Re: libguile thread safety

[Chris Vine <chris@cvine.freeserve.co.uk>]

On Sat, 4 Jan 2014 02:22:11 +0100
Panicz Maciej Godek <godek.maciek@gmail.com> wrote:
> I concluded that this might be a solution. I tested it with 2.0.5 from
> ubuntu repository. I had to add GC_allow_register_threads (and link
> against gc, accordingly), because otherwise the program aborted with
> the message "Threads explicit registering is not previously enabled"
> whenever it attempted to create a thread.
> 
> For some reason the initialization routine needs to call
> scm_c_eval_string; otherwise it doesn't help.
> 
> The solution doesn't fill me with joy, but I hope it works for you

Cool.

Your revised test case appears to work on guile-2.0.9 without calling
GC_allow_register_threads().  Subject to further testing the key seems
to be to allow one thread to complete a call to scm_with_guile() and
scm_c_eval_string() before any new one is allowed to do anything.  On
further testing (involving inserting my own synchronization and using
scm_init_guile()) it appears that none of scm_with_guile(),
scm_init_guile() and scm_c_eval_string() are thread safe on the first
call.  They seem to do some initial set up on first call which is not
done in a thread safe way.

By way of example, for me this also works (does it work for
guile-2.0.5?):

#include <libguile.h>
#include <pthread.h>

void *guile_wrapper (void *data) {
  scm_c_eval_string ("(display \"Hello\n\")");
  return NULL;
}

void *thread_func (void *data) {
  scm_with_guile (&guile_wrapper, NULL);
  return NULL;
}

int main () {
  pthread_t thread1;
  pthread_t thread2;
  pthread_t thread3;

  /* initialize */
  pthread_create (&thread1, NULL, thread_func, NULL);
  pthread_join (thread1, NULL);

  /* proceed with other threads */
  pthread_create (&thread2, NULL, thread_func, NULL);
  pthread_create (&thread3, NULL, thread_func, NULL);
  pthread_join (thread2, NULL);
  pthread_join (thread3, NULL);
  return 0;
}

Unfortunately, from what Mark Weaver says, the module system doesn't
initialize itself in a thread safe way either :(.  That is more
problematic because you might not know in advance what modules are to
be loaded by any particular code.  The task model isn't suppose to
import cross-dependencies of that kind.

Chris