The problem is that unless someone has encountered your exact scenario--which is quite unlikely--the only way to find a solution would be to try a few things out.

But in order to try things out, anyone considering doing so would have to try and reproduce your code from your description. And that's a hard thing to do and rather expecting to much of us. cdarke had a good attempt, but fell short.

If you want further help, you're much more likely to get it if you would post a minimal, runnable example that can be downloaded and tried out. Perhaps you could start with cdarke's example and modify it to reflect your requirements to the point where it produces the error message your getting.

Thanks for the comments BrowserUk. I understand. I was hoping my situation wasnt so unique..that cant be good for my project long term (ie more surprises probably lay ahead). Anycase, I'll post a full sample shortly. I'm not comfortable with XS (used SWIG), so may take a bit.

Note: Completely rewrote this post as I got some sleep and have the code to demo the issue. Hoping you can now really see the cause/problem and offer ideas.

Thanks in advance!

PS: I think there some leaks etc in that code, but didnt care about those at this point.

myModule.xs

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#include <pthread.h>
static PerlInterpreter *orig_perl=NULL;
static PerlInterpreter *cb_perl=NULL;

SV* cb_ptr = NULL;

void InvokeCB ()
{
   static int val = 0;
   val++;
   dTHX;
   dSP;
   ENTER;
   SAVETMPS;
   PUSHMARK(SP);
   SV * sv = cb_ptr;
   if (my_perl != orig_perl)
   {
      CLONE_PARAMS clone_param;
      clone_param.stashes = NULL;
      clone_param.flags = CLONEf_COPY_STACKS | CLONEf_KEEP_PTR_TABLE;
      clone_param.proto_perl = cb_perl;
      sv = sv_dup(sv, &clone_param);
   }

   XPUSHs(sv_2mortal(newSViv(val)));

   PUTBACK;
   call_sv(sv, G_DISCARD);

   FREETMPS;
   LEAVE;
}

void * BGThread(void * dontcare)
{
   PERL_SET_CONTEXT(orig_perl);
   cb_perl = perl_clone(orig_perl, CLONEf_COPY_STACKS | CLONEf_KEEP_PT
+R_TABLE);
   PERL_SET_CONTEXT(cb_perl);
   while (1)
   {
      sleep(5);
      InvokeCB();
   }
}

MODULE = myModule               PACKAGE = myModule

int
RegisterCB (SV *SubRef)
    CODE:
       pthread_t tid;
       pthread_create(&tid, NULL, BGThread, NULL);
       orig_perl = PERL_GET_CONTEXT;
       cb_ptr = newSVsv(SubRef);
       SvSHARE(cb_ptr);
       RETVAL = 1;
    OUTPUT:
       RETVAL
[download]

test.pl

#! /usr/local/bin/perl

use myModule;
use warnings;

#shared scalar
$cb_done = 0;
@results = ();

#This cb would be invoked by the cloned interpreter in the
#C library.  This works fine.
sub cb_one
{
  ($value) = @_;
   print "CB called. val received : ", $value, "\n";
   $results[scalar(@results)] = $value;

   if ($value == 5)
   {
      print "cb_done changed to one.\n";
      $cb_done = 1;
   }
}

print "Registered CB...\n";

$status = myModule::RegisterCB(\&main::cb_one);
do
{
   print "Waiting for CB to be done...\n";
   sleep (5);
} until ($cb_done == 1);

#These line should be printed when $cb_done becomes 1
#in the main of the perl script
print "CB was invoked : $cb_done\n";
print "results are : @results \n";
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script output

Name "main::status" used only once: possible typo at ./test.pl line 27
+.
Registered CB...
Waiting for CB to be done...
Waiting for CB to be done...
CB called. val received : 1
Waiting for CB to be done...
CB called. val received : 2
Waiting for CB to be done...
CB called. val received : 3
Waiting for CB to be done...
CB called. val received : 4
Waiting for CB to be done...
CB called. val received : 5
cb_done changed to one.
Waiting for CB to be done...
CB called. val received : 6
Waiting for CB to be done...
CB called. val received : 7
[download]