Creating Threaded callbacks in XS

Question

EDIT: I have created a ticket for this which has data on an alternative to this way of doing things.

I have updated the code in an attempt to use MY_CXT's callback as gcxt was not storing across threads. However this segfaults at ENTER.

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

#ifndef aTHX_
#define aTHX_
#endif

#ifdef USE_THREADS
#define HAVE_TLS_CONTEXT
#endif

/* For windows  */
#ifndef SDL_PERL_DEFINES_H
#define SDL_PERL_DEFINES_H

#ifdef HAVE_TLS_CONTEXT
PerlInterpreter *parent_perl = NULL;
extern PerlInterpreter *parent_perl;
#define GET_TLS_CONTEXT parent_perl =  PERL_GET_CONTEXT;
#define ENTER_TLS_CONTEXT \
        PerlInterpreter *current_perl = PERL_GET_CONTEXT; \
            PERL_SET_CONTEXT(parent_perl); { \
                                PerlInterpreter *my_perl = parent_perl;
#define LEAVE_TLS_CONTEXT \
                                        } PERL_SET_CONTEXT(current_perl);
#else
#define GET_TLS_CONTEXT         /* TLS context not enabled */
#define ENTER_TLS_CONTEXT       /* TLS context not enabled */
#define LEAVE_TLS_CONTEXT       /* TLS context not enabled */
#endif

#endif


#include <SDL.h>

#define MY_CXT_KEY "SDL::Time::_guts" XS_VERSION 


 typedef struct {
 void* data;
 SV* callback;
 Uint32 retval;
 } my_cxt_t;

static my_cxt_t gcxt;

START_MY_CXT 


static Uint32 add_timer_cb ( Uint32 interval, void* param )
{

        ENTER_TLS_CONTEXT
        dMY_CXT;
        dSP;
        int back;
        ENTER; //SEGFAULTS RIGHT HERE!
        SAVETMPS;
        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(interval)));
        PUTBACK;

        if (0 != (back = call_sv(MY_CXT.callback,G_SCALAR))) {
     SPAGAIN;
     if (back != 1 ) Perl_croak (aTHX_ "Timer Callback failed!");
     MY_CXT.retval = POPi;     
        } else {
     Perl_croak(aTHX_ "Timer Callback failed!");
        }

        FREETMPS;
        LEAVE;

        LEAVE_TLS_CONTEXT
        dMY_CXT;
        return MY_CXT.retval;

}

MODULE = SDL::Time  PACKAGE = SDL::Time    PREFIX = time_

BOOT:
{
  MY_CXT_INIT;
}


SDL_TimerID
time_add_timer ( interval, cmd )
    Uint32 interval
    void *cmd
    PREINIT:
     dMY_CXT;
    CODE:
     MY_CXT.callback=cmd; 
     gcxt = MY_CXT;
     RETVAL = SDL_AddTimer(interval,add_timer_cb,(void *)cmd);    
    OUTPUT:
     RETVAL

void
CLONE(...)
  CODE:
    MY_CXT_CLONE;

This segfaults as soon as I go into ENTER for the callback.

use SDL;
use SDL::Time;

SDL::init(SDL_INIT_TIMER);
my $time = 0;
SDL::Timer::add_timer(100, sub { $time++; return $_[0]} );
sleep(10);
print "Never Prints";

Output is

$

it should be

$ Never Prints

Answer 1

$time needs to be a shared variable - otherwise perl works with separate copies of the variable.

Answer 2

Quick comments:

• Do not use Perl structs (SV, AV, HV, ...) outside of the context of a Perl interpreter object. I.e. do not use it as C-level static data. It will blow up in a threading context. Trust me, I've been there.
• Check out the "Safely Storing Static Data in XS" section in the perlxs manpage.
• Some of that stuff you're doing looks rather non-public from the point of view of the perlapi. I'm not quite certain, though.

Answer 3

My preferred way of handling this is storing the data in the PL_modglobal hash. It's automatically tied to the current interpreter.

Answer 4

We have found a solution to this using Perl interpreter threads and threads::shared. Please see these

Time.xs

Also here is an example of a script using this code.

TestTimer.pl