (1)
    Scheme! (2)
    C thread (2)
    Scheme! (3)
    C thread (3)
    ....

I hope this helps (and that I haven't completely misunderstood your
question.)

-- 
Eric Cooper             e c c @ c m u . e d u