Hi all.

A former college professor of mine and I were discussing memory leaks and garbage collection in various scripting languages the other day, and he asked me how this issue was handled in perl ( he bought 'Learning Perl - 3rd edition' last week ). After explaining what I knew about references ( I'm still learning ), he asked me if I could research the topic so we could discuss it in more depth.

So far, I have learned that perl handles this topic through a mechanism called reference counting. It seems each block of memory has an internal reference 'count' that stores the number of references which contain an address. If / When this number becomes zero, the memory that occupied that space is released ( freed ). 


{ # Start of naked block.

 my $ref;
 {
   my $num = 20;
   $ref = \$num;
 }
} #End of naked block.

[download]

When $num is created, the reference count becomes 1. Then the reference count is incremented to 2 when $ref is assigned a reference to $num. When we reach the end of the inner block, $num goes out of scope and we can no longer reference it through $num. This has the effect of decrementing the reference count by 1. At the end of the outer block, $ref goes out of scope thereby dropping the reference count to zero. This will free any memory previously occupied.

The problem occurs when one reference refers to another. 

{
  my $ref;
  my $ref2 = \$ref;
  $ref = \$ref2;
}

[download]

The reference cannot drop to zero and the memory it is occupying will never be freed ( the dreaded memory leak ). This can bring a machine to it's knees if left 'unattended'. Does anyone know of an efficient way to break circular references? Aside from assigning a value to the scalar $ref ( i.e. $ref = 15 ), I don't see a solution.

Thanks in advance,
-Katie.

In reply to Circular references and Garbage collection. by DigitalKitty

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