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garbage collection

psu1991
stranger

Jul 24, 2001, 2:22 PM

Post #1 of 4 (1103 views)

garbage collection	Can't Post

$a = 123
$b = \$a
$c = $b
What is the reference count of $a? I think it should be
3, but want to verify with someone that may know for sure.

mhx
Enthusiast

Jul 24, 2001, 10:36 PM

Post #2 of 4 (1092 views)

Re: garbage collection [In reply to]	Can't Post

It is 3, at least with my HP-UX build of Perl 5.6.0. I'm not sure if there's an easier way to get the value of the reference counter, but mine isn't too hard anyway. (Took me only some minutes.)

If you have a look at the perlguts manpage, there's a section Reference Counts and Mortality. There's also an internal routine to retrieve the reference counter value of an SV called SvREFCNT. With this knowledge, one can easily write a small XSUB with the following code:

Code

MODULE = RefCount		PACKAGE = RefCount		 
  
int 
GetRefCount( sv ) 
	SV *sv 
	CODE: 
	RETVAL = SvREFCNT( sv ); 
	OUTPUT: 
	RETVAL

You can then use this XSUB from some Perl code like this:

Code

#!/bin/perl -w 
use RefCount; 
  
$a = 123; 
$b = \$a; 
$c = $b; 
printf "RefCount = %d\n", RefCount::GetRefCount( $a );

And in this case, it will print RefCount = 3.

There are also some interesting resources besides the perlguts manpage on the web. There is for example an article called Perl Guts Illustrated that graphically illustrates Perl's internal data types. Also, there's interesting reply by Nathan Torkington concerning circular references and the related problems.

Hope this helps.

-- Marcus

Code

s$$ab21b8d15c3d97bd6317286d$;$"=547269736;split'i',join$,,map{chr(($*+= 
($">>=1)&1?-hex:hex)+0140)}/./g;$"=chr$";s;.;\u$&;for@_[0,2];print"@_,"

psu1991
stranger

Jul 25, 2001, 8:06 AM

Post #3 of 4 (1088 views)

Re: garbage collection [In reply to]	Can't Post

Very cool stuff! thanks.
-Jason

mhx
Enthusiast

Aug 3, 2001, 8:06 AM

Post #4 of 4 (1069 views)

Re: garbage collection [In reply to]	Can't Post

Hehe, it's even easier to get the reference count of a variable in Perl than through writing an XS yourself. At least if one has a look at the standard Perl modules... Wink

Code

use Devel::Peek; 
$a = 123; 
$b = \$a; 
$c = $b; 
Dump $a;

That's it. It prints something similar to

Code

SV = IV(0x17637c4) at 0x1765374 
  REFCNT = 3 
  FLAGS = (IOK,pIOK,IsUV) 
  UV = 123

-- Marcus

Code

s$$ab21b8d15c3d97bd6317286d$;$"=547269736;split'i',join$,,map{chr(($*+= 
($">>=1)&1?-hex:hex)+0140)}/./g;$"=chr$";s;.;\u$&;for@_[0,2];print"@_,"

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