perlmonk1729 has asked for the wisdom of the Perl Monks concerning the following question:

Hello Perl Gurus,

I am having trouble updating a variable in a perl sub and using the updated value in the main body of the perl script. Ultimately, I'd like to share more complex data between the sub/callback and main. Any help would be greatly appreciated!

I am extending perl (with SWIG) to access a C library and also embedding a perl interepreter (via perl_clone) so that the library can invoke perl subroutines as callbacks.

The perl subs are getting called correctly. I need to wait in the main perl script until the callback is executed before other actions are taken. I initially initialized a simple scalar to "0" and changed its value in the subroutine and waited (sleep() ) until the value is updated. Though the callback updates the value, it is never reflected in the main body of the script. I'm not sure what is the reason?

As a possible solution, I declared the scalar as ":shared", but then I get this error on a unrelated line of code. The offending line is registering the perl-sub as a callback into the C library via a SWIG generated wrapper. Not sure of the link between "use threads:shared" and this message.

Perl version 5.8.8 threads version: 1.07, threads::shared version: 0.94

Original/ideal looking code sample.. 

#This is the SWIG generated module to my C library
use MyModule;
#shared scalar
$cb_done = 0;

#This cb would be invoked by the cloned interpreter in the
#C library.  This works fine.
sub cb_one
{
  ($event, $pdata) = @_;
  #This line is printed.
  print "event : ", $event, "\n";
  $cb_done = 1;
}

$status = MyModule::RegisterCB(\&main::cb_one);

do
{
   sleep (5);
} until ($cb_done == 1)

#This line is not printed as $cb_done never becomes 1 
#in the main of the perl script
print "CB was invoked : $cb_done\n";

[download]

Sample code using threads::shared 

use threads;
use threads::shared;

use MyModule;
#shared scalar
$cb_done : shared = 0;

[download]
But I get "Invalid value for shared scalar" for this line of code

 $status = MyModule::RegisterCB(\&main::cb_one);

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Re: Extending & Embedding Perl simultaneously: How to share a scalar between perl callback sub and main body?
by BrowserUk (Patriarch) on Dec 01, 2009 at 13:22 UTC

    Try prefixing you global var with our: 

    our $cb_done :shared = 0;

    [download]
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
Re: Extending & Embedding Perl simultaneously: How to share a scalar between perl callback sub and main body?
by cdarke (Prior) on Dec 01, 2009 at 13:41 UTC
    Works for me (ActiveState Perl 10.1 on Windoze).

    Did you miss the syntax error in your code (missing ; after until)? Try use warnings;

    Here is my C:
    #include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#include "const-c.inc"

MODULE = MyModule        PACKAGE = MyModule        

INCLUDE: const-xs.inc

int
RegisterCB (SV *SubRef)

    CODE:
    
    ENTER;
    SAVETMPS;
    PUSHMARK(SP);
    
    XPUSHs(sv_2mortal(newSVpv("Some event",0)));
    XPUSHs(sv_2mortal(newSVpv("Some pdata",0)));
    
    PUTBACK;
    call_sv(SubRef, G_DISCARD);
    
    FREETMPS;
    LEAVE;
    
    RETVAL = 1;
    OUTPUT:
        RETVAL

    [download]
    Here is the Perl (its a long time since I didn't use strict;):
    use MyModule;
use warnings;

#shared scalar
$cb_done = 0;

#This cb would be invoked by the cloned interpreter in the
#C library.  This works fine.
sub cb_one
{
  ($event, $pdata) = @_;
  #This line is printed.
  print "event : ", $event, "\n";
  $cb_done = 1;
}

$status = MyModule::RegisterCB(\&main::cb_one);

do
{
   sleep (5);
} until ($cb_done == 1);

#This line is printed as $cb_done becomes 1 
#in the main of the perl script
print "CB was invoked : $cb_done\n";

    [download]
    and here is the output:
    C:\gash\MyModule>test.pl
Name "main::pdata" used only once: possible typo at C:\gash\MyModule\t
+est.pl line 11.
Name "main::status" used only once: possible typo at C:\gash\MyModule\
+test.pl line 17.
event : Some event
CB was invoked : 1

    [download]

    Update: original code was not passing parameters to the callback.
    Update 2: I hadn't freed the temporaries and closed the scope
[reply]
[d/l]
[select]

      My guess would be the problem is that the OP is using perl_clone() (which you are not), and that the callback code is being executed in the wrong (cloned) interpreter.   (As usual, it would help to see the OP's actual code, though...)

[reply]
[d/l]
        Thanks for all the comments!

        Apologies for typo in the code sample. I tried prefixing "our", but that didnt change the result.

        As almut said, I am using a cloned interpreter and the perl sub/callback is executed in the cloned interpreter context. So, is the problem in how I'm cloning it? Or does cloning by definition mean, I cannot "share". Here is a post describing how I've done the embedding.

        One "crude" way I can think of is to implement a "shared memory" API on C side to save/retrieve data between the subs and main. Perhaps CPAN has modules like this already..but I'm really hoping there is a more "natural" solution.

        http://markmail.org/thread/cjbhybjfikuirvud

        http://markmail.org/thread/inetppjegt5iecl5

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