Re: [Chicken-users] Chicken + swig + C unions = infinite loop

[felix]

Dave wrote:
> Chicken looks really promising since I have been looking for something 
> that can wrap C++ apis and let me play around with them in a decent 
> language like scheme.
> In the course of trying it out by interfacing to SDL , I think I am 
> encountering a problem with C "union" types (SDL_Event is a union of all 
> the event types).
> I have a small test case below that seems to demonstrate the problem 
> which only seems to occur when allocating unions and  manifests as an 
> infinite loop. I assume that it is in the GC cycle, but I'm not sure.
> Here are the commands I used:
>
> swig -chicken crash.i
> csc crash_wrap.c crash.scm crash_test.scm
>
> in crash_test.scm, the first and second loops complete, but the third
> drops into a 100% cpu tailspin and never seems to recover. I tried this 
> with the chicken 1.0 release and  the 1.9 development version.
>
> I'd really appreciate any pointers anyone can give me with this!

Hi!

Thanks for the code. I was able to reproduce the problem pretty quickly.
The reason for the loop are calls to the interrupt-checking routine, that the
SWIG wrapper inserts. This routine (C_check_for_interrupt) is *not*
reentrant, since it saves the current limit for the allocation area
and sets it to an artificial value, that is guaranteed to trigger the
garbage-collector at the next check. In your test-code, you create
a very tight sequence of calls to crash:new-combo and crash:delete-combo.
Now, when the timer interrupt is signalled (for multithreading), the
interrupt-check in crash_wrap_new_combo saves the old limit and resets the
limit, then immediately afterwards (without a stack-check),
crash_wrap_delete_combo does the same (and also saves the old limit, which
was invalidated by C_check_for_interrupt' before) and...
Now GC and interrupt-handling go on forever.

I propose to remove the interrupt-checks in Source/Modules/chicken.cxx
in the SWIG code, those checks really should only done at the Scheme-side,
here we always make sure the stack-check happens right after the interrupt-
check.

A diff is attached.


cheers,
felix

cd ~/tmp/SWIG-1.3.18/Source/Modules/
diff -c /home/felix/tmp/SWIG-1.3.18/Source/Modules/chicken.cxx\~ 
/home/felix/tmp/SWIG-1.3.18/Source/Modules/chicken.cxx
*** /home/felix/tmp/SWIG-1.3.18/Source/Modules/chicken.cxx~     Sun Mar 23 
21:43:20 2003
--- /home/felix/tmp/SWIG-1.3.18/Source/Modules/chicken.cxx      Fri May 16 
10:24:52 2003
***************
*** 528,535 ****
    }
  
    // Check for interrupts
!   Printv (f->code, "C_check_for_interrupt;\n",
!         "C_trace(\"",scmname,"\");\n", NIL);
  
    Printv(f->def,
  #ifndef BINDING
--- 528,534 ----
    }
  
    // Check for interrupts
!   Printv (f->code, "C_trace(\"",scmname,"\");\n", NIL);
  
    Printv(f->def,
  #ifndef BINDING
***************
*** 952,959 ****
    namify(proc_name);
  
    // Check for interrupts
!   Printv (f->code, "C_check_for_interrupt;\n",
!         "C_trace(\"",scmname,"\");\n", NIL);
  
    closargs     = NewString("");
    closwrapargs = NewString("");
--- 951,957 ----
    namify(proc_name);
  
    // Check for interrupts
!   Printv (f->code, "C_trace(\"",scmname,"\");\n", NIL);
  
    closargs     = NewString("");
    closwrapargs = NewString("");
***************
*** 1224,1231 ****
    namify(proc_name);
      
    // Check for interrupts
!   Printv (f->code, "C_check_for_interrupt;\n",
!         "C_trace(\"",scmname,"\");\n", NIL);
  
    if (1 || (SwigType_type(t) != T_USER) || (isPointer(t))) {
        
--- 1222,1228 ----
    namify(proc_name);
      
    // Check for interrupts
!   Printv (f->code, "C_trace(\"",scmname,"\");\n", NIL);
  
    if (1 || (SwigType_type(t) != T_USER) || (isPointer(t))) {
        

Diff finished at Fri May 16 10:29:40